电话目录程序的“指针与整数之间的比较”错误

Question

So I'm pretty new too C, and programming. I'm learning little by little. A friend of mine gave me a good idea for a first project. The goal is too allow the user too select a department (Phone Directory for a Casino), and once that department has been selected, it will show all the numbers for that department.

Eventually I want too add more too this, but for now, I'm going too keep it simple.

#include <stdio.h>
main()
{
   printf("Please Select Department [Security, Food & Beverage, Hotel]\n");

   char str[20];

   gets(str);

   if (gets(str) == 8)
   {
       printf("You have selected Security\n");
       printf("218-760-1122, 218-760-5564\n");
   }

   else if (gets(str) == 5)
   {
       printf("You have Selected Hotel\n");
       printf("218-443-9810");
   }

   else if (gets(str) == 15)
   {
       printf("You have selected Security\n");
       printf("218-550-9818, 218-443-1231\n");
   }

   return 0;
}

The problem with this program is when I compile and build the program, it prints the "selection" part of the script, where get(str) is used. After I select which one I would like too use, it goes blank, and nothing is displayed, but the script itself is still running.

I'm really new too this, and I'm assuming its something too do with how I'm determining which department is selected, in the if/else statements. I cant seem too figure out what I'm doing wrong. I've tried different solutions, usually getting different results, the closest being by adding a semi-colon too the last line of the else if statement, which runs the script, skips the first two else statements, and prints the security code.

3 warnings pop up for lines 10, 16, and 22

warning: comparison between pointer and integer [enabled by default]

This is a learning experience for me, and I would appreciate any help that can be offered.

Answer 1

So, the first and most important error is already in the comments, but repeating it here for completeness:

Do not use gets() ! It is broken and therefore removed from the current C standard. With gets() , you have no control over how much input is actually read, so any buffer can overflow (and attackers will use that). The replacement is fgets() , which has a size argument.

Note the same holds for things like scanf("%s", ...) . If you ever need to use a conversion to a string with the scanf family of functions, make sure to read the documentation, it tells you how to use a field width .

Back to your code, assuming you use fgets() like you should:

char str[20];

fgets(str, 20, stdin);

if (fgets(str, 20, stdin) == 8)
{

You have two errors here:

1. You call fgets() twice. Each call will attempt to read a line of input and store it in str . That's not what you want.
2. fgets() doesn't return the length but a pointer to str if it was successfull . If it failed, it returns NULL

So, the code should start like this:

char str[256];

if (!fgets(str, 256, stdin))
{
    fputs("Error reading input.", stderr);
    return 1;
}

Only after that error check, you should examine the contents of str , ie, what the user actually entered.

Note I also increased buffer size. People might enter longer lines by accident and there's really no need to be that tight with bytes nowadays.

Finally, just checking the length probably isn't a good idea. Instead, include string.h and use the strcmp() function.