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.
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
volume 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
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
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
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.
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.
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.
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.
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 Electrical 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
IEEE/The Open Group 2013 STRERROR(3P)