menu function error: comparison between pointer and integer [enabled by default] - c

Question

I have this function which is a menu. After compiling, the following error keeps showing up: error: comparison between pointer and integer [enabled by default]. why is this happening?
char choice;
printf ("Welcome to the Customer menu! \n");
printf ("Please select option from below\n");
printf ("a. Add customer\n");
printf ("b. Modify customer\n");
printf ("c. List customers\n");
printf ("d. Go back to main menu");
while ((gets(&choice)) != 'q')
{
if (choice == '\n')
continue;
switch (choice)
{
case 'a' : add_customer();
break;
case 'b' : printf ("products_main ()");
break;
case 'c' : printf ("orders_main ()");
break;
default : printf ("Invalid input. Please enter an option from the above menu\n");
continue;
}
printf ("END PROGRAM");
Thank you!!

Answer 1

The gets() function returns a char *, whereas you're comparing that return value to a char:
if (gets(&choice)) != 'q')
Also note that this is wrong in two levels, since gets() reads from stdin until it encounters a newline, so if you pass it the address of one char, it will likely cause a buffer overrun error. Why not use fgets() instead?
char buf[128];
fgets(buf, sizeof(buf), stdin);
if (buf[0] == 'q') {
/* etc */
}

Answer 2

You can't use gets() to do that, and afterall gets() is very dangerous, doesn't checks how much characters to read so can cause a very bad runtime buffer overflow.
You should use fgets() like H2CO3, it has a limit of characters to read, so is more secure.
char * input(const char *message, size_t quantity)
{
const int BUFFER_SIZE = 512;
char buf[BUFFER_SIZE], *res = NULL;
if(quantity > BUFFER_SIZE || quantity == 0)
quantity = BUFFER_SIZE - 1;
if(message)
printf("%s",message);
if(fgets(buf, quantity + 1, stdin) > 0)
{
char *end = strchr(buf, '\n');
if(end){
*end = '\0';
}
res = malloc(strlen(buf) + 1);
if(!res)
{
fprintf(stderr, "input(): MEM alloc error\n");
return NULL;
}
strcpy(res, buf);
}
return res;
}
Try with that function, just pass the message you want, and the exact quantity of characters of input that you want. :)
If you want to try it alone, here you have a test program:
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
char * input(const char *message, size_t quantity)
{
const int BUFFER_SIZE = 512;
char buf[BUFFER_SIZE], *res = NULL;
if(quantity > BUFFER_SIZE || quantity == 0)
quantity = BUFFER_SIZE - 1;
if(message)
printf("%s",message);
if(fgets(buf, quantity + 1, stdin) > 0)
{
char *end = strchr(buf, '\n');
if(end){
*end = '\0';
}
res = malloc(strlen(buf) + 1);
if(!res)
{
fprintf(stderr, "input(): MEM alloc error\n");
return NULL;
}
strcpy(res, buf);
}
return res;
}
int main()
{
char *a = input("Input:", 4);
if(a)
{
printf("%s\n",a);
free(a);
return 0;
}
printf("Got NULL input\n");
return -1;
}
When you have a doubt about a particular function, what are their arguments, their return value, you could look it up in Google, and you will find plenty of examples and the function definition. With the time you will learn to easily understand the definitions and memorize some function names and their parameters.
Good Luck!

Answer 3

This line:
while ((gets(&choice)) != 'q')
gets() reads a string, not a char, and returns that string (i.e. it fills a buffer you pass to it via the char pointer). You then compare the pointer returned (which is the same as the one you passed in) with a char.
You probably just want to read a single character. If you want a whole string, you need to read it into a char array, and not pass the address of a single char.

Answer 4

After doing some reading I found that including
#include <unistd.h>
helps get ride of the warning. I'm new to unix c and I've never seen it before. I'm also still testing my code so I'll get back to you when I figure out if this works or not.
Hope this helps.
In the end the warning came back and it ended up going in an infinite loop so something is wrong with my logic.
Sorry I wasn't any help.