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perror(3)                  Library Functions Manual                  perror(3)

NAME
       perror - print a system error message

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #include <stdio.h>

       void perror(const char *s);

       #include <errno.h>

       int errno;       /* Not really declared this way; see errno(3) */

       [[deprecated]] const char *const sys_errlist[];
       [[deprecated]] int sys_nerr;

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

       sys_errlist, sys_nerr:
           From glibc 2.19 to glibc 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  func-
       tion.

       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 dep-
       recated; 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  fol-
       lowed by a call to perror(), the value of errno should be saved.

VERSIONS
       Since  glibc  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  at-
       tributes(7).

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

STANDARDS
       perror(), errno: POSIX.1-2001, POSIX.1-2008, 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)

Linux man-pages 6.03              2023-02-05                         perror(3)

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