=head1 NAME

perlclib - Interacting with standard C library functions

=head1 DESCRIPTION

The perl interpreter is written in C; XS code also expands to C.
Inevitably, this code will call some functions from the C library,
C<libc>.  This document gives some guidance on interfacing with that
library.

One thing Perl porters should note is that F<perl> doesn't tend to use that
much of the C standard library internally; you'll see very little use of,
for example, the F<ctype.h> functions in there. This is because Perl
tends to reimplement or abstract standard library functions, so that we
know exactly how they're going to operate.

=head1 libc functions to avoid

There are many many libc functions.  Most of them are fair game to use,
but some are not.  Some of the possible reasons are:

=over

=item *

They likely will interfere with the perl interpreter's functioning,
such as its bookkeeping, or signal handling, or memory allocation,
or any number of harmful things.

=item *

They aren't implemented on all platforms, but there is an alternative
that is.

Or they may be buggy or deprecated on some or all platforms.

=item *

They aren't suitable for multi-threaded operation, but there is an
alternative that is, and is just as easily usable.

You may not expect your code to ever be used under threads, but code has
a way of being adapted beyond our initial expectations.  If it is just
as easy to use something that can be used under threads, it's better to
use that now, just in case.

Perl furnishes macros that can "cure" many cases of multi-thread
problems.  The functions they are generated for are listed in
L<perlapi/Mutex locking macros>.  The macros themselves are listed
alphabetically in F<perl_lock_definitions.h>.

=item *

In functions that deal with strings, complications may arise because the
string may be encoded in different ways, for example in UTF-8.  For
these, it is likely better to place the string in a SV and use the Perl
SV string handling functions that contain extensive logic to deal with
this.

=item *

In functions that deal with numbers, complications may arise because the
numbers get too big or small, and what those limits are depends on the
current platform.  Again, the Perl SV numeric data types have extensive
logic to take care of these kinds of issues.

=item *

They are locale-aware, and your caller may not want this.

=back

The following commentary and tables give some functions in the first
column that shouldn't be used in C or XS code, with the preferred
alternative (if any) in the second column.

=head2 Conventions

In the following tables:

=over 3

=item C<~>

marks the function as deprecated; it should not be used regardless.

=item C<t>

is a type.

=item C<p>

is a pointer.

=item C<n>

is a number.

=item C<s>

is a string.

=back

C<sv>, C<av>, C<hv>, etc. represent variables of their respective types.

=head2 File Operations

Instead of the F<stdio.h> functions, you should use the Perl abstraction
layer. Instead of C<FILE*> types, you need to be handling C<PerlIO*>
types.  Don't forget that with the new PerlIO layered I/O abstraction
C<FILE*> types may not even be available. See also the C<perlapio>
documentation for more information about the following functions:

  Instead Of:                 Use:

  stdin                       PerlIO_stdin()
  stdout                      PerlIO_stdout()
  stderr                      PerlIO_stderr()

  fopen(fn, mode)             PerlIO_open(fn, mode)
  freopen(fn, mode, stream)   PerlIO_reopen(fn, mode, perlio) (Dep-
                                recated)
  fflush(stream)              PerlIO_flush(perlio)
  fclose(stream)              PerlIO_close(perlio)

=head2 File Input and Output

  Instead Of:                 Use:

  fprintf(stream, fmt, ...)   PerlIO_printf(perlio, fmt, ...)

  [f]getc(stream)             PerlIO_getc(perlio)
  [f]putc(stream, n)          PerlIO_putc(perlio, n)
  ungetc(n, stream)           PerlIO_ungetc(perlio, n)

Note that the PerlIO equivalents of C<fread> and C<fwrite> are slightly
different from their C library counterparts:

  fread(p, size, n, stream)   PerlIO_read(perlio, buf, numbytes)
  fwrite(p, size, n, stream)  PerlIO_write(perlio, buf, numbytes)

  fputs(s, stream)            PerlIO_puts(perlio, s)

There is no equivalent to C<fgets> (or the deprecated C<gets>); one
should use C<sv_gets> instead:

  fgets(s, n, stream)         sv_gets(sv, perlio, append)

=head2 File Positioning

  Instead Of:                 Use:

  feof(stream)                PerlIO_eof(perlio)
  fseek(stream, n, whence)    PerlIO_seek(perlio, n, whence)
  rewind(stream)              PerlIO_rewind(perlio)

  fgetpos(stream, p)          PerlIO_getpos(perlio, sv)
  fsetpos(stream, p)          PerlIO_setpos(perlio, sv)

  ferror(stream)              PerlIO_error(perlio)
  clearerr(stream)            PerlIO_clearerr(perlio)

=head2 Memory Management and String Handling

  Instead Of:                    Use:

  t* p = malloc(n)               Newx(p, n, t)
  t* p = calloc(n, s)            Newxz(p, n, t)   (But see [1] below)
  p = realloc(p, n)              Renew(p, n, t)

It is not portable to try to allocate 0 bytes; allocating 1 or more is
portable.  Never pass pointers between C<Newx>, C<Renew>, C<Safefree> and
I<libc> equivalents C<malloc>, C<realloc>, C<free>.  They are not from the
same memory pool or allocator.  Either an instant or delayed I<SEGV> will
occur, or subtle memory leaks or subtle heap corruption.

  memcpy(dst, src, n)            Copy(src, dst, n, t)
  memmove(dst, src, n)           Move(src, dst, n, t)
  memcpy(dst, src, sizeof(t))    StructCopy(src, dst, t)

Notice the different order of arguments to C<Copy> and C<Move> than used
in C<memcpy> and C<memmove>.

  memset(dst, 0, n * sizeof(t))  Zero(dst, n, t)      (But see [1] below)
  memzero(dst, 0)                Zero(dst, n, char)   (But see [1] below)
  free(p)                        Safefree(p)

  strdup(p)                      savepv(p)
  strndup(p, n)                  savepvn(p, n) (Hey, strndup doesn't
                                                exist!)

  strstr(big, little)            instr(big, little)
  memmem(big, blen, little, len) ninstr(big, bigend, little, little_end)
  strcmp(s1, s2)                 strLE(s1, s2) / strEQ(s1, s2)
                                               / strGT(s1,s2)
  strncmp(s1, s2, n)             strnNE(s1, s2, n) / strnEQ(s1, s2, n)

  memcmp(p1, p2, n)              memNE(p1, p2, n)
  !memcmp(p1, p2, n)             memEQ(p1, p2, n)

Most of the time, though, you'll want to be dealing with SVs internally
instead of raw C<char *> strings:

  strlen(s)                   sv_len(sv)
  strcpy(dt, src)             sv_setpv(sv, s)
  strncpy(dt, src, n)         sv_setpvn(sv, s, n)
  strcat(dt, src)             sv_catpv(sv, s)
  strncat(dt, src)            sv_catpvn(sv, s)

If you do need raw strings, use these instead:

  my_strnlen(s, maxlen)
  my_strlcpy(dt, src, sizeof(dt))
  my_strlcat(dt, src, sizeof(dt))

Similarly, you can use SVs for creating strings from formats

  sprintf(s, fmt, ...)         sv_setpvf(sv, fmt, ...)
  vsprintf(str, fmt, va_list)  sv_vsetpvf(sv, fmt, va_list)

Or for raw strings,

  my_snprintf(dt, len, fmt, ...)
  my_vsnprintf(dt, len, fmt, va_list)
  vsprintf(str, fmt, va_list)  sv_vsnprintf(sv, fmt, va_list)

Note also the existence of C<sv_catpvf> and C<sv_vcatpvfn>, combining
concatenation with formatting; and L<C<Perl_form>()|perlapi/form> for
another form of formatted populating.

