Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
STRERROR(3)	       FreeBSD Library Functions Manual		   STRERROR(3)

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

     Standard C	Library	(libc, -lc)

     #include <stdio.h>

     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);

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

     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 re-
     turns a pointer to	the corresponding message string in the	current	lo-
     cale.  strerror() is not thread-safe.  It returns a pointer to an inter-
     nal static	buffer that could be overwritten by a strerror() call from an-
     other 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	cur-
     rent value	of the global variable errno (intro(2))	and writes it, fol-
     lowed by a	newline, to the	standard error file descriptor.	 If the	argu-
     ment 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

     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 mes-
     sages in sys_errlist.  The	use of these variables is deprecated;
     strerror(), strerror_l(), or strerror_r() should be used instead.

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

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

	       int fd;

	       if ((fd = open("/nonexistent", O_RDONLY)) == -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'.

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

     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

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

     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 com-
     pile because they declare it inconsistently.  Size	of the sys_errlist ob-
     ject might	increase during	FreeBSD	lifetime, breaking some	ABI stability

FreeBSD	13.0		       December	17, 2020		  FreeBSD 13.0


Want to link to this manual page? Use this URL:

home | help