Concatenating file with path to get full path in C

Question

Using C, I\'m trying to concatenate the filenames in a directory with their paths so that I can call stat() for each, but when I try to do using strcat inside the loop it co

Answer 1

You must not try to increase the size of argv[1] by strcat'ing to it (and changing it at all is a bad idea) - that will cause undefined behaviour. Instead, copy argv[1] into a suitable sized buffer and work with that:

char path[1000];   // or some suitable size;
strcpy( path, argv[1] );

Answer 2

Where is the memory that you are copying to? path is allocated on the stack to contain the arguments you need yo allocate the memory yourself e.g.

char path[1024] ;   // or some other number
strcpy( path, argv[1] );
// then separator
strcat( path, "/" ) ; // or "\\" in Windows
strcat( path, ep->d_name);

In production code se strncat etc to stop overflows

Answer 3

You shouldn't modify argv[i]. Even if you do, you only have one argv[1], so doing strcat() on it is going to keep appending to whatever you had in it earlier.

You have another subtle bug. A directory name and file names in it should be separated by the path separator, / on most systems. You don't add that in your code.

To fix this, outside of your while loop:

size_t arglen = strlen(argv[1]);

You should do this in your while loop:

/* + 2 because of the '/' and the terminating 0 */
char *fullpath = malloc(arglen + strlen(ep->d_name) + 2);
if (fullpath == NULL) { /* deal with error and exit */ }
sprintf(fullpath, "%s/%s", path, ep->d_name);
/* use fullpath */
free(fullpath);

Answer 4

The problem is that the line

char* fullpath = strcat(path, ep->d_name);

keeps appending the name of the current file to path. Try creating a new string for the full path each time, like

char* fullpath = calloc(strlen(path) + strlen(ep->d_name) + 1);
strcat(fullpath, path);
strcat(fullpath, ep->d_name);
/* .. */
free(fullpath);