kBisla kBisla - 11 months ago 96
C Question

How to properly use scandir() in c?

I am trying to store list of files in a char** variable.

scandir() finishes properly but I get a segmentation fault when trying to print the char**.

Here's the code:

int main()
char** fileList;
int noOfFiles;
char* path = ".";
makeList(&fileList, &noOfFiles, path);
return 0;

void makeList(char ***fileList, int* noOfFiles, char* path){
struct dirent **fileListTemp;
*noOfFiles = scandir(path, &fileListTemp, NULL, alphasort);
int i;
fileList = (char***)malloc(sizeof(char***));
*fileList = (char**)malloc(*noOfFiles * sizeof(char*));
printf("total: %d files\n",*noOfFiles);
for(i = 0; i < *noOfFiles; i++){
*fileList[i] = (char*)malloc(strlen(fileListTemp[i] -> d_name) *sizeof(char));
strcpy(*fileList[i], fileListTemp[i] -> d_name);

This gives a segmentation fault after printing 2 file names.


total: 27 files.



Segmentation fault (core dumped)

Answer Source

The function scandir() allocates the memory for you.

You do not need to allocate ANY memory. You DO need to free the memory returned to you by scandir().

Your code calls: *noOfFiles = scandir(path, &fileListTemp, NULL, alphasort);

On return, noOfFiles will contain the number of directory entries in the path directory, and fileListTemp will point to an allocated array of pointers to allocated strings each of which points to an allocated block containing the null-terminated name of a file.

If your directory contains the files "FirstFile.txt", "AnotherFile.txt", "ThirdFile.txt", for example, with your call, upon return from scandir(), noOfFiles will be set to 5 for the three files plus two more for the "." and ".." directory entries. THE ENTRIES WILL BE IN NO PARTICULAr ORDER IF YOU DO NOT PASS 'alphasort'. (Actually that's a little incorrect. They will be in the order of the directory filename entries which depends on the order in which the files were originally created.)

Because you passed 'alphasort' you should see the entries in the following order (I am explicitly showing the null-byte-string-terminator:

fileListTemp[0] == ".\0"
fileListTemp[1] == "..\0"
fileListTemp[2] == "AnotherFile.txt\0"
fileListTemp[3] == "FirstFile.txt\0"
fileListTemp[4] == "ThirdFile.txt\0"

So fileListTemp points to a block of allocated memory holding five (char *) pointers. Each of the five (char *) pointers points to a block of allocated memory containing a null-terminated directory entry name.

That is SIX blocks of allocated memory.

You can use this allocated memory until you are done with it, and then you call free() on EACH entry in the array followed by free() of the array itself.

You MUST free every entry as well as the array itself. They are all independently allocated blobs of memory.

When you are done with the list you should:

for (int i = 0; i < noOfFiles; i++)