strerror(3p) — Linux manual page

PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       strerror, strerror_l, strerror_r — get error message string

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

       None.

APPLICATION USAGE         top

       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         top

       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         top

       None.

SEE ALSO         top

       perror(3p)

       The Base Definitions volume of POSIX.1‐2017, string.h(0p)

COPYRIGHT         top

       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(3P)

Pages that refer to this page: string.h(0p)perror(3p)setlocale(3p)