Question

I read the other articles for finding the file extension from a filename in C and I’ve tried them but the problem is that they don’t work correctly.

This is my code :

void optionOne()
{
char filenameSrc[101],filenameDest[101];
strcpy(filenameSrc,"");
do
{
    printf("Enter source filename (*.c) : ");
    scanf("%s",filenameSrc);
}while (check_file_ext(filenameSrc) != 0);
fflush(stdout);
printf("Enter destination filename : ");
scanf("%s",&filenameDest);
char line[80];
FILE* fp = fopen("data.inp","r");
while(fgets(line,sizeof(line),fp))
{
   // do something
}
fclose(fp);
}

and the function check_file_ext :

const char *get_file_ext(const char *filename)
{
   const char *dot = strrchr(filename, '.');
   if(!dot || dot == filename) return "";
   return dot + 1;
}
int check_file_ext(const char* filename)
{
   return strcmp(get_file_ext(filename),"c") == 0;
}

The problem is in the check method for the file extension?
Could you tell me where is the problem in the code?

Answer 1

Don’t return "", return a pointer to '\0' byte instead:

// gcc -std=c99
#include <stdio.h>
#include <string.h>

static const char*
get_file_ext(const char *filename) {
   const char *ext = strrchr(filename, '.');
   return (ext && ext != filename) ? ext : (filename + strlen(filename));
}

int main() {
  char *files[] = {"a.c", ".a", "a", NULL };

  for (char** f = files; *f != NULL; ++f)
    printf("ext: '%s'\n", get_file_ext(*f));
}

Note: it ncludes . in the extension for consistency.

Output

ext: '.c'
ext: ''
ext: ''

Reverse condition: do{ ... }while(strcmp(get_file_ext(filename), ".c") != 0);