'\" et
.TH PERROR "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
perror
\(em write error messages to standard error
.SH SYNOPSIS
.LP
.nf
#include <stdio.h>
.P
void perror(const char *\fIs\fP);
.fi
.SH DESCRIPTION
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\(hy2017 defers to the ISO\ C standard.
.P
The
\fIperror\fR()
function shall map the error number accessed through the symbol
.IR errno
to a language-dependent error message, which shall be written to the
standard error stream as follows:
.IP " *" 4
First (if
.IR s
is not a null pointer and the character pointed to by
.IR s
is not the null byte), the string pointed to by
.IR s
followed by a
<colon>
and a
<space>.
.IP " *" 4
Then an error message string followed by a
<newline>.
.P
The contents of the error message strings shall be the same as those
returned by
\fIstrerror\fR()
with argument
.IR errno .
.P
The
\fIperror\fR()
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
\fIexit\fR(),
\fIabort\fR(),
or the completion of
\fIfflush\fR()
or
\fIfclose\fR()
on
.IR stderr .
.P
The
\fIperror\fR()
function shall not change the orientation of the standard error stream.
.P
On error,
\fIperror\fR()
shall set the error indicator for the stream to which
.IR stderr
points, and shall set
.IR errno
to indicate the error.
.P
Since no value is returned, an application wishing to check for error
situations should call
.IR clearerr ( stderr )
before calling
\fIperror\fR(),
then if
.IR ferror ( stderr )
returns non-zero, the value of
.IR errno
indicates which error occurred.
.SH "RETURN VALUE"
The
\fIperror\fR()
function shall not return a value.
.SH ERRORS
Refer to
.IR "\fIfputc\fR\^(\|)".
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Printing an Error Message for a Function"
.P
The following example replaces
.IR bufptr
with a buffer that is the necessary size. If an error occurs, the
\fIperror\fR()
function prints a message and the program exits.
.sp
.RS 4
.nf

#include <stdio.h>
#include <stdlib.h>
\&...
char *bufptr;
size_t szbuf;
\&...
if ((bufptr = malloc(szbuf)) == NULL) {
    perror("malloc"); exit(2);
}
\&...
.fi
.P
.RE
.SH "APPLICATION USAGE"
Application writers may prefer to use alternative interfaces instead of
\fIperror\fR(),
such as
\fIstrerror_r\fR()
in combination with
\fIfprintf\fR().
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIfprintf\fR\^(\|)",
.IR "\fIfputc\fR\^(\|)",
.IR "\fIpsiginfo\fR\^(\|)",
.IR "\fIstrerror\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<stdio.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .