Incompatible pointer type error [C]

Question

I keep getting told that in this line of code passing argument from incompatible pointer type.
Here's the line of code:

if (linear_search (size_of_A, argv[i]))  

What does this mean and how do I fix it? Here is the whole program:

int linear_search ( int size_of_A, char*argv[]){  
   int i;  
   i = 2;  
   while (i <= size_of_A - 1){  
      if (!strcmp (argv [1], argv[i])){  
      return 1;  
      }  
   }  
   return 0;  
}  

int main (int argc, char*argv []){  
   int size_of_A = argc - 2;  
   int i = 2;  
      if (linear_search (size_of_A, argv)){  
         printf ("%s not found\n", argv [1]);  
         return 1;  
      } else{  
         printf ("%s found\n", argv[1]);  
         return 0;  
      }  
      i = i + 1;  
   }  
}   

Ok, that fixes the warning, but now when I run the program through the compiler nothing happens. It's supposed to tell me if the first argument is repeated or not.
For example the output would look like:

./a 3 hso 8 3  
3 found 

Answer 1

oldfart: Beware of strcmp and all unlimited string functions (like strcpy--but be careful with strncpy: it doesn't add a trailing NULL).

youngun: But it's ok since argv has a bunch of NULL terminated strings!

of: That way of thinking leads to a heaping helping of buffer overflow. argv was initialized with NULLS. But it could have changed since then.

y: But this is a small program and it will never happen.

of: Sure. Programs grow and 'never' happens more often than you think.

y: OK, well Mr. Paranoid, what size should I use for the limit on my strncmp? There's no obvious number to use! Are you suggesting I make something up?

of: See limits.h, in particular ARG_MAX and _POSIX_ARG_MAX for possible maximum values. You could use smaller values if you so desire. If you do, document it.

y: So I should write strncmp( arg, target, 5000 )? OK, mister paranoid.

of: Don't do that! #define APP_MAX_ARG 5000 and then use strncmp( arg, target, APP_MAX_ARG ). Don't get me started on magic numbers. Kids these days.

of: Hey, kid, get off my lawn.

Answer 2

Your linear search function only compares argv[1] with argv[2] as i is never incremented. A possible solution (using a for loop instead of a while, which is more common) would be:

int linear_search ( int size_of_A, char*argv[]){  
   int i = 0;  // should always initialize in construction
   for ( i = 2; i < size_of_A; ++i ) {
      if (!strcmp (argv [1], argv[i])){  
        return 1;  
      }  
   }  
   return 0;  
}

Variable i is never really used in main, you can safely remove the two lines that deal with it.

Answer 3

This seems to be great place to use the fact that argv is NULL-terminated. You don't need to pass the count of items in it, but just do like this:

/* Searches through the NULL-aterminated array for the.
 * given needle string. Returns 1 if found, 0 if not.
*/
int linear_search (char **argv, const char *needle)
{
  int i;

  for(i = 0; argv[i] != NULL; i++)
  {
     if(strcmp(argv[i], needle) == 0)
       return 1;
  }
  return 0;
}

Btw, I recommend comparing the return value of strcmp() nagainst literal 0, rather than using the ! notation, since the return value is in fact not boolean; it returns a int that expresses the relation between the compared elements.

I realize the use of ! here is common enough to be almost idiomatic, since many people think that C must always be as terse as possible, but ... I still recommend against it.

Answer 4

I don't know the logic of your program but compiler error is solved. See this:

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

int size_of_A = argc - 2;  
int i = 2;  
if (linear_search (size_of_A, argv[])){  
     printf ("%s not found\n", argv [1]);  
     return 1;  
} 

Answer 5

You have a logic error in linear_search. You have a while loop based on the value of i, but i never changes. You might be missing a i++; instruction.

Also (although this won't change your program's behaviour): the variable i in main is never really used. You can remove it and the i = i + 1; instruction.

Answer 6

Your linear_search() accepts an "array of strings" (pointer to array of characters), while you are passing it a "string" (argv[i]). What you want to do I think is to call linear_search as:

if (linear_search (size_of_A, (argv + i)))

Although I do not completely understand the logic of your program... Are you trying to search argv[1] in the later arguments...?

Answer 7

int linear_search ( int size_of_A, char*argv[]){ 

char*argv[] means "pointer to an array of char"; as a function call parameter, that's the same as char**.

argv is a char**, but argv[i] is a char*, because C defines argv[i] as *(argv + i ) or, in English "dereference (argv plus i)". Dereferencing a char** leaves you with a char*, and that''s not what linear_search is declared to take.

Take a look here if you're still confused.
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Answer 8

Your function expects a char*[] (which should be equivalent in this case to char**). However, by calling it with

linear_search (size_of_A, argv[i])

you are just passing a char* as argument (since [i] dereferences one pointer). From what I see in your code you could try using

linear_search (size_of_A, argv+i)

but I'm not quite sure whether that will yield the intended behaviour. Still too early to understand C :)

Answer 9

linear_search is expecting a data type "char **". You are passing in argv[i], which is just a char*. Try passing in "argv" like this:

if (linear_search(size_of_A, argv))

Answer 10

You are passing a parameter (the second one) to the linear_search function which does not match the expected type of the argument. argv[i] has the type char *, which is commonly referred to as a string, while the linear_search function expects char * [], which is an array of strings.