C - Why is my function returning NULL?

Question

I think my function is returning NULL since I initilize it to it. But I get compiling error if I dont.

This is just a prototype that I made in a test.c file to test it. So when I get it to work I will copy back the lookup function back into the correct file.

This is part of pset6 of cs50 if that helps anyone.

const char* lookup(const char* extension);

int main(void)
{
    const char* type = "css";
    const char* ending = lookup(type);  
    printf("the exstension: %s\nis of type = %s\n", type, ending);
}

const char* lookup(const char* extension)
{

    char temp[strlen(extension)];

    for (int i = 0; i < strlen(temp); i++)
    {
        if (isalpha(extension[i]))
            temp[i] = tolower(extension[i]);
    }

    printf("temp = %s\n", temp);

    char* filetype = NULL;

    if (strcmp(temp,  "html") == 0)
        strcpy(filetype, "text/html"); 

    else if(strcmp(temp, "css") == 0)
        strcpy(filetype, "text/css");

    else if(strcmp(temp, "js") == 0)
        strcpy(filetype, "text/js");

    else if(strcmp(temp, "jpg") == 0)
        strcpy(filetype, "image/jpg");

    else if(strcmp(temp, "ico" ) == 0)
        strcpy(filetype, "image/x-icon");

    else if(strcmp(temp, "gif") == 0)
        strcpy(filetype, "image/gif");

    else if(strcmp(temp, "png") == 0)
        strcpy(filetype, "image/png");

    else
        return NULL;

    return filetype;
}

I'm using all the correct libraries, it screwed up my code preview when I tried to include them!

Answer 1

 char temp[strlen(extension)];

You don't reserve the space for the trailing null character and you never set it! For example, char temp[strlen(extension) + 1] = {0};.

Then:

char* filetype = NULL;

if (strcmp(temp,  "html") == 0)
    strcpy(filetype, "text/html"); 

filetype pointed object must be allocated, for example using malloc, otherwise strcpy is copying with a null pointer.

Answer 2

Are you sure that extension contains only extension without .? I can prefer to use _stricmp, strcmpi to compare case insensitive. And why you do strcpy to filetype instead of assignment? You have only pointer without malloc:

const char* lookup(const char* extension)
{
const char* filetype = NULL;

if (_stricmp(extension, "html") == 0)
    filetype = "text/html"; 
else if(_stricmp(extension, "css") == 0)
    filetype = "text/css";

else if(_stricmp(extension, "js") == 0)
    filetype = "text/js";

else if(_stricmp(extension, "jpg") == 0)
    filetype = "image/jpg";

else if(_stricmp(extension, "ico" ) == 0)
    filetype = "image/x-icon";

else if(_stricmp(extension, "gif") == 0)
    filetype = "image/gif";

else if(_stricmp(extension, "png") == 0)
    filetype = "image/png";

return filetype;
}

Or better:

const char* lookup(const char* extension)
{
  char * ext[] = { "html", "text/html", "css", "text/css", "js", "text/js", "jpg", "image/jpg", NULL };


  for ( int i = 0; ext[i]; i += 2 )
  {
    if ( !stricmp( extension, ext[i] ) )
      return ext[i+1];
  }
  return NULL;
}