fwrite() - 大小和计数对性能的影响 [英] fwrite() - effect of size and count on performance

问题描述

关于 fwrite() 中两个参数 'size' 和 'count' 的用途似乎有很多混淆.我想弄清楚哪个会更快 -

There seems to be a lot of confusion regarding the purpose of the two arguments 'size' and 'count' in fwrite(). I am trying to figure out which will be faster -

fwrite(source, 1, 50000, destination);

或

fwrite(source, 50000, 1, destination);

这是我代码中的一个重要决定，因为此命令将执行数百万次.

This is an important decision in my code as this command will be executed millions of times.

现在，我可以直接进行测试并使用能够提供更好结果的方法，但问题是该代码适用于许多平台.

Now, I could just jump to testing and use the one which gives better results, but the problem is that the code is intended for MANY platforms.

所以，

• 对于跨平台哪个更好，我如何才能得到明确的答案?

• How can I get a definitive answer to which is better across platforms?

fwrite() 的实现逻辑会因平台而异吗?

Will implementation logic of fwrite() vary from platform to platform?

我意识到有类似的问题(fread/fwrite 将大小和计数作为参数的基本原理是什么?，fwrite 和 write size 的性能) 但要明白这是关于同一问题的不同问题.在这种情况下，类似问题的答案是不够的.

I realize there are similar questions (What is the rationale for fread/fwrite taking size and count as arguments?, Performance of fwrite and write size) but do understand that this is a different question regarding the same issue. The answers in similar questions do not suffice in this case.

推荐答案

性能不应该依赖于任何一种方式，因为任何实现 fwrite 的人都会乘以 size 和 count 来确定要执行多少 I/O.

The performance should not depend on either way, because anyone implementing fwrite would multiply size and count to determine how much I/O to do.

以 FreeBSD 的 fwrite.c 的 libc 实现为例，它完整地读取(省略了包含指令):

This is exemplified by FreeBSD's libc implementation of fwrite.c, which in its entirety reads (include directives elided):

/*
 * Write `count' objects (each size `size') from memory to the given file.
 * Return the number of whole objects written.
 */
size_t
fwrite(buf, size, count, fp)
    const void * __restrict buf;
    size_t size, count;
    FILE * __restrict fp;
{
    size_t n;
    struct __suio uio;
    struct __siov iov;

    /*
     * ANSI and SUSv2 require a return value of 0 if size or count are 0.
     */
    if ((count == 0) || (size == 0))
        return (0);

    /*
     * Check for integer overflow.  As an optimization, first check that
     * at least one of {count, size} is at least 2^16, since if both
     * values are less than that, their product can't possible overflow
     * (size_t is always at least 32 bits on FreeBSD).
     */
    if (((count | size) > 0xFFFF) &&
        (count > SIZE_MAX / size)) {
        errno = EINVAL;
        fp->_flags |= __SERR;
        return (0);
    }

    n = count * size;

    iov.iov_base = (void *)buf;
    uio.uio_resid = iov.iov_len = n;
    uio.uio_iov = &iov;
    uio.uio_iovcnt = 1;

    FLOCKFILE(fp);
    ORIENT(fp, -1);
    /*
     * The usual case is success (__sfvwrite returns 0);
     * skip the divide if this happens, since divides are
     * generally slow and since this occurs whenever size==0.
     */
    if (__sfvwrite(fp, &uio) != 0)
        count = (n - uio.uio_resid) / size;
    FUNLOCKFILE(fp);
    return (count);
}

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