PERLCLIB(1) Perl Programmers Reference Guide PERLCLIB(1)
NAME
perlclib - Interacting with standard C library functions
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,
"libc". This document gives some guidance on interfacing with that
library.
One thing Perl porters should note is that perl doesn't tend to use that
much of the C standard library internally; you'll see very little use
of, for example, the 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.
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:
• 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.
• 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.
• 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.
• 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.
• 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.
• They are locale-aware, and your caller may not want this.
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.
Conventions
In the following tables:
"~"
marks the function as deprecated; it should not be used regardless.
"t"
is a type.
"p"
is a pointer.
"n"
is a number.
"s"
is a string.
"sv", "av", "hv", etc. represent variables of their respective types.
File Operations
Instead of the stdio.h functions, you should use the Perl abstraction
layer. Instead of "FILE*" types, you need to be handling "PerlIO*"
types. Don't forget that with the new PerlIO layered I/O abstraction
"FILE*" types may not even be available. See also the "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)
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 "fread" and "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 "fgets"; one should use "sv_gets" instead:
fgets(s, n, stream) sv_gets(sv, perlio, append)
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)
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)
p = realloc(p, n) Renew(p, n, t)
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)
memset(dst, 0, n * sizeof(t)) Zero(dst, n, t)
memzero(dst, 0) Zero(dst, n, char)
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)
Notice the different order of arguments to "Copy" and "Move" than used
in "memcpy" and "memmove".
Most of the time, though, you'll want to be dealing with SVs internally
instead of raw "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)
sprintf(s, fmt, ...) sv_setpvf(sv, fmt, ...)
If you do need raw strings, some platforms have safer interfaces, and
Perl makes sure a version of these are available on all platforms:
strlcat(dt, src, sizeof(dt)) my_strlcat(dt, src, sizeof(dt))
strlcpy(dt, src, sizeof(dt)) my_strlcpy(dt, src, sizeof(dt))
strnlen(s) my_strnlen(s, maxlen)
Note also the existence of "sv_catpvf" and "sv_vcatpvfn", combining
concatenation with formatting.
Sometimes instead of zeroing the allocated heap by using Newxz() you
should consider "poisoning" the data. This means writing a bit pattern
into it that should be illegal as pointers (and floating point numbers),
and also hopefully surprising enough as integers, so that any code
attempting to use the data without forethought will break sooner rather
than later. Poisoning can be done using the Poison() macros, which have
similar arguments to Zero():
PoisonWith(dst, n, t, b) scribble memory with byte b
PoisonNew(dst, n, t) equal to PoisonWith(dst, n, t, 0xAB)
PoisonFree(dst, n, t) equal to PoisonWith(dst, n, t, 0xEF)
Poison(dst, n, t) equal to PoisonFree(dst, n, t)
Character Class Tests
There are several types of character class tests that Perl implements.
All are more fully described in "Character classification" in perlapi
and "Character case changing" in perlapi.
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 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 iswupper(), etc., use
isUPPER_uvchr() for non-locale; or isUPPER_LC_uvchr() for locale. And
use toLOWER_uvchr() instead of towlower(), etc.. There are no direct
equivalents for locale; best to put the string into an SV.
Don't use any of the functions like 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 "_A" to each of the macros in the ASCII column: "isALPHA_A",
"isDIGIT_A", and so on.
(There is no entry in the Latin1 column for "isascii" even though there
is an "isASCII_L1", which is identical to "isASCII"; the latter name is
clearer. There is no entry in the Latin1 column for "toupper" because
the result can be non-Latin1. You have to use "toUPPER_uvchr", as
described in "Character case changing" in perlapi.)
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 comparision. Similarly for collation.
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 these are subject to locale; see "Dealing with locales".
Typical use is to do range checks on "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 "grok_bin", "grok_hex", and "grok_oct" functions in
numeric.c for converting strings representing numbers in the respective
bases into "NV"s. Note that grok_atoUV() doesn't handle negative
inputs, or leading whitespace (being purposefully strict).
