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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.1‐2008 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 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.1‐2008, <string.h>

COPYRIGHT

       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 https://www.kernel.org/doc/man-pages/reporting_bugs.html .