osx man page for warn

ERR(3)							   BSD Library Functions Manual 						    ERR(3)

NAME
     err, verr, errc, verrc, errx, verrx, warn, vwarn, warnc, vwarnc, warnx, vwarnx, err_set_exit, err_set_exit_b, err_set_file -- formatted error
     messages

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <err.h>

     void
     err(int eval, const char *fmt, ...);

     void
     err_set_exit(void (*exitf)(int));

     void
     err_set_exit_b(void (^exitb)(int));

     void
     err_set_file(void *vfp);

     void
     errc(int eval, int code, const char *fmt, ...);

     void
     errx(int eval, const char *fmt, ...);

     void
     warn(const char *fmt, ...);

     void
     warnc(int code, const char *fmt, ...);

     void
     warnx(const char *fmt, ...);

     #include <stdarg.h>

     void
     verr(int eval, const char *fmt, va_list args);

     void
     verrc(int eval, int code, const char *fmt, va_list args);

     void
     verrx(int eval, const char *fmt, va_list args);

     void
     vwarn(const char *fmt, va_list args);

     void
     vwarnc(int code, const char *fmt, va_list args);

     void
     vwarnx(const char *fmt, va_list args);

DESCRIPTION
     The err() and warn() family of functions display a formatted error message on the standard error output, or on another file specified using
     the err_set_file() function.  In all cases, the last component of the program name, a colon character, and a space are output.  If the fmt
     argument is not NULL, the printf(3) -like formatted error message is output.  The output is terminated by a newline character.

     The err(), errc(), verr(), verrc(), warn(), warnc(), vwarn(), and vwarnc() functions append an error message obtained from strerror(3) based
     on a code or the global variable errno, preceded by another colon and space unless the fmt argument is NULL.

     In the case of the errc(), verrc(), warnc(), and vwarnc() functions, the code argument is used to look up the error message.

     The err(), verr(), warn(), and vwarn() functions use the global variable errno to look up the error message.

     The errx() and warnx() functions do not append an error message.

     The err(), verr(), errc(), verrc(), errx(), and verrx() functions do not return, but exit with the value of the argument eval.  It is recom-
     mended that the standard values defined in sysexits(3) be used for the value of eval.  The err_set_exit() function can be used to specify a
     function which is called before exit(3) to perform any necessary cleanup; passing a null function pointer for exitf resets the hook to do
     nothing.  The err_set_exit_b() function is like err_set_exit() except it takes a block pointer instead of a function pointer.

	   Note: The Block_copy() function (defined in <Blocks.h>) is used by err_set_exit_b() to make a copy of the block, especially for the
	   case when a stack-based block might go out of scope when the subroutine returns.

     The err_set_file() function sets the output stream used by the other functions.  Its vfp argument must be either a pointer to an open stream
     (possibly already converted to void *) or a null pointer (in which case the output stream is set to standard error).

EXAMPLES
     Display the current errno information string and exit:

	   if ((p = malloc(size)) == NULL)
		   err(1, NULL);
	   if ((fd = open(file_name, O_RDONLY, 0)) == -1)
		   err(1, "%s", file_name);

     Display an error message and exit:

	   if (tm.tm_hour < START_TIME)
		   errx(1, "too early, wait until %s", start_time_string);

     Warn of an error:

	   if ((fd = open(raw_device, O_RDONLY, 0)) == -1)
		   warnx("%s: %s: trying the block device",
		       raw_device, strerror(errno));
	   if ((fd = open(block_device, O_RDONLY, 0)) == -1)
		   err(1, "%s", block_device);

     Warn of an error without using the global variable errno:

	   error = my_function();  /* returns a value from <errno.h> */
	   if (error != 0)
		   warnc(error, "my_function");

SEE ALSO
     exit(3), fmtmsg(3), printf(3), strerror(3), sysexits(3)

HISTORY
     The err() and warn() functions first appeared in 4.4BSD.  The err_set_exit() and err_set_file() functions first appeared in FreeBSD 2.1.  The
     errc() and warnc() functions first appeared in FreeBSD 3.0.  The err_set_exit_b() function first appeared in Mac OS X 10.6.

BSD
								   May 20, 2008 							       BSD