perror(3) -- Linux man page
NAME
perror - print a system error message
SYNOPSIS
#include <stdio.h>
void perror(const char *s);
#include <errno.h>
const char *sys_errlist[];
int sys_nerr;
int errno;
DESCRIPTION
The routine
perror()
produces a message on the standard error output, describing the last
error encountered during a call to a system or library function.
First (if
s
is not NULL and
*s
is not NUL) the argument string
s
is printed, followed by a colon and a blank.
Then the message and a new-line.
To be of most use, the argument string should include the name
of the function that incurred the error. The error number is taken from
the external variable
errno,
which is set when errors occur but not
cleared when non-erroneous calls are made.
The global error list
sys_errlist[]
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[].
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 library 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.
CONFORMING TO
The function
perror()
and the external
errno
(see
errno(3))
conform to ANSI C, BSD 4.3, POSIX, X/OPEN.
The externals
sys_nerr
and
sys_errlist
conform to BSD.
NOTE
The externals
sys_nerr
and
sys_errlist
are defined by glibc, but in
<stdio.h>.
SEE ALSO
errno(3),
strerror(3)
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