NAME

perror — write error messages to standard error

SYNOPSIS

#include <stdio.h>

void perror(const char *s);


DESCRIPTION

^[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The perror() function shall map the error number accessed through the symbol errno to a language-dependent error message, which shall be written to the standard error stream as follows:

• First (if s is not a null pointer and the character pointed to by s is not the null byte), the string pointed to by s followed by a <colon> and a <space>.

• Then an error message string followed by a <newline>.

The contents of the error message strings shall be the same as those returned by strerror() with argument errno.

^[CX] The perror() function shall mark for update the last data modification and last file status change timestamps of the file associated with the standard error stream at some time between its successful completion and exit(), abort(), or the completion of fflush() or fclose() on stderr.

The perror() function shall not change the orientation of the standard error stream.

On error, perror() shall set the error indicator for the stream to which stderr points, and shall set errno to indicate the error.

Since no value is returned, an application wishing to check for error situations should call clearerr(stderr) before calling perror(), then if ferror(stderr) returns non-zero, the value of errno indicates which error occurred.

RETURN VALUE

The perror() function shall not return a value.

ERRORS

^[CX] Refer to fputc.

EXAMPLES

Printing an Error Message for a Function

The following example replaces bufptr with a buffer that is the necessary size. If an error occurs, the perror() function prints a message and the program exits.

#include <stdio.h>
#include <stdlib.h>
...
char *bufptr;
size_t szbuf;
...
if ((bufptr = malloc(szbuf)) == NULL) {
    perror("malloc"); exit(2);
}
...

APPLICATION USAGE

Application writers may prefer to use alternative interfaces instead of perror(), such as strerror_r() in combination with fprintf().

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

fprintf, fputc, psiginfo, strerror

XBD <stdio.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

A paragraph is added to the DESCRIPTION indicating that perror() does not change the orientation of the standard error stream.

Issue 6

Extensions beyond the ISO C standard are marked.

Issue 7

SD5-XSH-ERN-95 is applied.

Changes are made related to support for finegrained timestamps.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0429 [389,401], XSH/TC1-2008/0430 [389], and XSH/TC1-2008/0431 [389,401] are applied.