Note that glibc C<printf()>, C<sprintf()>, etc. are buggy before glibc
version 2.17.  They won't allow a C<%.s> format with a precision to
create a string that isn't valid UTF-8 if the current underlying locale
of the program is UTF-8.  What happens is that the C<%s> and its
operand are simply skipped without any notice.
L<https://sourceware.org/bugzilla/show_bug.cgi?id=6530>.

[1] Sometimes instead of zeroing the allocated memory, you
should consider "poisoning" the data to catch uninitialized uses of it.
See L<perlhacktips/Catching hidden flaws>.

=head2 Character Class Tests

There are several types of character class tests that Perl implements.
All are more fully described in L<perlapi/Character classification> and
L<perlapi/Character case changing>.

The C library routines listed in the table below return values based on
the current locale.  Use the entries in the final column for that
functionality.  The other two columns always assume a POSIX (or C)
locale.  The entries in the ASCII column are only meaningful for ASCII
inputs, returning FALSE for anything else.  Use these only when you
B<know> that is what you want.  The entries in the Latin1 column assume
that the non-ASCII 8-bit characters are as Unicode defines them, the
same as ISO-8859-1, often called Latin 1.

  Instead Of:  Use for ASCII:   Use for Latin1:      Use for locale:

  isalnum(c)  isALPHANUMERIC(c) isALPHANUMERIC_L1(c) isALPHANUMERIC_LC(c)
  isalpha(c)  isALPHA(c)        isALPHA_L1(c)        isALPHA_LC(u )
  isascii(c)  isASCII(c)                             isASCII_LC(c)
  isblank(c)  isBLANK(c)        isBLANK_L1(c)        isBLANK_LC(c)
  iscntrl(c)  isCNTRL(c)        isCNTRL_L1(c)        isCNTRL_LC(c)
  isdigit(c)  isDIGIT(c)        isDIGIT_L1(c)        isDIGIT_LC(c)
  isgraph(c)  isGRAPH(c)        isGRAPH_L1(c)        isGRAPH_LC(c)
  islower(c)  isLOWER(c)        isLOWER_L1(c)        isLOWER_LC(c)
  isprint(c)  isPRINT(c)        isPRINT_L1(c)        isPRINT_LC(c)
  ispunct(c)  isPUNCT(c)        isPUNCT_L1(c)        isPUNCT_LC(c)
  isspace(c)  isSPACE(c)        isSPACE_L1(c)        isSPACE_LC(c)
  isupper(c)  isUPPER(c)        isUPPER_L1(c)        isUPPER_LC(c)
  isxdigit(c) isXDIGIT(c)       isXDIGIT_L1(c)       isXDIGIT_LC(c)

  tolower(c)  toLOWER(c)        toLOWER_L1(c)
  toupper(c)  toUPPER(c)

For the corresponding functions like C<iswupper()>, I<etc.>, use
C<isUPPER_uvchr()> for non-locale; or C<isUPPER_LC_uvchr()> for locale.
And use C<toLOWER_uvchr()> instead of C<towlower()>, I<etc.>.  There are
no direct equivalents for locale; best to put the string into an SV.

Don't use any of the functions like C<isalnum_l()>.  Those are
non-portable, and interfere with Perl's internal handling.

To emphasize that you are operating only on ASCII characters, you can
append C<_A> to each of the macros in the ASCII column: C<isALPHA_A>,
C<isDIGIT_A>, and so on.

(There is no entry in the Latin1 column for C<isascii> even though there
is an C<isASCII_L1>, which is identical to C<isASCII>;  the
latter name is clearer.  There is no entry in the Latin1 column for
C<toupper> because the result can be non-Latin1.  You have to use
C<toUPPER_uvchr>, as described in L<perlapi/Character case changing>.)

Note that the libc caseless comparisons are crippled; Unicode
provides a richer set, using the concept of folding.  If you need
more than equality/non-equality, it's probably best to store your
strings in an SV and use SV functions to do the comparison.  Similarly
for collation.

=head2 F<stdlib.h> functions

  Instead Of:                 Use:

  atof(s)                     my_atof(s) or Atof(s)
  atoi(s)                     grok_atoUV(s, &uv, &e)
  atol(s)                     grok_atoUV(s, &uv, &e)
  strtod(s, &p)               Strtod(s, &p)
  strtol(s, &p, n)            Strtol(s, &p, b)
  strtoul(s, &p, n)           Strtoul(s, &p, b)

But note that even the alternative functions are subject to locale; see
L</Dealing with locales>.

Typical use is to do range checks on C<uv> before casting:

   int i; UV uv;
   char* end_ptr = input_end;
   if (grok_atoUV(input, &uv, &end_ptr)
       && uv <= INT_MAX)
     i = (int)uv;
     ... /* continue parsing from end_ptr */
   } else {
     ... /* parse error: not a decimal integer in range 0 .. MAX_IV */
   }

Notice also the C<grok_bin>, C<grok_hex>, and C<grok_oct> functions in
F<numeric.c> for converting strings representing numbers in the respective
bases into C<NV>s.  Note that grok_atoUV() doesn't handle negative inputs,
or leading whitespace (being purposefully strict).

=head2 Miscellaneous functions

You should not even B<want> to use F<setjmp.h> functions, but if you
think you do, use the C<JMPENV> stack in F<scope.h> instead.

