Parse out the file extension from a file-path in C

Question

I was previously using the following code to determine if a file was an .exe or .o file and thus set binFile to 1:

if(strstr(fpath,".exe") != NULL || strstr(fpath,".o") != NULL)
          binFile = 1;

Through debugging, I noticed that this method will also set binFile to 1 with files like foo.out or foo.execute. What I really want is to match '.exe\0' and '.o\0' but strstr() says it ignores the terminating NUL bytes. How should I go about this?

Thanks

Answer 1

You could check one past the result of strstr (taking into account the length of the search string) to see if it is NULL. Example:

const char* p = strstr(fpath,".exe");

if (p != NULL && *(p + 4 + 1) == 0) // 4 is the length of ".exe"; +1 should get you to \0
    binFile = 1;

Answer 2

int iLen = strlen(fpath);
if  ((iLen >= 4 && strcmp(&fpath[iLen - 4], ".exe") == 0)
  || (iLen >= 2 && strcmp(&fpath[iLen - 2], ".o") == 0))
   binfile = 1;

Edit added test on length, to handle very short file names.

Answer 3

one way of doing it:

char * suffix = fpath;
char * i = fpath;
while (*i++) {
    if (*i == '.') {
        suffix = i;
    }
}

if (!strcmp(suffix, ".o") || !strcmp(suffix, ".exe")) {
    /* do stuff */
}

Answer 4

#include <stdio.h>
#include <string.h>

int endswith(const char* haystack, const char* needle)
{
    size_t hlen;
    size_t nlen;
    /* find the length of both arguments - 
    if needle is longer than haystack, haystack can't end with needle */
    hlen = strlen(haystack); 
    nlen = strlen(needle);
    if(nlen > hlen) return 0;

    /* see if the end of haystack equals needle */
    return (strcmp(&haystack[hlen-nlen], needle)) == 0;
}

int main(int argc, char** argv) {

    if(argc != 3) {
     printf("Usage: %s <string> <test-ending>\n", argv[0]);
     return 1;
    }

    printf("Does \"%s\" end with \"%s\"? ", argv[1], argv[2]);

    if(endswith(argv[1], argv[2])) {
     printf("Yes!\n");
    } else {
     printf("No!\n");
    }

    return 0;
}

Answer 5

char *ext = strrchr(fpath, '.');

if (ext && (!strcmp(ext, ".exe") || !strcmp(ext, ".o")))
   binfile = 1;

If your system has the BSD/POSIX strcasecmp, you should probably use that instead of strcmp.

Answer 6

I like to get the extension and then check it.

char *suffix = strrchr(fpath,'.');

if (suffix)
{
  suffix++;
  if (strcasecmp(suffix,"exe"))
  {
    // you got it
  }
}

Incrementing suffix is okay since you know it points at a found a period at that point. Incrementing it will at worst make it point at the null termination character, which will not bother strcasecmp at all.

You can easily check against a list of extensions this way too.