Provided by: manpages-posix-dev_2017a-2_all 
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 .