 ~asctime()              Perl_sv_strftime_tm()
 ~asctime_r()            Perl_sv_strftime_tm()
  chsize()               my_chsize()
 ~ctime()                Perl_sv_strftime_tm()
 ~ctime_r()              Perl_sv_strftime_tm()
 ~cuserid()              DO NOT USE; see its man page
  dirfd()                my_dirfd()
  duplocale()            Perl_setlocale()
 ~ecvt()                 my_snprintf()
 ~endgrent_r()           endgrent()
 ~endhostent_r()         endhostent()
 ~endnetent_r()          endnetent()
 ~endprotoent_r()        endprotoent()
 ~endpwent_r()           endpwent()
 ~endservent_r()         endservent()
 ~endutent()             endutxent()
  exit(n)                my_exit(n)
 ~fcvt()                 my_snprintf()
  freelocale()           Perl_setlocale()
 ~ftw()                  nftw()
  getenv(s)              PerlEnv_getenv(s)
 ~gethostbyaddr()        getaddrinfo()
 ~gethostbyname()        getnameinfo()
 ~getpass()              DO NOT USE; see its man page
 ~getpw()                getpwuid()
 ~getutent()             getutxent()
 ~getutid()              getutxid()
 ~getutline()            getutxline()
 ~gsignal()              DO NOT USE; see its man page
  localeconv()           Perl_localeconv()
  mblen()                mbrlen()
  mbtowc()               mbrtowc()
  newlocale()            Perl_setlocale()
  pclose()               my_pclose()
  popen()                my_popen()
 ~pututline()            pututxline()
 ~qecvt()                my_snprintf()
 ~qfcvt()                my_snprintf()
  querylocale()          Perl_setlocale()
  int rand()             double Drand01()
  srand(n)               { seedDrand01((Rand_seed_t)n);
                           PL_srand_called = TRUE; }
 ~readdir_r()            readdir()
  realloc()              saferealloc(), Renew() or Renewc()
 ~re_comp()              regcomp()
 ~re_exec()              regexec()
 ~rexec()                rcmd()
 ~rexec_af()             rcmd()
  setenv(s, val)         my_setenv(s, val)
 ~setgrent_r()           setgrent()
 ~sethostent_r()         sethostent()
  setlocale()            Perl_setlocale()
  setlocale_r()          Perl_setlocale()
 ~setnetent_r()          setnetent()
 ~setprotoent_r()        setprotoent()
 ~setpwent_r()           setpwent()
 ~setservent_r()         setservent()
 ~setutent()             setutxent()
  sigaction()            rsignal(signo, handler)
 ~siginterrupt()         rsignal() with the SA_RESTART flag instead
  signal(signo, handler) rsignal(signo, handler)
 ~ssignal()              DO NOT USE; see its man page
  strcasecmp()           a Perl foldEQ-family function
  strerror()             sv_string_from_errnum()
  strerror_l()           sv_string_from_errnum()
  strerror_r()           sv_string_from_errnum()
  strftime()             Perl_sv_strftime_tm()
  strtod()               my_strtod() or Strtod()
  system(s)              Don't. Look at pp_system or use my_popen.
 ~tempnam()              mkstemp()
 ~tmpnam()               mkstemp()
  tmpnam_r()             mkstemp()
  uselocale()            Perl_setlocale()
  vsnprintf()            my_vsnprintf()
  wctob()                wcrtomb()
  wctomb()               wcrtomb()
  wsetlocale()           Perl_setlocale()

The Perl-furnished alternatives are documented in L<perlapi>, which you
should peruse anyway to see what all is available to you.

The lists are incomplete.  Think when using an unlisted function if it
seems likely to interfere with Perl.

=head1 Dealing with locales

Like it or not, your code will be executed in the context of a locale,
as are all C language programs.  See L<perllocale>.  Most libc calls are
not affected by the locale, but a surprising number are:

 addmntent()           getspent_r()        sethostent()
 alphasort()           getspnam()          sethostent_r()
 asctime()             getspnam_r()        setnetent()
 asctime_r()           getwc()             setnetent_r()
 asprintf()            getwchar()          setnetgrent()
 atof()                glob()              setprotoent()
 atoi()                gmtime()            setprotoent_r()
 atol()                gmtime_r()          setpwent()
 atoll()               grantpt()           setpwent_r()
 btowc()               iconv_open()        setrpcent()
 catopen()             inet_addr()         setservent()
 ctime()               inet_aton()         setservent_r()
 ctime_r()             inet_network()      setspent()
 cuserid()             inet_ntoa()         sgetspent_r()
 daylight              inet_ntop()         shm_open()
 dirname()             inet_pton()         shm_unlink()
 dprintf()             initgroups()        snprintf()
 endaliasent()         innetgr()           sprintf()
 endgrent()            iruserok()          sscanf()
 endgrent_r()          iruserok_af()       strcasecmp()
 endhostent()          isalnum()           strcasestr()
 endhostent_r()        isalnum_l()         strcoll()
 endnetent()           isalpha()           strerror()
 endnetent_r()         isalpha_l()         strerror_l()
 endprotoent()         isascii()           strerror_r()
 endprotoent_r()       isascii_l()         strfmon()
 endpwent()            isblank()           strfmon_l()
 endpwent_r()          isblank_l()         strfromd()
 endrpcent()           iscntrl()           strfromf()
 endservent()          iscntrl_l()         strfroml()
 endservent_r()        isdigit()           strftime()
 endspent()            isdigit_l()         strftime_l()
 err()                 isgraph()           strncasecmp()
 error()               isgraph_l()         strptime()
 error_at_line()       islower()           strsignal()
 errx()                islower_l()         strtod()
 fgetwc()              isprint()           strtof()
 fgetwc_unlocked()     isprint_l()         strtoimax()
 fgetws()              ispunct()           strtol()
 fgetws_unlocked()     ispunct_l()         strtold()
 fnmatch()             isspace()           strtoll()
 forkpty()             isspace_l()         strtoq()
 fprintf()             isupper()           strtoul()
 fputwc()              isupper_l()         strtoull()
 fputwc_unlocked()     iswalnum()          strtoumax()
 fputws()              iswalnum_l()        strtouq()
 fputws_unlocked()     iswalpha()          strverscmp()
 fscanf()              iswalpha_l()        strxfrm()
 fwprintf()            iswblank()          swprintf()
 fwscanf()             iswblank_l()        swscanf()
 getaddrinfo()         iswcntrl()          syslog()
 getaliasbyname_r()    iswcntrl_l()        timegm()
 getaliasent_r()       iswdigit()          timelocal()
 getdate()             iswdigit_l()        timezone
 getdate_r()           iswgraph()          tolower()
 getfsent()            iswgraph_l()        tolower_l()
 getfsfile()           iswlower()          toupper()
 getfsspec()           iswlower_l()        toupper_l()
 getgrent()            iswprint()          towctrans()
 getgrent_r()          iswprint_l()        towlower()
 getgrgid()            iswpunct()          towlower_l()
 getgrgid_r()          iswpunct_l()        towupper()
 getgrnam()            iswspace()          towupper_l()
 getgrnam_r()          iswspace_l()        tzname
 getgrouplist()        iswupper()          tzset()
 gethostbyaddr()       iswupper_l()        ungetwc()
 gethostbyaddr_r()     iswxdigit()         vasprintf()
 gethostbyname()       iswxdigit_l()       vdprintf()
 gethostbyname2()      isxdigit()          verr()
 gethostbyname2_r()    isxdigit_l()        verrx()
 gethostbyname_r()     localeconv()        versionsort()
 gethostent()          localtime()         vfprintf()
 gethostent_r()        localtime_r()       vfscanf()
 gethostid()           MB_CUR_MAX          vfwprintf()
 getlogin()            mblen()             vprintf()
 getlogin_r()          mbrlen()            vscanf()
 getmntent()           mbrtowc()           vsnprintf()
 getmntent_r()         mbsinit()           vsprintf()
 getnameinfo()         mbsnrtowcs()        vsscanf()
 getnetbyaddr()        mbsrtowcs()         vswprintf()
 getnetbyaddr_r()      mbstowcs()          vsyslog()
 getnetbyname()        mbtowc()            vwarn()
 getnetbyname_r()      mktime()            vwarnx()
 getnetent()           nan()               vwprintf()
 getnetent_r()         nanf()              warn()
 getnetgrent()         nanl()              warnx()
 getnetgrent_r()       nl_langinfo()       wcrtomb()
 getprotobyname()      openpty()           wcscasecmp()
 getprotobyname_r()    printf()            wcschr()
 getprotobynumber()    psiginfo()          wcscoll()
 getprotobynumber_r()  psignal()           wcsftime()
 getprotoent()         putpwent()          wcsncasecmp()
 getprotoent_r()       putspent()          wcsnrtombs()
 getpw()               putwc()             wcsrchr()
 getpwent()            putwchar()          wcsrtombs()
 getpwent_r()          regcomp()           wcstod()
 getpwnam()            regexec()           wcstof()
 getpwnam_r()          res_nclose()        wcstoimax()
 getpwuid()            res_ninit()         wcstold()
 getpwuid_r()          res_nquery()        wcstombs()
 getrpcbyname_r()      res_nquerydomain()  wcstoumax()
 getrpcbynumber_r()    res_nsearch()       wcswidth()
 getrpcent_r()         res_nsend()         wcsxfrm()
 getrpcport()          rpmatch()           wctob()
 getservbyname()       ruserok()           wctomb()
 getservbyname_r()     ruserok_af()        wctrans()
 getservbyport()       scandir()           wctype()
 getservbyport_r()     scanf()             wcwidth()
 getservent()          setaliasent()       wordexp()
 getservent_r()        setgrent()          wprintf()
 getspent()            setgrent_r()        wscanf()

