C 언어에서 파일 권한 이해 및 chmod 시스템 콜 활용법

#include <sys/stat.h>
#include <unistd.h>

int stat(const char *path, struct stat *buf);

#include <stdio.h>
#include <sys/stat.h>
#include <unistd.h>

void check_permissions(const char *path) {
    struct stat fileStat;

    if (stat(path, &fileStat) < 0) {
        perror("stat");
        return;
    }

    printf("File: %s\n", path);
    printf("Permissions: ");
    printf((S_ISDIR(fileStat.st_mode)) ? "d" : "-");
    printf((fileStat.st_mode & S_IRUSR) ? "r" : "-");
    printf((fileStat.st_mode & S_IWUSR) ? "w" : "-");
    printf((fileStat.st_mode & S_IXUSR) ? "x" : "-");
    printf((fileStat.st_mode & S_IRGRP) ? "r" : "-");
    printf((fileStat.st_mode & S_IWGRP) ? "w" : "-");
    printf((fileStat.st_mode & S_IXGRP) ? "x" : "-");
    printf((fileStat.st_mode & S_IROTH) ? "r" : "-");
    printf((fileStat.st_mode & S_IWOTH) ? "w" : "-");
    printf((fileStat.st_mode & S_IXOTH) ? "x" : "-");
    printf("\n");
}

File: example.txt  
Permissions: -rw-r--r--  

#include <sys/stat.h>

int chmod(const char *path, mode_t mode);

#include <stdio.h>
#include <sys/stat.h>

int main() {
    const char *filename = "example.txt";
    mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;  // rw-r--r--

    if (chmod(filename, mode) < 0) {
        perror("chmod");
        return 1;
    }

    printf("Permissions for %s have been changed successfully.\n", filename);
    return 0;
}

-rw-r--r--  1 user group  12345 Jan 6 12:00 example.txt

#include <stdio.h>
#include <sys/stat.h>
#include <errno.h>

void change_permissions(const char *path, mode_t mode) {
    if (chmod(path, mode) < 0) {
        perror("chmod");
        printf("Error code: %d\n", errno);
    } else {
        printf("Permissions for %s changed successfully.\n", path);
    }
}

int main() {
    const char *filename = "example.txt";
    mode_t new_mode = S_IRUSR | S_IWUSR | S_IRGRP;  // rw-r-----

    change_permissions(filename, new_mode);
    return 0;
}

Permissions for example.txt changed successfully.

mode_t readonly_mode = S_IRUSR | S_IRGRP | S_IROTH;  // r--r--r--
change_permissions("readonly_file.txt", readonly_mode);

-r--r--r--  1 user group  5678 Jan 6 13:00 readonly_file.txt

#include <stdio.h>
#include <sys/stat.h>
#include <errno.h>

void change_permissions_with_error_handling(const char *path, mode_t mode) {
    if (chmod(path, mode) < 0) {
        switch (errno) {
            case ENOENT:
                printf("Error: File '%s' does not exist.\n", path);
                break;
            case EACCES:
                printf("Error: Permission denied for '%s'.\n", path);
                break;
            case EINVAL:
                printf("Error: Invalid mode value provided for '%s'.\n", path);
                break;
            case EROFS:
                printf("Error: Read-only file system for '%s'.\n", path);
                break;
            default:
                perror("chmod");
        }
    } else {
        printf("Permissions for %s changed successfully.\n", path);
    }
}

change_permissions_with_error_handling("nonexistent.txt", S_IRUSR | S_IWUSR);

Error: File 'nonexistent.txt' does not exist.

#include <unistd.h>

void debug_permissions(const char *path, mode_t mode) {
    printf("Attempting to change permissions for: %s\n", path);
    printf("UID: %d, EUID: %d\n", getuid(), geteuid());
    printf("Requested mode: %o\n", mode);
    change_permissions_with_error_handling(path, mode);
}

#include <sys/stat.h>
#include <stdio.h>

void secure_log_file(const char *log_file) {
    mode_t readonly_mode = S_IRUSR | S_IRGRP;  // r--r-----
    if (chmod(log_file, readonly_mode) == 0) {
        printf("Log file %s is now read-only.\n", log_file);
    } else {
        perror("chmod");
    }
}

int main() {
    secure_log_file("secure_log.txt");
    return 0;
}

Log file secure_log.txt is now read-only.

#include <sys/stat.h>
#include <stdio.h>

void restrict_project_dir(const char *dir_path) {
    mode_t restricted_mode = S_IRUSR | S_IRGRP | S_IXUSR | S_IXGRP;  // r-xr-x---
    if (chmod(dir_path, restricted_mode) == 0) {
        printf("Directory %s is now restricted to read and execute.\n", dir_path);
    } else {
        perror("chmod");
    }
}

int main() {
    restrict_project_dir("project");
    return 0;
}

Directory project is now restricted to read and execute.

#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>

void create_temp_file_with_permissions(const char *temp_file) {
    FILE *file = fopen(temp_file, "w");
    if (file) {
        fprintf(file, "Temporary data\n");
        fclose(file);

        mode_t secure_mode = S_IRUSR | S_IWUSR;  // rw-------
        if (chmod(temp_file, secure_mode) == 0) {
            printf("Temporary file %s permissions set to secure.\n", temp_file);
        } else {
            perror("chmod");
        }
    } else {
        perror("fopen");
    }
}

int main() {
    create_temp_file_with_permissions("temp.txt");
    return 0;
}

Temporary file temp.txt permissions set to secure.