[原]write和fwrite选哪个?底层操作很快吗?

说是过几天到水利研究所参加连调,可惜这几天等着一点信号也没有,有点慢下来了.搞搞自己的...
实验室的arm开发箱虽然样子很大,但是一个开发箱居然都不给原理图.这让人怎么搞啊,想编译个内核,发现连液晶是哪个公司的型号都不知道...还是一步一步来吧...有C8051F做arm的过渡我感觉还不错哈...

晚上回来早点.继续上次的学习.fwrite和write,一个是标准I/O,一个是底层调用.谁快呢?我起初觉得即然用底层调用应该是很快的.不过实践是检验真理的好办法.

下面给出实际测试情况:
先上fwrite:
===========================
代码:
--------------------------------------------------

#include <stdio.h>

#define N (1024 * 1024 * 5)

int main(void)
{
char block[N] = {0};
FILE *ofp;
int cnt;

ofp = fopen("1G_file.out", "w");
for (cnt = 0; cnt < 200; cnt++)
fwrite(block, N, 1, ofp);

fclose(ofp);
printf("1G file creat complete!\n");
return 0;
}

 

-------------------------------------------------------
有多快呢?
测试结果:
--------------------------------------------------------
1G file creat complete!
0.00user 3.65system 0:36.67elapsed 9%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+2048016outputs (0major+1418minor)pagefaults 0swaps

casio$ ls -lh

总用量 1001M
-rw-r--r-- 1 casio casio 1000M 2010-05-15 22:35 1G_file.out
-rwxr-xr-x 1 casio casio 8.9K 2010-05-15 22:34 1G_fpr
-rw-r--r-- 1 casio casio 269 2010-05-15 22:25 1G_fpr.c

===============================================
下面是write:
代码,和上次经过了点修改:
------------------------------------------------------------------

#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>

#define N (1024 * 1024 * 5)

int main(void)
{
char block[N] = {0};
int out,cnt;

out = open("1G_file.out", O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR);
for (cnt = 0; cnt < 200; cnt++)
write(out, block, N);
printf("1G file creat complete!\n");
exit(0);
}

------------------------------------------------
测试结果如下:
--------------------------------------------------
1G file creat complete!
0.00user 3.42system 0:37.51elapsed 9%CPU (0avgtext+0avgdata 0maxresident)k
8inputs+2048000outputs (0major+1401minor)pagefaults 0swaps

casio:$ ls -lh

总用量 1001M
-rw------- 1 casio casio 1000M 2010-05-15 22:40 1G_file.out
-rwxr-xr-x 1 casio casio 7.7K 2010-05-15 22:39 1G_write
-rw-r--r-- 1 casio casio 359 2010-05-15 22:27 1G_write.c

=======================================
为此我经过了好几次测试,有时候write点,有时候fwrite快点,不过底层的时候稍微多那么几次.但是最大的差距不会超过1秒.所以底层调用和标准I/O看,性能都差不多.

stdio库在FILE结构里使用了一个内部缓冲区,只有在缓冲区满时才进行底层系统调用.