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PERROR(3)                                                               Linux Programmer's Manual                                                              PERROR(3)

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 correspond‐
       ing 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 pro‐
       vided  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)

                                                                               2021-03-22                                                                      PERROR(3)

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