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

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       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)