(The list doesn't include functions that manipulate the locale, such as
C<setlocale()>.)

If any of these functions are called directly or indirectly from your
code, you are affected by the current locale.

The first thing to know about this list is that there are better
alternatives to many of the functions, which it's highly likely that you
should be using instead.  See L</libc functions to avoid> above.
This includes using Perl IO L<perlapio>.

The second thing to know is that Perl is documented to not pay attention
to the current locale except for code executed within the scope of a
S<C<use locale>> statement.  If you violate that, you may be creating
bugs, depending on the application.

The next thing to know is that many of these functions depend only on
the locale in regards to numeric values.  Your code is likely to have
been written expecting that the decimal point (radix) character is a dot
(U+002E: FULL STOP), and that strings of integer numbers are not
separated into groups (1,000,000 in an American locale means a million;
your code is likely not expecting the commas.)  The good news is that
normally (as of Perl v5.22), your code will get called with the locale
set so those expectations are met.  Explicit action has to be taken to
change this (described a little ways below).  This is accomplished by
Perl not actually switching into a locale that doesn't conform to these
expectations, except when explicitly told to do so.  The Perl
input/output and formatting routines do this switching for you
automatically, if appropriate, and then switch back.  If, for some
reason, you need to do it yourself, the easiest way from C and XS code
is to use the macro L<perlapi/C<WITH_LC_NUMERIC_SET_TO_NEEDED>>.  You
can wrap this macro around an entire block of code that you want to be
executed in the correct environment.  The bottom line is that your code
is likely to work as expected in this regard without you having to take
any action.

This leaves the remaining functions.  Your code will get called with all
but the numeric locale portions set to the underlying locale.  Often,
the locale is of not much import to your code, and you also won't have
to take any action; things will just work out.  But you should examine
the man pages of the ones you use to verify this.  Often, Perl has
better ways of doing the same functionality.  Consider using SVs and
their access routines rather than calling the low level functions that,
for example, find how many bytes are in a UTF-8 encoded character.

You can determine if you have been called from within the scope of a
S<C<use locale>> by using the boolen macro L<perlapi/C<IN_LOCALE>>.

If you need to not be in the underlying locale, you can call
L<perlapi/C<Perl_setlocale>> to change it temporarily to the one you
need (likely the "C" locale), and then change it back before returning.
This can be B<very> problematic on threaded perls on some platforms. See
L</Dealing with embedded perls and threads>.

A problem with changing the locale of a single category is that mojibake
can arise on some platforms if the C<LC_CTYPE> category and the changed one
are not the same.  On the platforms that that isn't an issue, the
preprocessor directive C<LIBC_HANDLES_MISMATCHED_CTYPE> will be defined.
Otherwise, you may have to change more than one category to correctly
accomplish your task.  And, there will be many locale combinations where
the mojibake likely won't happen, so you won't be confronted with this
until the code gets executed in the field by someone who doesn't speak
your language very well.

Earlier we mentioned that explicit action is required to have your code
get called with the numeric portions of the locale not meeting the
typical expectations of having a dot for the radix character and no
punctuation separating groups of digits.  That action is to call the
function L<perlapi/C<switch_to_global_locale>>.

C<switch_to_global_locale()> was written initially to cope with the
C<Tk> library, but is general enough for other similar situations.  C<Tk>
changes the global locale to match its expectations (later versions of
it allow this to be turned off).  This presents a conflict with Perl
thinking it also controls the locale.  Calling this function tells Perl to
yield control.  Calling L<perlapi/C<sync_locale>> tells Perl to take
control again, accepting whatever the locale has been changed to in the
interim.  If your code is called during that interim, all portions of
the locale will be the raw underlying values.  Should you need to
manipulate numbers, you are on your own with regard to the radix
character and grouping.  If you find yourself in this situation, it is
generally best to make the interval between the calls to these two
functions as short as possible, and avoid calculations until after perl
has control again.

It is important for perl to know about all the possible locale
categories on the platform, even if they aren't apparently used in your
program.  Perl knows all of the Linux ones.  If your platform has
others, you can submit an issue at
L<https://github.com/Perl/perl5/issues> for inclusion of it in the next
release.  In the meantime, it is possible to edit the Perl source to
teach it about the category, and then recompile.  Search for instances
of, say, C<LC_PAPER> in the source, and use that as a template to add
the omitted one.

There are further complications under multi-threaded operation.  Keep on
reading.

=head1 Dealing with embedded perls and threads

It is possible to embed a Perl interpreter within a larger program.  See
L<perlembed>.

