This header implements the functions from BSD's <err.h>. In short:

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

Output a warning to stderr, consisting of the program name (as determined
by getprogname() -- if your system doesn't have this, you can use my
getprogname library, or #define getprogname() "you program's name"), followed
by printf() like output (if fmt is not NULL -- otherwise this portion is
skipped). Finally, warn() prints a string version of errno, and warnc() prints
the string if errno were code (warnx() simply returns after printf() handling).

In addition to those:

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

These print the same output as above, and then exit with a status of eval.

There are also versions of all of the above, prefixed with v, that take a
va_list parameter rather than variadic arguments.