C: Passing pointers to a recursive function

Question

So I'm struggling on a recursive function to enter a directory and it's subdirectories to save all the file names and a counter for the number of files. I don't know if this is the best way to save the file names, but I don't see any other way. I'm having problems passing the array of pointers and the file counter to the function.

Sorry for any absurd code.

Here's what I have:

#include <stdio.h> #include <string.h> #include <stdlib.h> #include <errno.h> #include <dirent.h> int main(int argc, char *argv[]) { char * path = argv[1]; char ** tempFiles = NULL; int fileCounter = 0; findFilesRecursive(&fileCounter, &tempFiles, path); for (int i = 0; i < fileCounter; i++) printf("\n[%d]: %s", i, tempFiles[i]); } void findFilesRecursive(int * fileCounter, char ** tempFiles, char * path) { DIR * dirStream; struct dirent * entry; if(!(dirStream = opendir(path))) return; while ((entry = readdir(dirStream)) != NULL) { if (entry->d_type == DT_DIR) { if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0) continue; char fullPath[strlen(path) + strlen(entry->d_name) + 1]; sprintf(fullPath, "%s%s/", path, entry->d_name); findFilesRecursive(&(*fileCounter), &tempFiles, fullPath); } else if (entry->d_type == DT_REG) { char filePath[strlen(path) + strlen(entry->d_name)]; sprintf(filePath, "%s%s", path, entry->d_name); printf("FILE: %s\n", filePath); tempFiles = (char **) realloc(tempFiles, (*fileCounter + 1) * sizeof(char *)); tempFiles[*fileCounter] = (char *) MALLOC(strlen(filePath) * sizeof(char)); strcpy(tempFiles[*fileCounter], filePath); (*fileCounter)++; } } closedir(dirStream); }

You may define a new counter variable in the if (entry->d_type == DT_DIR) { and pass a pointer to it, instead of a shared variable. That way you don't need to rely on it to be initialised properly. And to avoid too many reallocs you could have created a structure that holds a pointer to a pointer to a char and additional fields that could hold say current capacity and number of elements in a collection. — zerkms
– zerkms, Commented Oct 24, 2017 at 22:03
You never initialize tempFiles to any sane value. You pass its address to findFilesRecursive, and then pass that address to realloc. That doesn't make any sense -- you can only use realloc to either get a brand new block (in which case, you must pass it NULL) or to enlarge an existing block you got from malloc or realloc. — David Schwartz
– David Schwartz, Commented Oct 24, 2017 at 22:18

mevets · Accepted Answer · 2017-10-24 22:20:45Z

1

Your compiler should have warned you that &tempFiles, as in:

findFilesRecursive(&(*fileCounter), &tempFiles, fullPath);

is a char ***, yet you promised the compiler that tempFiles would be a char **. It is very tempting to be “&” happy in C, but really you should restrict it to an ‘as needed’ basis - it makes the code more understandable. The construct &(*fileCounter) had me scratching my head, after 30+ years of programming in C.

answered Oct 24, 2017 at 22:20

mevets

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2 Comments

José Pereira Over a year ago

I honestly was changing and trying different possibilities, because, as I stated, I'm not very experient in using char ** or char ***, I'm sorry for that. How would I correctly pass tempFiles to the function?

André Santos Over a year ago

Just pass tempfiles as is. I think this should work: findFilesRecursive(fileCounter, tempFiles, fullPath). Think like this: my argument tempFiles is a char ** and now i want to give it to a function that expects a char **. But tempFiles is a char ** already so i have nothing to do. Same rationale goes for fileCounter.

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