MULTIPLICITY is the way this is accomplished internally; it is described in
L<perlguts/How multiple interpreters and concurrency are supported>.
Multiple Perl interpreters may be embedded.

It is also possible to compile perl to support threading.  See
L<perlthrtut>.  Perl's implementation of threading requires
MULTIPLICITY, but not the other way around.

MULTIPLICITY without threading means that only one thing runs at a time,
so there are no concurrency issues, but each component or instance can
affect the global state, potentially interfering with the execution of
other instances.  This can happen if one instance:

=over

=item *

changes the current working directory

=item *

changes the process's environment

=item *

changes the global locale the process is operating under

=item *

writes to shared memory or to a shared file

=item *

uses a shared file descriptor (including a database iterator)

=item *

raises a signal that functions in other instances are sensitive to

=back

If your code doesn't do any of these things, nor depends on any of their
values, then Congratulations!!, you don't have to worry about MULTIPLICITY
or threading.  But wait, a surprising number of libc functions do
depend on data global to the process in some way that may not be
immediately obvious.  For example, calling C<L<strtok(3)>> changes the
global state of a process, and thus needs special attention.

The section 3 libc uses that we know about that have MULTIPLICITY and/or
multi-thread issues are:

 addmntent()             getrpcent_r()        re_exec()
 alphasort()             getrpcport()         regcomp()
 asctime()               getservbyname()      regerror()
 asctime_r()             getservbyname_r()    regexec()
 asprintf()              getservbyport()      res_nclose()
 atof()                  getservbyport_r()    res_ninit()
 atoi()                  getservent()         res_nquery()
 atol()                  getservent_r()       res_nquerydomain()
 atoll()                 getspent()           res_nsearch()
 basename()              getspent_r()         res_nsend()
 btowc()                 getspnam()           rexec()
 catgets()               getspnam_r()         rexec_af()
 catopen()               getttyent()          rpmatch()
 clearenv()              getttynam()          ruserok()
 clearerr_unlocked()     getusershell()       ruserok_af()
 crypt()                 getutent()           scandir()
 crypt_gensalt()         getutid()            scanf()
 crypt_r()               getutline()          secure_getenv()
 ctermid()               getutxent()          seed48()
 ctermid_r()             getutxid()           seed48_r()
 ctime()                 getutxline()         setaliasent()
 ctime_r()               getwc()              setcontext()
 cuserid()               getwchar()           setenv()
 daylight                getwchar_unlocked()  setfsent()
 dbm_clearerr()          getwc_unlocked()     setgrent()
 dbm_close()             glob()               setgrent_r()
 dbm_delete()            gmtime()             sethostent()
 dbm_error()             gmtime_r()           sethostent_r()
 dbm_fetch()             grantpt()            sethostid()
 dbm_firstkey()          hcreate()            setkey()
 dbm_nextkey()           hcreate_r()          setlocale()
 dbm_open()              hdestroy()           setlocale_r()
 dbm_store()             hdestroy_r()         setlogmask()
 dirname()               hsearch()            setnetent()
 dlerror()               hsearch_r()          setnetent_r()
 dprintf()               iconv()              setnetgrent()
 drand48()               iconv_open()         setprotoent()
 drand48_r()             inet_addr()          setprotoent_r()
 ecvt()                  inet_aton()          setpwent()
 encrypt()               inet_network()       setpwent_r()
 endaliasent()           inet_ntoa()          setrpcent()
 endfsent()              inet_ntop()          setservent()
 endgrent()              inet_pton()          setservent_r()
 endgrent_r()            initgroups()         setspent()
 endhostent()            initstate_r()        setstate_r()
 endhostent_r()          innetgr()            setttyent()
 endnetent()             iruserok()           setusershell()
 endnetent_r()           iruserok_af()        setutent()
 endnetgrent()           isalnum()            setutxent()
 endprotoent()           isalnum_l()          sgetspent()
 endprotoent_r()         isalpha()            sgetspent_r()
 endpwent()              isalpha_l()          shm_open()
 endpwent_r()            isascii()            shm_unlink()
 endrpcent()             isascii_l()          siginterrupt()
 endservent()            isblank()            sleep()
 endservent_r()          isblank_l()          snprintf()
 endspent()              iscntrl()            sprintf()
 endttyent()             iscntrl_l()          srand48()
 endusershell()          isdigit()            srand48_r()
 endutent()              isdigit_l()          srandom_r()
 endutxent()             isgraph()            sscanf()
 erand48()               isgraph_l()          ssignal()
 erand48_r()             islower()            strcasecmp()
 err()                   islower_l()          strcasestr()
 error()                 isprint()            strcoll()
 error_at_line()         isprint_l()          strerror()
 errx()                  ispunct()            strerror_l()
 ether_aton()            ispunct_l()          strerror_r()
 ether_ntoa()            isspace()            strfmon()
 execlp()                isspace_l()          strfmon_l()
 execvp()                isupper()            strfromd()
 execvpe()               isupper_l()          strfromf()
 exit()                  iswalnum()           strfroml()
 __fbufsize()            iswalnum_l()         strftime()
 fcloseall()             iswalpha()           strftime_l()
 fcvt()                  iswalpha_l()         strncasecmp()
 fflush_unlocked()       iswblank()           strptime()
 fgetc_unlocked()        iswblank_l()         strsignal()
 fgetgrent()             iswcntrl()           strtod()
 fgetpwent()             iswcntrl_l()         strtof()
 fgetspent()             iswdigit()           strtoimax()
 fgets_unlocked()        iswdigit_l()         strtok()
 fgetwc()                iswgraph()           strtol()
 fgetwc_unlocked()       iswgraph_l()         strtold()
 fgetws()                iswlower()           strtoll()
 fgetws_unlocked()       iswlower_l()         strtoq()
 fnmatch()               iswprint()           strtoul()
 forkpty()               iswprint_l()         strtoull()
 __fpending()            iswpunct()           strtoumax()
 fprintf()               iswpunct_l()         strtouq()
 __fpurge()              iswspace()           strverscmp()
 fputc_unlocked()        iswspace_l()         strxfrm()
 fputs_unlocked()        iswupper()           swapcontext()
 fputwc()                iswupper_l()         swprintf()
 fputwc_unlocked()       iswxdigit()          swscanf()
 fputws()                iswxdigit_l()        sysconf()
 fputws_unlocked()       isxdigit()           syslog()
 fread_unlocked()        isxdigit_l()         system()
 fscanf()                jrand48()            tdelete()
 __fsetlocking()         jrand48_r()          tempnam()
 fts_children()          l64a()               tfind()
 fts_read()              lcong48()            timegm()
 ftw()                   lcong48_r()          timelocal()
 fwprintf()              lgamma()             timezone
 fwrite_unlocked()       lgammaf()            tmpnam()
 fwscanf()               lgammal()            tmpnam_r()
 gamma()                 localeconv()         tolower()
 gammaf()                localtime()          tolower_l()
 gammal()                localtime_r()        toupper()
 getaddrinfo()           login()              toupper_l()
 getaliasbyname()        login_tty()          towctrans()
 getaliasbyname_r()      logout()             towlower()
 getaliasent()           logwtmp()            towlower_l()
 getaliasent_r()         lrand48()            towupper()
 getchar_unlocked()      lrand48_r()          towupper_l()
 getcontext()            makecontext()        tsearch()
 getc_unlocked()         mallinfo()           ttyname()
 get_current_dir_name()  MB_CUR_MAX           ttyname_r()
 getdate()               mblen()              ttyslot()
 getdate_r()             mbrlen()             twalk()
 getenv()                mbrtowc()            twalk_r()
 getfsent()              mbsinit()            tzname
 getfsfile()             mbsnrtowcs()         tzset()
 getfsspec()             mbsrtowcs()          ungetwc()
 getgrent()              mbstowcs()           unsetenv()
 getgrent_r()            mbtowc()             updwtmp()
 getgrgid()              mcheck()             utmpname()
 getgrgid_r()            mcheck_check_all()   va_arg()
 getgrnam()              mcheck_pedantic()    valloc()
 getgrnam_r()            mktime()             vasprintf()
 getgrouplist()          mprobe()             vdprintf()
 gethostbyaddr()         mrand48()            verr()
 gethostbyaddr_r()       mrand48_r()          verrx()
 gethostbyname()         mtrace()             versionsort()
 gethostbyname2()        muntrace()           vfprintf()
 gethostbyname2_r()      nan()                vfscanf()
 gethostbyname_r()       nanf()               vfwprintf()
 gethostent()            nanl()               vprintf()
 gethostent_r()          newlocale()          vscanf()
 gethostid()             nftw()               vsnprintf()
 getlogin()              nl_langinfo()        vsprintf()
 getlogin_r()            nrand48()            vsscanf()
 getmntent()             nrand48_r()          vswprintf()
 getmntent_r()           openpty()            vsyslog()
 getnameinfo()           perror()             vwarn()
 getnetbyaddr()          posix_fallocate()    vwarnx()
 getnetbyaddr_r()        printf()             vwprintf()
 getnetbyname()          profil()             warn()
 getnetbyname_r()        psiginfo()           warnx()
 getnetent()             psignal()            wcrtomb()
 getnetent_r()           ptsname()            wcscasecmp()
 getnetgrent()           putchar_unlocked()   wcschr()
 getnetgrent_r()         putc_unlocked()      wcscoll()
 getopt()                putenv()             wcsftime()
 getopt_long()           putpwent()           wcsncasecmp()
 getopt_long_only()      putspent()           wcsnrtombs()
 getpass()               pututline()          wcsrchr()
 getprotobyname()        pututxline()         wcsrtombs()
 getprotobyname_r()      putwc()              wcstod()
 getprotobynumber()      putwchar()           wcstof()
 getprotobynumber_r()    putwchar_unlocked()  wcstoimax()
 getprotoent()           putwc_unlocked()     wcstold()
 getprotoent_r()         pvalloc()            wcstombs()
 getpw()                 qecvt()              wcstoumax()
 getpwent()              qfcvt()              wcswidth()
 getpwent_r()            querylocale()        wcsxfrm()
 getpwnam()              rand()               wctob()
 getpwnam_r()            random_r()           wctomb()
 getpwuid()              rcmd()               wctrans()
 getpwuid_r()            rcmd_af()            wctype()
 getrpcbyname()          readdir()            wcwidth()
 getrpcbyname_r()        readdir64()          wordexp()
 getrpcbynumber()        readdir64_r()        wprintf()
 getrpcbynumber_r()      readdir_r()          wscanf()
 getrpcent()             re_comp()            wsetlocale()

