C —不评估交换机中的多个Scanf语句

Question

I have a problem with some really basic C code for which I couldn't find a solution on any existing StackOverflow question.

The problem is simple: I have two scanf statements inside two different cases of a switch statement. The first statement is executed perfectly every time. The second, however, doesn't work, no matter which case the switch is evaluating.

Here's a tiny program I wrote to demonstrate the issue:

#include<stdio.h>
#include<string.h>

int main(void)
{
int i = 0;

char *str1;
char *str2;
int state1, state2;
switch(i)
{
    case 0:
        state1 = scanf("%s", str1);
        getchar();
        break;
    case 1:
        state2 = scanf("%s", str2);
        getchar();
        break;
    default:
        printf("Something weird happened.\n");
        break;
    }
    printf("String 1: %s\nString 2: %s\nReturn 1: %d\nReturn 2: %d\n", str1, 
    str2, state1, state2); 

    return 0;
}

The getchar s are there to make sure it isn't an input overflow problem.

If I run that with i = 0 with an input of "hello" then I get

String 1: hello
String 2: (null)
Return 1: 1
Return 2: 4195648

If run with i = 1 and "hello" it outputs

String 1: 
String 2: (null)
Return 1: 0
Return 2: 0

As you can see, String 2 is never given a value despite running through that case .

This, to me, implies that the switch statement somehow runs through every case up to the matching one without actually evaluating what's inside, since both state1 and state2 are 0 in the second instance.

Does anyone know why this happens and how I can fix this?

Answer 1

the variable i is only set to 0, so only the switch/case 0: will be evaluated.

The switch statement is never re-entered and the value of i is never updated, so no other switch/case will ever be evaluated/executed.

I suspect your idea of how a switch statement works is not quite correct.