文件复制函数的效率比较

使用read, write, scanf, printf等系统调用或库函数，可以实现简单的文件复制程序，但效率上有明显差别。

方法一：逐个字符读入并输出

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

int main(void){
    char c;
    int in, out;

    in=open("file.in", O_RDONLY);
    out=open("file.out", O_WRONLY|O_CREAT, S_IRUSR|S_IWUSR);

    while(read(in, &c, 1)==1){
        write(out, &c, 1);
    }

    exit(0);
}

执行效率：

[lujinhong@lujinhong BeginningLinuxProgramming]$ TIMEFORAMT="" time ./a.out
0.31user 8.07system 0:08.40elapsed 99%CPU (0avgtext+0avgdata 1392maxresident)k
0inputs+9032outputs (0major+110minor)pagefaults 0swaps

方法二：每次读入1K再输出

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

#define READ_SIZE 1024

int main(void){
    char c[READ_SIZE];
    int in, out;
    int nread;

    in=open("file.in", O_RDONLY);
    out=open("file.out", O_WRONLY|O_CREAT, S_IRUSR|S_IWUSR);

    while((nread=read(in, c, READ_SIZE))>0){
        write(out, c, nread);
    }

    exit(0);
}

执行效率：

[lujinhong@lujinhong BeginningLinuxProgramming]$ TIMEFORAMT="" time ./a.out
0.00user 0.01system 0:00.01elapsed 92%CPU (0avgtext+0avgdata 1408maxresident)k
0inputs+9032outputs (0major+111minor)pagefaults 0swaps