Why would scanf make my switch statement freak out? - c

Question

I can't seem to figure out why my program doesn't work if I call scanf() from within a case statement.
Everything works fine until I call scanf(). It's a really small console based menu. It reads a string that has just been entered and goes down the menu accordingly. It all works fine until I try scanf() in an integer that it should respond to.
I first thought that returning a value will break the Switch statement... then that didn't work so I placed break statements in just in case. But it still doesn't work though. As soon as the scanf() is not commented the whole sequence is just messed up.
void main(void) {
char MenuS;
MenuS = 's';
while (MenuS != 'q') {
MenuS = Menu();
}
}
char Menu() {
unsigned char Menu;
unsigned char UserInput[9];
unsigned int Offset;
Offset = 0;
printf("\nEnt:");
gets(UserInput);
trim(UserInput);
if (UserInput[0] != 'q') {
switch (UserInput[0]) {
case 'r':
scanf("%d", &Offset);
printf("\nIn R");
return ('r');
break;
case 'w':
// printf("Is w");
printf("\nIn W");
return ('w');
break;
default:
printf("\nSomething else");
break;
}
} else {
return 'q';
}
}

Answer 1

I think you're having a problem, because you declared an unsigned int variable, but you used a wrong format specifier here:
scanf("%d", &Offset);
Offset is an unsigned int variable.
You have to use %u for unsigned integer instead of %d
Try this, this might solve the problem:
scanf("%u", &Offset);

Answer 2

Learn to enable all warnings and debug info in your compiler. With GCC compile using gcc -Wall -g
Then use the debugger (e.g. gdb) to run your program step by step.
At last, read the documentation of functions that you are using. scanf(3) is documented to return an integer number:
These functions return the number of input items successfully matched
and assigned, which can be fewer than provided for, or even zero in
the event of an early matching failure.
The value EOF is returned if the end of input is reached before
either the first successful conversion or a matching failure occurs.
EOF is also returned if a read error occurs, in which case the error
indicator for the stream (see ferror(3)) is set, and errno is set
indicate the error.
So you should code probably
if (scanf("%d", &Offset)<1) { perror("scanf failed"); exit (EXIT_FAILURE); };
BTW, your printf(3) format strings look strange: they should end (not start) with a newline \n e.g. code printf("In R\n"); or even better
printf("line %d of %s: In R\n", __LINE__, __FILE__);
or at least call explicitly fflush(NULL); after them, read
about fflush(3) and remember that stdio(3) is buffered.
At last, gets(3) is deprecated and dangerous, use fgets(3) or preferably getline(3). So at least replace gets(UserInput); with
memset (UserInput, 0, sizeof(UserInput));
if (NULL == fgets(UserInput, sizeof(UserInput), stdin))
{ perror("fgets"); exit(EXIT_FAILURE); }
BTW, your main function is incorrectly declared. It should be int main (int argc, char**argv); preferably. It should not return void but EXIT_SUCCESS i.e. 0 on success and something else on failure. See also EXIT_SUCCESS and EXIT_FAILURE and exit(3)
PS. You are missing several #include directives at the beginning of your program, at least:
#include <stdio.h>
#include <stdlib.h>
I assume your code have them but you did not show them to us.
NB: Using capital letters in variables is not conventional in C. Generally, people
reserve upper-case letters for macros. But that is a convention.

Answer 3

when I call ScanF from within the Switch it seems to ignore the Return AND the Break and just to Default which just prints "Not Something"..
If it "ignored the return and the break" then you would fall through to the case 'w'. But that isn't happening. Therefore it is not ignoring the return and the break (not that that is possible anyway).
Actually the program flow is working just as you wrote it. The sequence is:
scanf("%d", &Offset);
You type some numbers and press enter
return ('r');
back to main: MenuS = Menu(); - MenuS is now 'r'
while (MenuS != 'q') { MenuS = Menu(); - call Menu again
gets(UserInput); - gets the blank string you typed earlier
if (UserInput[0] != 'q') - true, since UserInput[0] is 0, the end-of-string marker for your blank string
switch (UserInput[0]) { there is no case 0, so default: is entered
printf("\nSomething else");
To fix this: you need to discard the Enter that was pressed after you typed in the number. One way is to put this code straight after the scanf:
int c;
while ( (c = getchar()) != '\n' && c != EOF ) {}
Note, your code has a lot of other problems but this was the main issue.