"fmt2" is supposed to contain a format string. It's supposed to contain the format string from "fmt". That's how it prints the args from "ap". The bug is when

"fmt2" is supposed to contain a format string. It's supposed to contain the format string from "fmt". That's how it prints the args from "ap". The bug is when getprogname() also returns a string containing a format specifier (e.g. "%s"), which is not expected. Clearly the fix is: fprintf(stderr, "%s: ", getprogname()); va_start(ap, fmt); vfprintf(stderr, fmt, ap); va_end(ap); fprintf(stderr, "\n"); (Edit for formatting, clarity)