Linux系统编程手册——第4章 阅读记录及习题练习

习题1：实现 部分tee，实现 -a 追加选项

#include <stdio.h>
#include <readline/readline.h>
#include <readline/history.h>
#include "tlpi_hdr.h"
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#define MAX_READ 1024

int readLine(int fd, char *buf, int size)
{
        int count = 0, nbytes;
        char *ptr;

        ptr = buf;
        while (count < size) {
                nbytes = read(fd, ptr, 1);
                if (nbytes == -1)
                        errExit("read");
                if (nbytes == 0)
                        break;
                if (*ptr == '\n')
                        break;
                ptr++;
                count++;
        }

        return count;
}

int main(int argc, char **argv)
{
        int opt, flags, fd, nbytes;
        char buf[MAX_READ + 1], *file;

        if (argc > 3) {
                usageErr("%s [-a] <file>\n", argv[0]);
        }
        opt = getopt(argc, argv , "a:");
        if (opt == ':' || opt == '?')
                usageErr("%s [-a] <file>\n", argv[0]);
        switch(opt) {
                case 'a':
                        flags = O_APPEND | O_WRONLY | O_CREAT;
                        file = optarg;
                        break;
                default:
                        flags = O_WRONLY | O_TRUNC | O_CREAT;
                        file = argv[1];
                        break;
        }
        if (file == NULL)
                fd = STDOUT_FILENO;
        else
                fd = open(file, flags, 0644);

        while (1) {
                if (fd == -1)
                        errExit("open");
                nbytes = readLine(STDIN_FILENO, buf, MAX_READ);
                buf[nbytes] = '\0';
                if (write(fd, buf, nbytes) == -1)
                        errExit("write");
                if (write(fd, "\n", 1) == -1)
                        errExit("write");
        }
        close(fd);

        exit(EXIT_SUCCESS);
}

 　　总结：练习了 getopt，并纠正一个错误认识——close后，数据才会写入文件，实际上write后，数据已经写入驱动的缓存中，即数据是否写入文件与close无关，close只是释放文件描述符资源，由于文件描述符为有限资源，所以需要及时释放。

习题2：实现cp 命令，能复制空洞文件

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "tlpi_hdr.h"


int main(int argc, char **argv)
{
        int src_fd, des_fd, src_size, i, nbytes;
        struct stat src_st;
        char ch;


        if (argc != 3)
                usageErr("%s <source file> <des file>\n", argv[0]);
        src_fd = open(argv[1], O_RDONLY);
        if (src_fd == -1)
                errExit("open src");
        des_fd = open(argv[2], O_WRONLY | O_CREAT | O_TRUNC, 0644);
        if (des_fd == -1)
                errExit("open des");
        if (stat(argv[1], &src_st) == -1)
                errExit("stat src");
        src_size = src_st.st_size;
        for (i = 0; i < src_size; i++) {
                lseek(src_fd, i, SEEK_SET);
                lseek(des_fd, i, SEEK_SET);
                nbytes = read(src_fd, &ch, sizeof(ch));
                if (nbytes == -1)
                        errExit("read src");
                if (nbytes == 0)
                        continue;
                nbytes = write(des_fd, &ch, sizeof(ch));
                if (nbytes == -1)
                        errExit("write des");
        }

        exit(EXIT_SUCCESS);
}

总结：练习了下lseek的使用，直观感受是和写内存一样，另外文件偏移量是从0开始，和内存一样的设计。