NAME

       perror - print a system error message

SYNOPSIS

       #include <stdio.h>

       void perror(const char *s);

       #include <errno.h>

       const char * const sys_errlist[];
       int sys_nerr;
       int errno;       /* Not really declared this way; see errno(3) */

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       sys_errlist, sys_nerr:
           From glibc 2.19 to 2.31:
               _DEFAULT_SOURCE
           Glibc 2.19 and earlier:
               _BSD_SOURCE

DESCRIPTION

       The perror() function produces a message on standard error describing the last error encountered during a
       call to a system or library function.

       First (if s is not NULL and *s is not a null byte ('\0')), the argument string s is printed, followed  by
       a colon and a blank.  Then an error message corresponding to the current value of errno and a new-line.

       To be of most use, the argument string should include the name of the function that incurred the error.

       The  global  error  list  sys_errlist[],  which  can be indexed by errno, can be used to obtain the error
       message without the newline.  The largest message number provided in the table is sys_nerr-1.  Be careful
       when  directly  accessing  this  list, because new error values may not have been added to sys_errlist[].
       The use of sys_errlist[] is nowadays deprecated; use strerror(3) instead.

       When a system call fails, it usually returns -1 and sets the variable errno to a  value  describing  what
       went wrong.  (These values can be found in <errno.h>.)  Many library functions do likewise.  The function
       perror() serves to translate this error code into human-readable form.   Note  that  errno  is  undefined
       after  a  successful  system call or library function call: this call may well change this variable, even
       though it succeeds, for example because it internally used  some  other  library  function  that  failed.
       Thus,  if  a failing call is not immediately followed by a call to perror(), the value of errno should be
       saved.

VERSIONS

       Since glibc version 2.32, the  declarations  of  sys_errlist  and  sys_nerr  are  no  longer  exposed  by
       <stdio.h>.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────┬───────────────┬─────────────────────┐
       │Interface │ Attribute     │ Value               │
       ├──────────┼───────────────┼─────────────────────┤
       │perror()  │ Thread safety │ MT-Safe race:stderr │
       └──────────┴───────────────┴─────────────────────┘

CONFORMING TO

       perror(), errno: POSIX.1-2001, POSIX.1-2008, C89, C99, 4.3BSD.

       The externals sys_nerr and sys_errlist derive from BSD, but are not specified in POSIX.1.

NOTES

       The externals sys_nerr and sys_errlist are defined by glibc, but in <stdio.h>.

SEE ALSO

       err(3), errno(3), error(3), strerror(3)

COLOPHON

       This  page  is  part  of  release  5.10  of  the  Linux man-pages project.  A description of the project,
       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.

                                                   2020-11-01                                          PERROR(3)