Create Directory and Store File (C programming) - c

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.