Create Directory and Store File (C programming)

Question

I want to create a directory that has the name ends with the process ID (to make it unique) and then store the new files that I just wrote inside that directory.

What I want:

1) Create a new directory named : mydirectory.1923 (example of the process id number)

2) Store a file that I just created using

FILE * fPointer = fopen("new.txt",w+)

into mydirectory.1923

What I have so far is this:

int bufSize = 20;
int pid = getpid();
char *fileName = malloc(bufSize);
char *prefix = "that.rooms.";
snprintf(fileName, bufSize,"%s%d", prefix, pid);
printf("%s\n",fileName);
struct stat st = {0};
if (stat(fileName, &st) == -1) {
    mkdir(fileName, 0755);
}
DIR *dir = opendir (fileName);
if (dir != NULL) {

    FILE *fLib = fopen("library.txt" , "w+");
    fclose(fLib);

}
closedir(fileName);

return 0;

My Question:

This code doesn't work, apparently it says error on the DIR part.

Is this the right thing to do if I want to create directory, create file and store that file directly to the new directory?

Is there any suggestion or advice to do it better than this? Thank you.

Answer 1

Some comments:

• As commented above, you should really allocate more space for your dir's name, a 5 digit pid will break your code right away. There are the macros PATH_MAX and FILE_MAX in limits.h
• You don't need to open the directory for anything. This is usually used to iterate over items in directories. You only want to create a file in it.
• Even if you don't really need it, the closedir function receives a DIR * argument, which I suppose would be dir in your code.
• To create the file inside the new directory you should include your dir in the path on creation, or at least chdir to it before the fopen call. I don't recommend the latter as it affects the process wide working directory.

Below is a quick and dirty patched version of your code that creates the directory and the new file inside it taking into account the above:

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

int main(int argc, char **argv){
    int pid = getpid();
    char dirName[NAME_MAX+1];
    char *prefix = "that.rooms.";
    snprintf(dirName, NAME_MAX + 1,"%s%d", prefix, pid);
    printf("%s\n",dirName);
    struct stat st = {0};
    if (stat(dirName, &st) == -1) {
        if(mkdir(dirName, 0755) != -1){
            char libPath[PATH_MAX+1];
            snprintf(libPath, PATH_MAX + 1, "%s/library.txt", dirName);

            FILE *fLib = fopen(libPath , "w+");
            fclose(fLib);
        }else{
            perror("mkdir: ");
        }
    }
    return 0;
}

Changed the variable names a bit so it's clearer:

• dirName is used to hold the directory name. It uses NAME_MAX as this is the system limit for the length of a file name
• libPath is used to hold the path to the library.txt file you are creating. If your pid was 3123, libPath would read that.rooms.3123/library.txt after the snprintf call. It uses PATH_MAX as this is the system limit for a the length of a file's path.