(If you know of additional functions that are unsafe on some platform or
another, notify us via filing a bug report at
L<https://github.com/Perl/perl5/issues>.)

Some of these are safe under MULTIPLICITY, problematic only under threading.
If a use doesn't appear in the above list, we think it is MULTIPLICITY
and thread-safe on all platforms.

All the uses listed above are function calls, except for these:

 daylight  MB_CUR_MAX  timezone  tzname

There are various approaches to coping with issues involving these
constructs, each suitable for different circumstances:

=over

=item *

Don't use them.

Some of them are obsolete.  For example, most functions listed above that
deal with randomness or cryptography have been superceded by other
functions that are more secure, and tend to be thread-safe.  C<setkey>,
for example has even been removed from some distributions.

Others have preferred alternatives.  Use the list above in L</libc
functions to avoid> to replace your uses with ones that are
thread-friendly.  For example I/O, should be done via L<perlapio>.

Still others are implemented in the L<POSIX> module, and can be called
from XS code using the mechanisms described in L<perlcall>.  See the
next item.

=item *

Use them with mutexes.

Since v5.46, perl furnishes wrapper macros to make many of the above calls
thread-safe.  If you surround a function call with the macros designed
for it that are found in F<perl_lock_definitions.h>, you automatically
make it thread-safe, as long as all other executing threads have done
the same.

Note that if the function returns results in global storage, you will
need to save it to a per-instance area before unlocking.  Then you can
work on it at your leisure.  This might be an automatic C variable for
non-pointers, or something as described in C<L<perlxs/Safely Storing
Static Data in XS>>.

(If you're writing a module, you can't guarantee the behavior of other
modules in other threads, but you can advertise that your module is
thread-safe when used with any other thread-safe module(s).)

For example F<perl_lock_definitions.h> defines the C<PERL_TZSET_LOCK> and
C<PERL_TZSET_UNLOCK> macros to wrap C<tzset> with, making it
thread-safe.

There are a few functions that are callable from Perl code that use
mutexes somewhat prior to v5.46.  You can call them to get thread-safety
a little further back if you use L<perlcall> to access them.  An example
is C<tzset>.

Otherwise, you can say that your module is thread-safe when used with
perls 5.46 or later.

=item *

C<#define PERL_REENTRANT>

Some functions have reentrant versions on some or all systems and which don't
have races, or have fewer races than their base counterpart.  You gain
automatic conversion to use these when you define this symbol prior to
including F<perl.h>.
See L</Reentrant equivalent functions> below for details.

=item *

If the above items don't work for you, many, but certainly not all, of
the functions will be ok as long as their use is confined to a single
thread that has no interaction with conflicting uses in other threads.
You will need to closely examine their man pages for this, and be aware
that vendor documentation is often imprecise.

