如果使用追加标志打开一个文件以便读、写，能否仍用lseek在 任一位置开始读?能否用lseek更新文件中任一部分的数据?请编写一段 程序验证。

猜测

可以

验证代码

#include <iostream>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#define BUFFSIZE 4096
int main(int argc, char * argv[]) {
    std::cin.sync_with_stdio(false);
    std::cin.tie(NULL);
    printf("test append and lseek\n");

    try{
        int fd = open(argv[1], O_RDWR | O_CREAT |  O_APPEND);
        if (!fd) {
            std::cout << "open file failed" << std::endl;
        }
        int len = 0;
        char buf[BUFFSIZE];
        memset(buf, 0, BUFFSIZE);
        len = read(fd, buf, BUFFSIZE);
        printf("read %d bytesn\n", len);
        close(fd);

        //write
        fd = open(argv[1], O_RDWR | O_CREAT | O_APPEND);
        if(!fd) {
            std::cout << "open file failed" << std::endl;
        }
        char name[] = "oooooooooo";
        len = write(fd, name, strlen(name));
        if(!len) {
            std::cout << "write error" << std::endl;
        }
        std::cout << "length of write " << len << std::endl;
        lseek(fd, 0, SEEK_SET);
        len = read(fd, buf, 5);
        printf("after lseek read content %s\n", buf);

        lseek(fd, 0, SEEK_SET);
        len = write(fd, name, strlen(name));
        if(!len) {
            std::cout << "write failed after failed\n" << std::endl;
        }
        printf("write %s after lseek\n", buf);
    }
    catch (std::exception &e)
    {
        std::cout << e.what() << std::endl;
    } catch (...) {
        std::cout << "unknown error" << std::endl;
    }

    return 0;
}

hello.txt文件内容

wodanimade

image.png

测试后文件内容

wodanimadeoooooooooooooooooooo

也就是不能覆盖写

结论

如果使用追加标志O_APPEND打开一个文件以便读、写; 能用lseek在任一位置开始读,不能用lseek更新文件中任一部分的数据，只能从文件末尾更新数据。