Question

I’m currently writing a callback function in C:

static size_t writedata(void *ptr, size_t size, size_t nmemb, void *stream){

        size_t written = fwrite(ptr, size, nmemb, (FILE)*stream);
        return written;
}

This function is going to be used in another function, which does a HTTP request, retrieves the request, and writes it to the local machine. That writedata function will be used for the later part. The whole operation has to be multithreaded, so I was in doubt between write and fwrite. Could someone help me outlining the differences between write() and fwrite() in C, so I could choose which one best fits into my problem?

Answer 1

fwrite writes to a FILE*, i.e. a (potentially) buffered stdio stream. It’s specified by the ISO C standard. Additionally, on POSIX systems, fwrite is thread-safe to a certain degree.

write is a lower-level API based on file descriptors, described in the POSIX standard. It doesn’t know about buffering. If you want to use it on a FILE*, then fetch its file descriptor with fileno, but be sure to manually lock and flush the stream before attempting a write.

Use fwrite unless you know what you’re doing.