fread/fwrite

fread/fwrite第二个参数和第三个参数的区别以及与返回值的关系

size_t fwrite_unlocked(const void * __restrict ptr, size_t size,
                         size_t nmemb, register FILE * __restrict stream)
{
    __STDIO_STREAM_VALIDATE(stream);

    /* Note: If nmbem * size > SIZE_MAX then there is an application
     * bug since no array can be larger than SIZE_MAX in size. */

    if ((__STDIO_STREAM_IS_NARROW_WRITING(stream)
         || !__STDIO_STREAM_TRANS_TO_WRITE(stream, __FLAG_NARROW))
        && size && nmemb
        ) {

        if (nmemb <= (SIZE_MAX / size)) {
            return __stdio_fwrite((const unsigned char *) ptr,
                                  size*nmemb, stream) / size;
        }

        __STDIO_STREAM_SET_ERROR(stream);
        __set_errno(EINVAL);
    }

    return 0;
}

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