An example of this case is the C<dbm> set of functions.  Perl does not
generate locking macros for functions that are used atomically in
conjunction with other functions.  In the case of databases with
iterators, the open, traversal, and close must be done without
interference from other threads.  As long as just a single thread
accesses a given database, there is no need for locking.  If that cannot
be guaranteed, you'll have to carefully craft your own mutexes, and be
very worried about deadlock.

=item *

Do all your business at initialization, before any other code can change
things.

=back

Without threading, you don't have to worry about being interrupted by
the system giving control to another thread.  With threading, you will
have to use mutexes, and, if not using the automatically furnished ones,
be concerned with the possibility of deadlock.

=head2 Functions always unsuitable for use under multi-threads

A few functions are considered totally unsuited for use in a multi-thread
environment.  These must be called only during single-thread operation.

  endusershell()    @getaliasent()      muntrace()   rexec()
  ether_aton()      @getrpcbyname()     profil()     rexec_af()
  ether_ntoa()      @getrpcbynumber()   rcmd()       setusershell()
  fts_children()    @getrpcent()        rcmd_af()    ttyslot()
  fts_read()         getusershell()     re_comp()
 @getaliasbyname()   mtrace()           re_exec()

C<@> above marks the functions for which there are preferred alternatives
available on some platforms, and those alternatives may be suitable for
multi-thread use.

=head2 Functions which must be called at least once before starting threads

Some functions perform initialization on their first call that must be done
while still in a single-thread environment, but subsequent calls are
thread-safe when executed in a critical section controlled by mutexes.
Therefore, they must be called at least once before switching to
multi-threads:

 getutent()  getutline()  getutxid()    mallinfo()  valloc()
 getutid()   getutxent()  getutxline()  pvalloc()

=head2 Functions that are thread-safe when called with appropriate arguments

Some of the functions are thread-safe if called with arguments that
comply with certain (easily met) restrictions.  These are:

 ctermid()        mbrlen()      mbsrtowcs()  wcrtomb()
 cuserid()        mbrtowc()     tmpnam()     wcsnrtombs()
 error_at_line()  mbsnrtowcs()  va_arg()     wcsrtombs()

See the man pages of each for details.  (For completeness, the list
includes functions that you shouldn't be using anyway because of other
reasons.)

=head2 Functions vulnerable to signals

Some functions are vulnerable to asynchronous signals.  These are:

 getlogin()    getutid()    getutxid()    login()   pututline()  updwtmp()
 getlogin_r()  getutline()  getutxline()  logout()  pututxline() wordexp()
 getutent()    getutxent()  glob()        logwtmp() sleep()

Some libc's implement 'system()' thread-safely.  But in others, it also
has signal issues.

=head2 General issues with thread-safety

Some libc functions use and/or modify a global state, such as a database.
The libc functions presume that there is only one instance at a time
operating on that database.  Unpredictable results occur if more than one
does, even if the database is not changed.  For example, typically there is
a global iterator for such a data base and that iterator is maintained by
libc, so that each new read from any instance advances it, meaning that no
instance will see all the entries.  The only way to make these thread-safe
is to have an exclusive lock on a mutex from the open call all the way
through the close.  You are advised to not use such databases from more
than one instance at a time.

Other examples of functions that use a global state include pseudo-random
number generators.  Some libc implementations of 'rand()', for example, may
share the data across threads; and others may have per-thread data.  The
shared ones will have unreproducible results, as the threads will vary in
their timings and interactions.  This may be what you want; or it may not
be.  (This particular function is a candidate to be removed from the POSIX
Standard because of these issues.)

Functions that output to a stream also are considered thread-unsafe when
locking is not done.  But the typical consequences are just that the data
is output in an unpredictable order; that outcome may be totally
acceptable to you.

Since the current working directory is global to a process, all
instances depend on it.  One instance doing a chdir(2) affects all the
other instances.  In a multi-threaded environment, any libc call that
expects the directory to not change for the duration of its execution
will have undefined results if another thread interrupts it at just the
wrong time and changes the directory.  The man pages only list one such
call, nftw().  But there may be other issues lurking.

=head2 Reentrant equivalent functions

Some functions that are problematic with regard to MULTIPLICITY have
reentrant versions (on some or all platforms) that are better suited,
with fewer (perhaps no) races when run under threads.

Some of these reentrant functions that are available on all platforms
should always be used anyway; they are in the lists directly under
L<libc functions to avoid>.

Others may not be available on some platforms, or have issues that make
them undesirable to use even when they are available.  Or it may just be
more complicated and tedious to use the reentrant version.  For these,
perl has a mechanism for automatically substituting that reentrant
version when available and desirable, while hiding the complications
from your code.  This feature is enabled by default for code in the Perl
core and its extensions.  To enable it in other XS modules,

   #define PERL_REENTRANT

It is simpler for you to use the unpreferred version in your code, and
rely on this feature to do the better thing, in part because no
substitution is done if the alternative is not available or desirable on
the platform, nor if threads aren't enabled.  You just write as if there
weren't threads, and you get the better behavior without having to think
about it.

An example is C<localtime>.  It has various potential races with other
threads executing related functions.  POSIX 2008 added a C<localtime_r>
reentrant version which doesn't have such races (but it does require
that the environment not change during its execution, nor the locale if
it isn't executing in a per-thread locale).  If your code has defined
C<PERL_REENTRANT>, all calls to C<localtime> will automatically be
converted to C<localtime_r> on systems that implement POSIX 2008.
Further, if you have wrapped calls to C<localtime> with
C<PERL_LOCALTIME_LOCK>/C<PERL_LOCALTIME_UNLOCK> macros, it will be
thread-safe, as long as the returned struct is copied to a safe place
before the unlock.  Further, on systems with C<localtime_r>, the calls
will have been converted to use that, and the calls to the locking
macros will be converted to use the locking macros for that function,
which have fewer things that need to be locked.

On some platforms the safer library functions may fail if the result
buffer is too small (for example the user group databases may be rather
large, and the reentrant functions may have to carry around a full
snapshot of those databases).  Perl will start with a small buffer, but
keep retrying and growing the result buffer until the result fits.  If
this limitless growing sounds bad for security or memory consumption
reasons you can recompile Perl with C<PERL_REENTRANT_MAXSIZE> #defined
to the maximum number of bytes you will allow.

Below is a list of the non-reentrant functions and their reentrant
alternatives.  This substitution is done even on functions that you
shouldn't be using in the first place.  These are marked by a C<*>.  You
should instead use the alternate given in the lists directly under
L<libc functions to avoid>.

Even so, some of the preferred alternatives are considered obsolete or
otherwise unwise to use on some platforms.  These are marked with a '?'.
Also, some alternatives aren't Perl-defined functions and aren't in
the POSIX Standard, so won't be widely available.  These are marked with
'~'.  (Remember that the automatic substitution only happens when they
are available and desirable, so you can just use the unpreferred
alternative.)

