STRERROR(3)                POSIX Programmer's Manual               STRERROR(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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 ""uunnkknnoowwnnerror" or ""eerrrroorrnumberxxxxxx"" (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 ""uunnkknnoowwnnerror" or ""eerrrroorrnumberxxxxxx""
           (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 ""uunnkknnoowwnnerror" or ""eerrrroorrnumberxxxxxx"" 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(3)
