size_t fwrite(const void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream);

The fwrite function writes, from the array pointed to by ptr, up to nmemb elements whose size is specified by size, to the stream pointed to by stream. For each object, size calls are made to the fputc function, taking the values (in order) from an array of unsigned char exactly overlaying the object. The file position indicator for the stream (if defined) is advanced by the number of characters successfully written. If an error occurs, the resulting value of the file position indicator for the stream is indeterminate.