perror — print a system error message
#include <stdio.h>
void
perror( |
const char * | s); |
#include <errno.h> const char *sys_errlist[]; int sys_nerr; int errno;
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 a null byte ('\0'))
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.
The function perror() and
the external errno (see errno(3)) conform to C89,
C99, 4.3BSD, POSIX.1-2001. The externals sys_nerr and sys_errlist conform to
BSD.
The externals sys_nerr and sys_errlist are defined by
glibc, but in <stdio.h>. _GNU_SOURCE is defined, the symbols
err(3), errno(3), error(3), strerror(3)
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