性能不应该依赖于任何一种方式,因为执行fwrite的任何人都会乘以大小并计数以确定要执行多少I/O操作。
这是由FreeBSD的libc实现的fwrite.c
,在其全部读取例子(包括指令省略掉):
/*
* 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);
}
我刚刚在OS-X上做了一些测试,写了10个100MB的文件,参数的顺序和使用write(2)而不是fwrite之间没有区别。至于其他平台,我不能说。 –