How to read all files in a folder using C

Question

I wish to read all the text files in a particular folder. The files' names do not have any common pattern in them- else the task would have been easier.

//read a file from the directory  
//Perform a common operation  
//write output to a common file  
//read the next file

It will be good if I could work around with sub-folders as well, but even the basic implementation is sufficient.

I tried looking at the previously asked related questions (here, here, here and here), but none of them give a C and Linux specific answer which I need.

edit : So, this is what I wrote based on the answers received-

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <dirent.h>
#include <unistd.h>
#include <errno.h>


int main(int argc, char **argv)
{
    DIR* FD;
    struct dirent* in_file;
    FILE    *output_file;
    FILE    *entry_file;
    char    buffer[BUFSIZ];

    /* Opening common file for writing */
    output_file = fopen("/home/pnp/snort_rules_folder/rulesoutput.txt", "a+");
    if (output_file == NULL)
    {
        fprintf(stderr, "Error : Failed to open output_file\n");

        return 1;
    }

    /* Scanning the in directory */
    if (NULL == (FD = opendir ("/home/pnp/snort_rules_folder/rules"))) 
    {
        fprintf(stderr, "Error : Failed to open input directory\n");
        fclose(output_file);

        return 1;
    }
    while ((in_file = readdir(FD))) 
    {
        /* On linux/Unix we don't want current and parent directories
         * If you're on Windows machine remove this two lines
         */
        if (!strcmp (in_file->d_name, "."))
            continue;
        if (!strcmp (in_file->d_name, ".."))    
            continue;
        /* Open directory entry file for common operation */
        /* TODO : change permissions to meet your need! */
        entry_file = fopen(in_file->d_name, "r");
        if (entry_file == NULL)
        {
            fprintf(stderr, "Error : Failed to open entry file\n");
            fclose(output_file);

            return 1;
        }

        /* Doing some stuff with entry_file : */

        while (fgets(buffer, BUFSIZ, entry_file) != NULL)
        {
            /* Use fprintf or fwrite to write some stuff into common_file*/
        }

    fprintf(output_file, "reading file %s", in_file->d_name);

        /* When you finish with the file, close it */
        fclose(entry_file);
    }

    /* Don't forget to close common file before leaving */
    fclose(output_file);

    return 0;
     }

And the error received-
pnp@pnp-laptop:~/snort_rules_folder$ ./a.out
Error : Failed to open entry file

Answer 1

You can use this sample code and modify it if you need:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <dirent.h>
#include <unistd.h>
#include <errno.h>

/* This is just a sample code, modify it to meet your need */
int main(int argc, char **argv)
{
    DIR* FD;
    struct dirent* in_file;
    FILE    *common_file;
    FILE    *entry_file;
    char    buffer[BUFSIZ];

    /* Openiing common file for writing */
    common_file = fopen(path_to_your_common_file, "w");
    if (common_file == NULL)
    {
        fprintf(stderr, "Error : Failed to open common_file - %s\n", strerror(errno));

        return 1;
    }

    /* Scanning the in directory */
    if (NULL == (FD = opendir (in_dir))) 
    {
        fprintf(stderr, "Error : Failed to open input directory - %s\n", strerror(errno));
        fclose(common_file);

        return 1;
    }
    while ((in_file = readdir(FD))) 
    {
        /* On linux/Unix we don't want current and parent directories
         * On windows machine too, thanks Greg Hewgill
         */
        if (!strcmp (in_file->d_name, "."))
            continue;
        if (!strcmp (in_file->d_name, ".."))    
            continue;
        /* Open directory entry file for common operation */
        /* TODO : change permissions to meet your need! */
        entry_file = fopen(in_file->d_name, "rw");
        if (entry_file == NULL)
        {
            fprintf(stderr, "Error : Failed to open entry file - %s\n", strerror(errno));
            fclose(common_file);

            return 1;
        }

        /* Doing some struf with entry_file : */
        /* For example use fgets */
        while (fgets(buffer, BUFSIZ, entry_file) != NULL)
        {
            /* Use fprintf or fwrite to write some stuff into common_file*/
        }

        /* When you finish with the file, close it */
        fclose(entry_file);
    }

    /* Don't forget to close common file before leaving */
    fclose(common_file);

    return 0;
}

Hope this hellp.

Regards.