When does the write() system call write all of the requested buffer versus just doing a partial write?

Question

If I am counting on my write() system call to write say e.g., 100 bytes, I always put that write() call in a loop that checks to see if the length that gets returned is what I e

Answer 1

write may return partial write especially using operations on sockets or if internal buffers full. So good way is to do following:

while(size > 0 && (res=write(fd,buff,size))!=size) {
    if(res<0 && errno==EINTR) 
       continue;
    if(res < 0) {
        // real error processing
        break;
    }
    size-=res;
    buf+=res;
}

Never relay on what usually happens...

Note: in case of full disk you would get ENOSPC not partial write.