How can I check whether a string ends with ".csv" in C

How about:

char *dot = strrchr(str, '.');
if (dot && !strcmp(dot, ".csv"))
    /* ... */

The simplest (and most general) form of ThiefMaster's code would be:

int string_ends_with(const char * str, const char * suffix)
{
  int str_len = strlen(str);
  int suffix_len = strlen(suffix);

  return 
    (str_len >= suffix_len) &&
    (0 == strcmp(str + (str_len-suffix_len), suffix));
}

if(strlen(str) > 4 && !strcmp(str + strlen(str) - 4, ".csv"))