STRERROR(3P) POSIX Programmer's Manual STRERROR(3P)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.
NAME
strerror, strerror_l, strerror_r — get error message string
SYNOPSIS
#include <string.h>
char *strerror(int errnum);
char *strerror_l(int errnum, locale_t locale);
int strerror_r(int errnum, char *strerrbuf, size_t buflen);
DESCRIPTION
For strerror(): 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.1‐2008 defers to the ISO C standard.
The strerror() function shall map the error number in errnum to a
locale-dependent error message string and shall return a pointer to it.
Typically, the values for errnum come from errno, but strerror() shall
map any value of type int to a message.
The application shall not modify the string returned. The returned
string pointer might be invalidated or the string content might be
overwritten by a subsequent call to strerror(), or by a subsequent call
to strerror_l() in the same thread.
The string may be overwritten by a subsequent call to strerror_l() in
the same thread.
The contents of the error message strings returned by strerror() should
be determined by the setting of the LC_MESSAGES category in the current
locale.
The implementation shall behave as if no function defined in this vol‐
ume of POSIX.1‐2008 calls strerror().
The strerror() and strerror_l() functions shall not change the setting
of errno if successful.
Since no return value is reserved to indicate an error of strerror(),
an application wishing to check for error situations should set errno
to 0, then call strerror(), then check errno. Similarly, since str‐
error_l() is required to return a string for some errors, an applica‐
tion wishing to check for all error situations should set errno to 0,
then call strerror_l(), then check errno.
The strerror() function need not be thread-safe.
The strerror_l() function shall map the error number in errnum to a
locale-dependent error message string in the locale represented by
locale and shall return a pointer to it.
The strerror_r() function shall map the error number in errnum to a
locale-dependent error message string and shall return the string in
the buffer pointed to by strerrbuf, with length buflen.
If the value of errnum is a valid error number, the message string
shall indicate what error occurred; if the value of errnum is zero, the
message string shall either be an empty string or indicate that no
error occurred; otherwise, if these functions complete successfully,
the message string shall indicate that an unknown error occurred.
The behavior is undefined if the locale argument to strerror_l() is the
special locale object LC_GLOBAL_LOCALE or is not a valid locale object
handle.
RETURN VALUE
Upon completion, whether successful or not, strerror() shall return a
pointer to the generated message string. On error errno may be set,
but no return value is reserved to indicate an error.
Upon successful completion, strerror_l() shall return a pointer to the
generated message string. If errnum is not a valid error number, errno
may be set to [EINVAL], but a pointer to a message string shall still
be returned. If any other error occurs, errno shall be set to indicate
the error and a null pointer shall be returned.
Upon successful completion, strerror_r() shall return 0. Otherwise, an
error number shall be returned to indicate the error.
ERRORS
These functions may fail if:
EINVAL The value of errnum is neither a valid error number nor zero.
The strerror_r() function may fail if:
ERANGE Insufficient storage was supplied via strerrbuf and buflen to
contain the generated message string.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
Historically in some implementations, calls to perror() would overwrite
the string that the pointer returned by strerror() points to. Such
implementations did not conform to the ISO C standard; however, appli‐
cation developers should be aware of this behavior if they wish their
applications to be portable to such implementations.
RATIONALE
The strerror_l() function is required to be thread-safe, thereby elimi‐
nating the need for an equivalent to the strerror_r() function.
Earlier versions of this standard did not explicitly require that the
error message strings returned by strerror() and strerror_r() provide
any information about the error. This version of the standard requires
a meaningful message for any successful completion.
Since no return value is reserved to indicate a strerror() error, but
all calls (whether successful or not) must return a pointer to a mes‐
sage string, on error strerror() can return a pointer to an empty
string or a pointer to a meaningful string that can be printed.
Note that the [EINVAL] error condition is a may fail error. If an
invalid error number is supplied as the value of errnum, applications
should be prepared to handle any of the following:
1. Error (with no meaningful message): errno is set to [EINVAL], the
return value is a pointer to an empty string.
2. Successful completion: errno is unchanged and the return value
points to a string like "unknownerror" or "errornumberxxx" (where
xxx is the value of errnum).
3. Combination of #1 and #2: errno is set to [EINVAL] and the return
value points to a string like "unknownerror" or "errornumberxxx"
(where xxx is the value of errnum). Since applications frequently
use the return value of strerror() as an argument to functions like
fprintf() (without checking the return value) and since applica‐
tions have no way to parse an error message string to determine
whether errnum represents a valid error number, implementations are
encouraged to implement #3. Similarly, implementations are encour‐
aged to have strerror_r() return [EINVAL] and put a string like
"unknownerror" or "errornumberxxx" in the buffer pointed to by str‐
errbuf when the value of errnum is not a valid error number.
Some applications rely on being able to set errno to 0 before calling a
function with no reserved value to indicate an error, then call str‐
error(errno) afterwards to detect whether an error occurred (because
errno changed) or to indicate success (because errno remained zero).
This usage pattern requires that strerror(0) succeed with useful
results. Previous versions of the standard did not specify the behavior
when errnum is zero.
FUTURE DIRECTIONS
None.
SEE ALSOperror()
The Base Definitions volume of POSIX.1‐2008, <string.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 STRERROR(3P)
