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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:
           Since glibc 2.19:
               _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.

ATTRIBUTES

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

       ┌──────────┬───────────────┬─────────────────────┐
       │InterfaceAttributeValue               │
       ├──────────┼───────────────┼─────────────────────┤
       │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 4.16 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/.

                                            2017-09-15                                  PERROR(3)