Miscellaneous functions
You should not even want to use setjmp.h functions, but if you think you
do, use the "JMPENV" stack in 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() or tmpfile()
~tmpnam() mkstemp() or tmpfile()
tmpnam_r() mkstemp() or tmpfile()
uselocale() Perl_setlocale()
vsnprintf() my_vsnprintf()
wctob() wcrtomb()
wctomb() wcrtomb()
wsetlocale() Perl_setlocale()
The Perl-furnished alternatives are documented in 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.
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 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
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 "libc functions to avoid" above. This
includes using Perl IO 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
"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 ""WITH_LC_NUMERIC_SET_TO_NEEDED"" in perlapi. 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
"use locale" by using the boolen macro ""IN_LOCALE"" in perlapi.
If you need to not be in the underlying locale, you can call
""Perl_setlocale"" in perlapi to change it temporarily to the one you
need (likely the "C" locale), and then change it back before returning.
This can be very problematic on threaded perls on some platforms. See
"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 "LC_CTYPE" category and the changed
one are not the same. On the platforms that that isn't an issue, the
preprocessor directive "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 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 ""switch_to_global_locale"" in perlapi.
switch_to_global_locale() was written initially to cope with the "Tk"
library, but is general enough for other similar situations. "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 ""sync_locale"" in perlapi 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
<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, "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.
Dealing with embedded perls and threads
It is possible to embed a Perl interpreter within a larger program. See
perlembed.
MULTIPLICITY is the way this is accomplished internally; it is described
in "How multiple interpreters and concurrency are supported" in
perlguts. Multiple Perl interpreters may be embedded.
It is also possible to compile perl to support threading. See
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 instance. This can happen if one instance:
• changes the current working directory
• changes the process's environment
• changes the global locale the process is operating under
• writes to shared memory or to a shared file
• uses a shared file descriptor (including a database iterator)
• raises a signal that functions in other instances are sensitive to
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 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
<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 three main approaches to coping with issues involving these
constructs, each suitable for different circumstances:
• Don't use them. Some of them have preferred alternatives. Use the
list above in "libc functions to avoid" to replace your uses with
ones that are thread-friendly. For example I/O, should be done via
perlapio.
If you must use them, many, but not all, of them 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.
• Do all your business before any other code can change things. If
you make changes, change back before returning.
• Save the result of a query of global information to a per-instance
area before allowing another instance to execute. Then you can work
on it at your leisure. This might be an automatic C variable for
non-pointers, or something as described above in ""Safely Storing
Static Data in XS" in perlxs".
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 uses mutexes, and be concerned with the possibility of deadlock.
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()
"@" above marks the functions for which there are preferred alternatives
available on some platforms, and those alternatives may be suitable for
multi-thread use.
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. Therefore, they
must be called at least once before switching to multi-threads:
getutent() getutline() getutxid() mallinfo() valloc()
getutid() getutxent() getutxline() pvalloc()
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.)
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.
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 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.
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 "libc
functions to avoid".
Others may not be available on some platforms, or have issues that makes
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.
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 "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 "*". You
should instead use the alternate given in the lists directly under "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 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()
The Perl-furnished items are documented in perlapi.
The bottom line is:
For items marked "*"
Replace all uses of these with the preferred alternative given in
the lists directly under "libc functions to avoid".
For the remaining items
If you really need to use these functions, you have two choices:
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.
You should look at the man pages for both versions to find any
other gotchas.
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.
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.
These are the functions that the man pages list as being sensitive to
that.
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.
Perl defines mutexes "ENV_READ_LOCK" and "ENV_READ_UNLOCK" with which to
wrap calls to these functions. You need to consider the possibility of
deadlock. It is expected that a different mechanism will be in place
and preferred for Perl v5.42.
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 "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.
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 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
"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, often leading to crashes.
Perl has extensive code to work as well as possible on both types of
systems. You should always use Perl_setlocale() to change and query the
locale, as it portably works across the range of possibilities.
SEE ALSO
perlapi, perlapio, perlguts, perlxs
perl v5.40.1 2026-08-30 PERLCLIB(1)
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