assignment makes pointer from integer without cast [enabled by default]

I have googled this and see many answers, but none fits my situation. This is my main():

char * cString;
puts("Please enter data you want to encrypt.");
cString = getInput(cString, &iStringSize);
printf("The message is: %s\n\n", cString);

char * strEncrypt;
strEncrypt = encrypt(cString, offset);
printf("The encrypted message is: %s\n\n", strEncrypt);

return 0;

This program basically reads in an arbitrary input, then encrypt it. This is the getInput function:

char * getInput(char *cString, int * iStringSize)
{
    puts("Begin reading input.");
    char buffer[CHUNK];
    int iBufferSize;
    while(fgets(buffer, CHUNK - 1, stdin) != NULL)
    {
        iBufferSize = strlen(buffer);
        *iStringSize += iBufferSize;
        cString = realloc(cString, sizeof(char) * (*iStringSize + 1));
        strcat(cString, buffer);
    }
    printf("String size is: %d\n", *iStringSize);
    puts("Reading successful");
    return cString;
}

As shown above, cString is char pointer. The getInput function also returns char pointer. However, I keep getting the message: assignment makes pointer from integer without cast [enabled by default]

This happens when I compile the code.

The same happens to this function

char * encrypt(char *str, int offset)
{
    puts("Begin encryption.");
    char * strEncrypt = malloc(sizeof(char) * (strlen(str) + 1));
    int i;
    for(i = 0; i < strlen(str); i++)
    {
        //substitution algorithm using int offset variable.
        //I accessed str and strEncrypt using pointer arithmetic
    }

    puts("Encryption success!");
    return strEncrypt;
}

Please no suggestion on error handling of realloc.