How to calculate the length of output that sprintf will generate?

Question

Goal: serialize data to JSON.

Issue: i cant know beforehand how many chars long the integer is.

i thought a good way to do this

Answer 1

If you check the performance, you will running snprintf without an output buffer will take roughly the same time as a full invocation.

So I recommend you to use a smaller buffer just in case, and only call it a second time if the returned size exceeded the buffer size.

This uses C++'s std::string but I guess you can adapt it for your needs.

std::string format(const char* format, ...) {
    va_list args;
    va_start(args, format);
    char smallBuffer[1024];
    int size = vsnprintf(smallBuffer, sizeof smallBuffer, format, args);
    va_end(args);

    if (size < sizeof smallBuffer) 
        return std::string(smallBuffer);

    char buffer[size  + 1]; /* maybe malloc if it's too big */

    va_start(args, format);
    vsnprintf(buffer, sizeof buffer, format, args);
    va_end(args);
    return std::string(buffer);
}

This code will run 2x faster for strings under 1k compared to the longer ones.