STRERROR(3)            FreeBSD Library Functions Manual            STRERROR(3)

NAME
     perror, strerror, strerror_l, strerror_r, sys_errlist, sys_nerr - system
     error messages

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

     void
     perror(const char *string);

     extern const char * const sys_errlist[];
     extern const int sys_nerr;

     #include <string.h>

     char *
     strerror(int errnum);

     char *
     strerror_l(int errnum, locale_t);

     int
     strerror_r(int errnum, char *strerrbuf, size_t buflen);

DESCRIPTION
     The strerror(), strerror_l(), strerror_r(), and perror() functions look
     up the error message string corresponding to an error number.

     The strerror() function accepts an error number argument errnum and
     returns a pointer to the corresponding message string in the current
     locale.  strerror() is not thread-safe.  It returns a pointer to an
     internal static buffer that could be overwritten by a strerror() call
     from another thread.

     The strerror_l() function accepts errnum error number and locale locale
     handle arguments and returns a pointer to a string corresponding to the
     specified error in the given locale.  strerror_l() is thread-safe, its
     result can be only overwritten by another call to strerror_l() from the
     current thread.

     The strerror_r() function renders the same result into strerrbuf for a
     maximum of buflen characters and returns 0 upon success.

     The perror() function finds the error message corresponding to the
     current value of the global variable errno (intro(2)) and writes it,
     followed by a newline, to the standard error file descriptor.  If the
     argument string is non-NULL and does not point to the null character,
     this string is prepended to the message string and separated from it by a
     colon and space (": "); otherwise, only the error message string is
     printed.

     If the error number is not recognized, these functions return an error
     message string containing "Unknown error: " followed by the error number
     in decimal.  The strerror() and strerror_r() functions return EINVAL as a
     warning.  Error numbers recognized by this implementation fall in the
     range 0 < errnum < sys_nerr.  The number 0 is also recognized, although
     applications that take advantage of this are likely to use unspecified
     values of errno.

     If insufficient storage is provided in strerrbuf (as specified in buflen)
     to contain the error string, strerror_r() returns ERANGE and strerrbuf
     will contain an error message that has been truncated and NUL terminated
     to fit the length specified by buflen.

     The message strings can be accessed directly using the external array
     sys_errlist.  The external value sys_nerr contains a count of the
     messages in sys_errlist.  The use of these variables is deprecated;
     strerror(), strerror_l(), or strerror_r() should be used instead.

EXAMPLES
     The following example shows how to use perror() to report an error.

       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>

       int
       main(void)
       {
               int fd;

               if ((fd = open("/nonexistent", O_RDONLY)) == -1) {
                       perror("open()");
                       exit(1);
               }
               printf("File descriptor: %d\n", fd);
               return (0);
       }

     When executed, the program will print an error message along the lines of
     `open(): No such file or directory'.

SEE ALSO
     intro(2), err(3), psignal(3)

STANDARDS
     The perror() and strerror() functions conform to ISO/IEC 9899:1999
     ("ISO C99").  The strerror_r() function conforms to IEEE Std 1003.1-2001
     ("POSIX.1").  The strerror_l() function conforms to IEEE Std 1003.1-2008
     ("POSIX.1").

HISTORY
     The strerror() and perror() functions first appeared in 4.4BSD.  The
     strerror_r() function was implemented in FreeBSD 4.4 by Wes Peters
     <wes@FreeBSD.org>.  The strerror_l() function was added in FreeBSD 13.0.

BUGS
     The strerror() function returns its result in a static buffer which will
     be overwritten by subsequent calls.

     Programs that use the deprecated sys_errlist variable often fail to
     compile because they declare it inconsistently.  Size of the sys_errlist
     object might increase during FreeBSD lifetime, breaking some ABI
     stability guarantees.

FreeBSD 13.1-RELEASE-p6        December 17, 2020       FreeBSD 13.1-RELEASE-p6