Example of how to wrap C printf type functions to add a new format specifier

custom_fprintf.c

// adds the %hb specifer to print a number of bytes in human-readable number of bytes/kb/mb
// there's no way to modify or build a va_list after handling some of them ourselves
// so we get the standard function to ignore them by treating them as zero-length strings
// this could be a problem for compilers that evaluate a number as a string pointer before truncating
// but it works with clang on Xcode 15.3
// there's no buffer overflow handling to make the code easy to follow - don't deploy as-is in real code!
void custom_fprintf(FILE* stream, const char* format_orig, ...) {
    size_t format_orig_len = strlen(format_orig);
    
    va_list args_orig, args_copy;
    va_start(args_orig, format_orig);
    va_copy(args_copy, args_orig);

    char format_new[1024] = { 0 };
    size_t outlen = 0;

    const char *p = format_orig;
    while (p - format_orig < format_orig_len) {
        if (*p == '%') {
            int num = va_arg(args_orig, int);

            if (p + 2 < format_orig + format_orig_len && *(p + 1) == 'h' && *(p + 2) == 'b') {
                // the %%.0s specifier makes the standard printf function effectively ignore an arg!
                typedef struct { const char *u; int d; } ud;
                ud unidiv = { .u = NULL, .d = 0 };

                if (num < 1024)
                    unidiv = (ud) { .u = "bytes", .d = 1 };
                else if (num < 1024 * 1024)
                    unidiv = (ud) { .u = "KB", .d = 1024 };
                else if (num < 1024 * 1024 * 1024)
                    unidiv = (ud) { .u = "MB", .d = 1024 * 1024 };
                else
                    unidiv = (ud) { .u = "GB", .d = 1024 * 1024 * 1024 };

                outlen += snprintf(format_new + outlen, sizeof(format_new) - outlen, "%.2f %s%%.0s", (float)num / unidiv.d, unidiv.u);
                
                p += 3;
                continue;
            }
        }
        format_new[outlen++] = *p;
        p++;
    }
    format_new[outlen] = '\0';

    va_end(args_orig);
    vfprintf(stream, format_new, args_copy);
    va_end(args_copy);
}