原题：

编写一个类似 cp 的程序，它复制包含空洞的文件，但不将字节0写到输出文件中去。

源代码：

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>

#define BUF_SIZ 128

int my_cp( const char *file1, const char *file2 )
{
    int fd1, fd2;
    char buffer[BUF_SIZ];
    int res;
    int current_position = 0;
    int byte_count = 0;
    
    fd1 = open( file1, O_RDWR );
    if( -1 == fd1 ) {
        perror("open file1 failed");
        return -1;
    }

    fd2 = open( file2, O_RDWR | O_APPEND | O_CREAT | O_TRUNC, 0666 );
    if( -1 == fd2 ) {
        perror("open file2 failed");
        return -1;
    }

    memset( buffer, '\0', BUF_SIZ );
    res = read( fd1, buffer, BUF_SIZ );
    if( -1 == res ) {
        perror("file1 read error");
        return -1;
    }

    while( 0 != res ) {
        byte_count = 0;
        for( current_position=0; current_position<BUF_SIZ; current_position++ ) {
            if( '\0' != buffer[current_position] ) {
                buffer[byte_count] = buffer[current_position];
                byte_count++;
            }
        }
        
        res = write( fd2, buffer, byte_count );
        if( -1 == res ) {
            perror("file2 write failed");
            return -1;
        }

        memset( buffer, '\0', BUF_SIZ );
        res = read( fd1, buffer, BUF_SIZ );
        if( -1 == res ) {
            perror("file1 read failed");
            return -1;
        }
    }
}

int main(int argc, char *argv[])
{
    if( 3 != argc) {
        printf("correct usage: ./my_cp file1 file2\n");
        exit( EXIT_FAILURE );
    }
    
    //产生一个含有空洞的文件 file1
    int fd = open( argv[1], O_RDWR | O_CREAT | O_TRUNC, 0666 );
    if( -1 == fd ) {
        perror("open failed");
        exit( EXIT_FAILURE );
    }
    
    int res = write( fd, "Hey,man!", strlen("Hey,man!") );
    if( -1 == res ) {
        perror("file1 write failed");
        exit( EXIT_FAILURE );
    }

    off_t offset = lseek( fd, 10000, SEEK_SET );
    if( -1 == offset ) {
        perror("lseek failed");
        exit( EXIT_FAILURE );
    }

    res = write( fd, "How are you?", strlen("How are you?") );
    if( -1 ==res ) {
        perror("file1 write failed");
        exit( EXIT_FAILURE );
    }

    //应用 my_cp 函数拷贝文件
    res = my_cp( argv[1], argv[2] );
    if( -1 == res ) {
        perror("copy failed");
        exit( EXIT_FAILURE );
    }

    exit( EXIT_SUCCESS );
}

[zhang@localhost APUE]$ ./my_cp file1 file2
[zhang@localhost APUE]$ cat file2
Hey,man!How are you?

输出文件在本例中为file2，大小为 20 Bytes = strlen("Hey,man!") + strlen("How are you?")。测试通过。

APUE习题4.6源代码----实现自己的简易 cp 命令