umask系统调用及示例

#include <sys/stat.h> // 包含 umask 函数声明

// 设置文件权限掩码并返回旧的掩码
mode_t umask(mode_t mask);

#define _GNU_SOURCE // 启用 GNU 扩展
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h> // 包含 umask, mkdir, stat
#include <fcntl.h>    // 包含 open, O_* flags
#include <string.h>
#include <errno.h>

// 辅助函数：将 mode_t 转换为字符串表示 (简化版)
void print_permissions(mode_t mode) {
    printf( (mode & S_IRUSR) ? "r" : "-");
    printf( (mode & S_IWUSR) ? "w" : "-");
    printf( (mode & S_IXUSR) ? "x" : "-");
    printf( (mode & S_IRGRP) ? "r" : "-");
    printf( (mode & S_IWGRP) ? "w" : "-");
    printf( (mode & S_IXGRP) ? "x" : "-");
    printf( (mode & S_IROTH) ? "r" : "-");
    printf( (mode & S_IWOTH) ? "w" : "-");
    printf( (mode & S_IXOTH) ? "x" : "-");
}

// 辅助函数：获取并打印文件/目录的权限
void check_permissions(const char* path) {
    struct stat sb;
    if (stat(path, &sb) == 0) {
        printf("Permissions for '%s': ", path);
        print_permissions(sb.st_mode);
        printf(" (%o)\n", sb.st_mode & (S_IRWXU | S_IRWXG | S_IRWXO));
    } else {
        perror("stat");
    }
}

int main() {
    mode_t old_mask;
    int fd;

    printf("--- Demonstrating umask ---\n");
    printf("PID: %d\n", getpid());

    // 1. 查看当前的 umask 值
    // 通过设置 umask(0) 并立即恢复旧值来查询
    old_mask = umask(0);
    umask(old_mask); // 立即恢复
    printf("Initial umask: %03o\n", old_mask);

    // 2. 创建文件和目录，使用初始 umask
    printf("\n--- Creating files/dirs with initial umask ---\n");
    fd = open("file_with_initial_umask.txt", O_CREAT | O_WRONLY | O_TRUNC, 0666);
    if (fd != -1) close(fd);
    mkdir("dir_with_initial_umask", 0777);
    check_permissions("file_with_initial_umask.txt");
    check_permissions("dir_with_initial_umask");

    // 3. 改变 umask 为 077 (非常私密)
    printf("\n--- Changing umask to 077 ---\n");
    old_mask = umask(0077); // 返回旧的 umask
    printf("Old umask was: %03o\n", old_mask);
    printf("New umask is: 077\n");

    // 4. 再次创建文件和目录
    fd = open("file_with_umask_077.txt", O_CREAT | O_WRONLY | O_TRUNC, 0666);
    if (fd != -1) close(fd);
    mkdir("dir_with_umask_077", 0777);
    check_permissions("file_with_umask_077.txt");
    check_permissions("dir_with_umask_077");

    // 5. 改变 umask 为 002 (组协作)
    printf("\n--- Changing umask to 002 ---\n");
    old_mask = umask(0002);
    printf("Old umask was: %03o\n", old_mask);
    printf("New umask is: 002\n");

    // 6. 再次创建文件和目录
    fd = open("file_with_umask_002.txt", O_CREAT | O_WRONLY | O_TRUNC, 0666);
    if (fd != -1) close(fd);
    mkdir("dir_with_umask_002", 0777);
    check_permissions("file_with_umask_002.txt");
    check_permissions("dir_with_umask_002");

    // 7. 恢复原始 umask
    printf("\n--- Restoring original umask ---\n");
    umask(old_mask);
    printf("Restored umask to: %03o\n", old_mask);

    printf("\n--- Summary ---\n");
    printf("1. umask acts as a filter on default permissions (666 for files, 777 for dirs).\n");
    printf("2. Bits set to 1 in umask REMOVE the corresponding permission.\n");
    printf("3. umask 022: Owner has full access, Group/Others have read/execute (not write).\n");
    printf("4. umask 077: Only Owner has access (very private).\n");
    printf("5. umask 002: Owner/Group have full access, Others lack write (collaborative).\n");
    printf("6. The umask set in a program affects files/dirs it creates.\n");

    // 8. 清理 (可选)
    // unlink("file_with_initial_umask.txt");
    // rmdir("dir_with_initial_umask");
    // unlink("file_with_umask_077.txt");
    // rmdir("dir_with_umask_077");
    // unlink("file_with_umask_002.txt");
    // rmdir("dir_with_umask_002");

    return 0;
}

# 假设代码保存在 umask_example.c 中
gcc -o umask_example umask_example.c

# 运行程序
./umask_example

# 查看当前 umask
umask

# 设置 umask 为 077
umask 077

# 创建一个文件测试权限
touch test_file_umask_077.txt
ls -l test_file_umask_077.txt
# 输出类似：-rw------- 1 user user 0 date time test_file_umask_077.txt

# 恢复为常见的 022
umask 022
touch test_file_umask_022.txt
ls -l test_file_umask_022.txt
# 输出类似：-rw-r--r-- 1 user user 0 date time test_file_umask_022.txt