log message and exit

die.c

#include <errno.h>
#include <stdarg.h>
#include <stdio.h>

void die(const char *fmt, ...)
{
    va_list ap;
    va_start(ap, fmt);

    /* This buffer length is arbitrary. */
    const size_t buflen = 1024;
    static char message[buflen] = "ERROR: ";

    if (errno) {
        vsnprintf(message, buflen, fmt, ap);
        perror(message);
    } else {
        vsnprintf(strchr(message, '\0'), buflen - strlen(message), fmt, ap);
        fprintf(stderr, "%s\n", message);
    }

    va_end(ap);
    exit(1);
}