 *asctime()             ?asctime_r()
  crypt()               ~crypt_r()
  ctermid()             ~ctermid_r()
 *ctime()               ?ctime_r()
  endgrent()           ?~endgrent_r()
  endhostent()         ?~endhostent_r()
  endnetent()          ?~endnetent_r()
  endprotoent()        ?~endprotoent_r()
  endpwent()           ?~endpwent_r()
  endservent()         ?~endservent_r()
  getgrent()            ~getgrent_r()
  getgrgid()             getgrgid_r()
  getgrnam()             getgrnam_r()
  gethostbyaddr()       ~gethostbyaddr_r()
  gethostbyname()       ~gethostbyname_r()
  gethostent()          ~gethostent_r()
  getlogin()             getlogin_r()
  getnetbyaddr()        ~getnetbyaddr_r()
  getnetbyname()        ~getnetbyname_r()
  getnetent()           ~getnetent_r()
  getprotobyname()      ~getprotobyname_r()
  getprotobynumber()    ~getprotobynumber_r()
  getprotoent()         ~getprotoent_r()
  getpwent()            ~getpwent_r()
  getpwnam()             getpwnam_r()
  getpwuid()             getpwuid_r()
  getservbyname()       ~getservbyname_r()
  getservbyport()       ~getservbyport_r()
  getservent()          ~getservent_r()
  getspnam()            ~getspnam_r()
  gmtime()               gmtime_r()
  localtime()            localtime_r()
  readdir()             ?readdir_r()
  readdir64()           ~readdir64_r()
  setgrent()           ?~setgrent_r()
  sethostent()         ?~sethostent_r()
 *setlocale()          ?~setlocale_r()
  setnetent()          ?~setnetent_r()
  setprotoent()        ?~setprotoent_r()
  setpwent()           ?~setpwent_r()
  setservent()         ?~setservent_r()
 *strerror()             strerror_r()
 *tmpnam()              ~tmpnam_r()
  ttyname()              ttyname_r()

(There are other functions that could be added to have this behavior,
but nobody has.  Patches Welcome!  For example, some systems (but not
the POSIX Standard) have an C<lgamma_r> that doesn't change the global
C<signgam> that plain C<lgamma> does.  It would be fairly easy to add
that function.)

The Perl-furnished items are documented in perlapi.

The bottom line is:

=over

=item For items marked C<*>

Replace all uses of these with the preferred alternative given in the
lists directly under L<libc functions to avoid>.

=item For the remaining items

If you really need to use these functions, you have two choices:

=over

=item If you #define PERL_REENTRANT

Use the function in the first column as-is, and let perl do the work of
substituting the function in the right column if available on the
platform, and it is deemed suitable for use.  If there are locking
macros defined for it in F<perl_lock_definitions.h>, wrap your calls with
them.  Perl will automatically substitute whatever locking mechanism is
needed for the replacement function.

You should look at the man pages for both versions to find any other
gotchas.

=item If you don't enable automatic substitution

You should examine the application's code to determine if the column 1
function presents a real problem under threads given the circumstances
it is used in.  You can go directly to the column 2 replacement, but
beware of the ones that are marked.  Some of those may be nonexistent or
flaky on some platforms.  If there are macros defined for the
replacement in F<perl_lock_definitions.h>, wrap your calls with them.

=back

=back

=head2 Functions that need the environment to be constant

Since the environment is global to a process, all instances depend on
it.  One instance changing the environment affects all the other
instances.  Under threads, any libc call that expects the environment to
not change for the duration of its execution will have undefined results
if another thread interrupts it at just the wrong time and changes it.

Any locking macros for the functions in the list below, in
F<perl_lock_definitions.h>, automatically cause the environment to be held
constant while locked, so you don't otherwise need to be concerned about
them.

 catopen()               gethostbyname2()    newlocale()
 ctime()                 gethostbyname2_r()  regerror()
 ctime_r()               gethostbyname_r()   secure_getenv()
 endhostent()            gethostent()        sethostent()
 endhostent_r()          gethostent_r()      sethostent_r()
 endnetent()             gethostid()         setlocale()
 endnetent_r()           getnameinfo()       setlocale_r()
 execlp()                getnetbyname()      setnetent()
 execvp()                getnetent()         setnetent_r()
 execvpe()               getopt()            strftime()
 fnmatch()               getopt_long()       strptime()
 getaddrinfo()           getopt_long_only()  sysconf()
 get_current_dir_name()  getrpcport()        syslog()
 getdate()               glob()              tempnam()
 getdate_r()             gmtime()            timegm()
 getenv()                gmtime_r()          timelocal()
 gethostbyaddr()         localtime()         tzset()
 gethostbyaddr_r()       localtime_r()       vsyslog()
 gethostbyname()         mktime()

Many of these functions are problematic under threads for other reasons
as well.  See the man pages for any you use.

=head2 Locale-specific issues

C language programs originally had a single locale global to the entire
process.  This was later found to be inadequate for many purposes, so later
extensions changed that, first with Windows, and then POSIX 2008.  In
Windows, you can change any thread at any time to operate either with a
per-thread locale, or with the global one, using a special new libc
function.  In POSIX, the original API operates only on the global
locale, but there is an entirely new API to manipulate either per-thread
locales or the global one.  As with Windows (but using the new API), a
thread can be switched at any time to operate on the global locale, or a
per-thread one.

When one instance changes the global locale, all other instances using
the global locale are affected.  Almost all the locale-related functions
in the list directly under L</Dealing with embedded perls and threads>
have undefined behavior if another thread interrupts their execution and
changes the locale.  Under threads, another thread could do exactly that.
Starting in v5.46, if all threads wrap those function calls with the
locking macros designed for each and found in F<perl_lock_definitions.h>,
that interruption doesn't happen.  But, the thread may still find itself
executing in an unexpected locale, with bad results.

But, on systems that have per-thread locales, starting with Perl v5.28,
perl uses them after initialization; the global locale is not used
except if XS code has called C<switch_to_global_locale()>.  Doing so
affects only the thread that called it.  If a maximum of one instance is
using the global locale, no other instances are affected, the locale of
concurrently executing functions in other threads is not changed, and
this becomes a non-issue.  The C preprocessor symbol
C<USE_THREAD_SAFE_LOCALE> will be defined if per-thread locales are
available and perl has been compiled to use them.  The implementation of
per-thread locales on some platforms, like most *BSD-based ones, is so
buggy that the perl hints files for them deliberately turn off the
possibility of using them.

The converse is that on systems with only a global locale, having
different threads using different locales is not likely to work well;
and changing the locale is dangerous, and without use of the locking
macros, often leads to crashes.

Perl has extensive code to work as well as possible on both types of
systems.  You should always use C<Perl_setlocale()> to change and query
the locale, as it portably works across the range of possibilities.

=head1 SEE ALSO

L<perlapi>, L<perlapio>, L<perlguts>, L<perlxs>

