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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.1‐2017 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 returned pointer and the string content might also be invalidated if
the calling thread is terminated.
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 volume of POSIX.1‐2017 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
strerror_l() is required to return a string for some errors, an application 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,
application 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 eliminating 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 message 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 applications 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 encouraged to have strerror_r() return [EINVAL] and put a string like
"unknownerror" or "errornumberxxx" in the buffer pointed to by strerrbuf 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 strerror(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 ALSO
perror()
The Base Definitions volume of POSIX.1‐2017, <string.h>
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 .
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 .
IEEE/The Open Group 2017 STRERROR(3POSIX)