C how to find executable files only within given directory?

Question

I'm working on a c programming mimicking UNIX shell and I wonder if there's a way to find an executable file(command file) in a specific directory

Answer 1

By executable file, I am assuming you are looking for any file that is not a directory that has the execution flag set.
One way you could go about achieving your goal is to:

1. Get a directory stream corresponding to the directory by using the opendir.
2. Iterate through the directory stream using readdir .
3. At each iteration, find the absolute path of the file, which can be computed by concatenating the directory path and file name (can be obtained by accessing the d_name of the dirent struct), pass this path to stat, and use bit mask specified in inode documentation to check for the execution permission.

Code:

#include <stdio.h>
#include <sys/types.h> //opendir, stat
#include <dirent.h> //opendir
#include <errno.h>
#include <string.h>
#include <sys/stat.h> //stat
#include <stdlib.h> //free, malloc

int main(int argc, char** argv) {
    if (argc < 2) {
        fprintf(stderr, "Usage: ./a.out path_to_a_directory \n");
        return 1;
    }
    char* path_to_directory = argv[1];
    int path_length = strlen(path_to_directory);
    int modified = 0;
    //Modify path so that a dash is at the end.
    if (path_to_directory[path_length - 1] != '/') {
        char* modified_path = (char *) malloc(sizeof(char) * (strlen(path_to_directory) + 2));
        //Copies the null character as well.
        strcpy(modified_path, path_to_directory);
        modified_path[path_length] = '/';
        modified_path[path_length + 1] = '\0';
        path_to_directory = modified_path;
        //Set flag to true so that the dynamically allocated memory is freed later.
        modified = 1;
    }

    //Get the directory stream corresponding to the directory. 
    DIR* in_directory_stream = opendir(path_to_directory);
    if (in_directory_stream == NULL) {
        fprintf(stderr, "Error: the specified directory cannot be found or opened. \n", errno);
        if (modified) free(path_to_directory);
        return 1;
    }
    dirent* entry = NULL; 
    printf("Files that are executable by at least one of the permission classes (owner, group, others) are: \n");
    while ((entry = readdir(in_directory_stream)) != NULL) {
        //All directories contain . and .., which corresponds to current and parent directory respectively,
        //in unix systems. Since we are looking for only executable files, we can ignore them. 
        if (!strcmp(".", entry->d_name)) {
            continue;
        }
        if (!strcmp("..", entry->d_name)) {
            continue;
        }
        //Get file information. 
        struct stat entry_info;
        /* Create the absolute path of the entry.
         * Without it, as mentioned by Shawn below, 
         * stat will look for a file with the entry's name in current working directory 
         * instead of the specified directory. 
         */
        char* entry_absolute_path  = (char *) malloc(sizeof(char) * (strlen(path_to_directory) 
                    + strlen(entry->d_name) + 1));
        strcat(entry_absolute_path, path_to_directory);
        strcat(entry_absolute_path, entry->d_name);       
       if (stat(entry_absolute_path, &entry_info) == -1) {
            fprintf(stderr, "Error in obtaining file information about %s\n", entry->d_name);
       } else {
           // Check if the file is not a directory and 
           // is executable by one of the permission classes (owner, group, others). 
           if (((entry_info.st_mode & S_IFMT) != S_IFDIR) && 
                   ((entry_info.st_mode & S_IXUSR) || (entry_info.st_mode & S_IXGRP) 
                   || (entry_info.st_mode & S_IXOTH))) {
               printf("%s\n", entry->d_name);
           }
       } 
           free(entry_absolute_path);
    }
    //Close directory stream.
    closedir(in_directory_stream);    
    if (modified) free(path_to_directory);
    return 0;


}

EDIT: Corrected the program to pass the absolute path of the file to stat. Prior to this edit, the stat was given only the file name resulting in the program searching only the current working directory. Error identified by Shawn