Assignment makes pointer from integer without cast

Question

Coming from a Java background I'm learning C, but I find those vague compiler error messages increasingly frustrating. Here's my code:

/*
 * PURPOSE
 *      Do case-insensetive string comparison.
 */
#include <stdio.h>
#include <string.h>
#include <ctype.h>

int compareString(char cString1[], char cString2[]);
char strToLower(char cString[]);

int main() {
    // Declarations
    char cString1[50], cString2[50];
    int isEqual;

    // Input
    puts("Enter string 1: ");
    gets(cString1);
    puts("Enter string 2: ");
    gets(cString2);

    // Call
    isEqual = compareString(cString1, cString2);
    if (isEqual == 0)
        printf("Equal!\n");
    else
        printf("Not equal!\n");

    return 0;
}

// WATCH OUT
//      This method *will* modify its input arrays.
int compareString(char cString1[], char cString2[]) {
    // To lowercase
    cString1 = strToLower(cString1);
    cString2 = strToLower(cString2);

    // Do regular strcmp
    return strcmp(cString1, cString2);
}

// WATCH OUT
//      This method *will* modify its input arrays.
char strToLower(char cString[]) {
    // Declarations
    int iTeller;

    for (iTeller = 0; cString[iTeller] != '\0'; iTeller++)
        cString[iTeller] = (char)tolower(cString[iTeller]);

    return cString;
}

This generates two warnings.

• assignment makes pointer from integer without a cast
  • cString1 = strToLower(cString1);
  • cString2 = strToLower(cString2);
• return makes integer from pointer without a cast
  • return cString;

Can someone explain these warnings?

Answer 1

You don't need these two assigments:

cString1 = strToLower(cString1); 
cString2 = strToLower(cString2);

you are modifying the strings in place.

Warnings are because you are returning a char, and assigning to a char[] (which is equivalent to char*)

Answer 2

C strings are not anything like Java strings. They're essentially arrays of characters.

You are getting the error because strToLower returns a char. A char is a form of integer in C. You are assigning it into a char[] which is a pointer. Hence "converting integer to pointer".

Your strToLower makes all its changes in place, there is no reason for it to return anything, especially not a char. You should "return" void, or a char*.

On the call to strToLower, there is also no need for assignment, you are essentially just passing the memory address for cString1.

In my experience, Strings in C are the hardest part to learn for anyone coming from Java/C# background back to C. People can get along with memory allocation (since even in Java you often allocate arrays). If your eventual goal is C++ and not C, you may prefer to focus less on C strings, make sure you understand the basics, and just use the C++ string from STL.

Answer 3

strToLower should return a char * instead of a char. Something like this would do.

char *strToLower(char *cString)

Answer 4

You are returning char, and not char*, which is the pointer to the first character of an array.

If you want to return a new character array instead of doing in-place modification, you can ask for an already allocated pointer (char*) as parameter or an uninitialized pointer. In this last case you must allocate the proper number of characters for new string and remember that in C parameters as passed by value ALWAYS, so you must use char** as parameter in the case of array allocated internally by function. Of course, the caller must free that pointer later.

Answer 5

• 1) Don't use gets! You're introducing a buffer-overflow vulnerability. Use fgets(..., stdin) instead.

• 2) In strToLower you're returning a char instead of a char-array. Either return char* as Autopulated suggested, or just return void since you're modifying the input anyway. As a result, just write

 

 strToLower(cString1);
 strToLower(cString2);

• 3) To compare case-insensitive strings, you can use strcasecmp (Linux & Mac) or stricmp (Windows).

Answer 6

strToLower's return type should be char* not char (or it should return nothing at all, since it doesn't re-allocate the string)