CODEWITHSUNDEEP
c
Programmer
Programmer

Reputation: 423

C question about passing a string as a function agrument

I would like to pass a string to function like this..

 long_var = get_value("long_value");
 short_var = get_value("short_value");

Inside the function, I did this..

double get_value(char *get_type){

   if  (*get_type == "short_value")
   {
        //calculate and return
    }
   else if  (*get_type == "long_value")
   {
         //calculate and return
    }  
}

However, i have an error Error: main.c(334): function argument #1 of type 'flash unsigned char [11]' is incompatible with required parameter of type 'unsigned char *'

I thought that a string was just an array of char and I can call it..

Also, is there a better way to do this..

Thanks..

Upvotes: 0

Views: 1204

Answers (3)

Kerrek SB
Kerrek SB

Reputation: 476950

String literals are of type const char[], which decays to const char * in the function call, so you should make the signature of your function double get_value(const char *).

Second, when you dereference *get_type, you only get one char, not the entire string! And then you're trying to compare that char to an array (which again decays to a pointer) -- that doesn't work. What you need is strcmp (or a variant version thereof):

if (!strcmp(get_type, "short_value")) { ... }`

If you prefer, you can say strncmp(get_type, "short_value", 12) and only compare the initial 12 characters, which is the length of "short_value" including its terminating null byte - not strictly necessary, but it's good to be aware of one's string lengths when using string manipulation functions.

Upvotes: 3

Ray Toal
Ray Toal

Reputation: 88378

You are going to need to include <string.h> and change your code to say

 if (!strcmp(get_type, "short_value"))

The way things are now, you are comparing a character (*get_type) with a pointer to a character ("short_value").

Upvotes: 6

Mihails Strasuns
Mihails Strasuns

Reputation: 3803

You should pass constant strings as "const char*" or even "const char * const". Also, c-strings are plain arrays and using == operation on them will just compare pointers, not strings. You should use strcmp function for comparison.

Upvotes: 4

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