Symbolic Constant Problem in Xcode

Answer 1

The first line should be

#include <stdio.h>

Answer 2

The code is perfectly fine. There's something wrong with your XCode setup (may be related: unable to read unknown load command.

cristi:tmp diciu$ cat test.c
#include <stdio.h>

#define LOWER    0
#define UPPER    300
#define STEP 20

main()
{
    int fahr;

    for (fahr = LOWER; fahr <= UPPER; fahr = fahr + STEP)
        printf("%3d %6.1f\n", fahr, (5.0/9.0)*(fahr-32));
}
cristi:tmp diciu$ gcc test.c
cristi:tmp diciu$ ./a.out 
  0  -17.8
 20   -6.7
 40    4.4
[..]

Answer 3

Works for me in XCode - the only warning/error I got was

Control reaches end of non-void function

As defining main() defaults to int it should return something, eg. 0 for a successful program. Convention suggests that 0 means a program runs correctly and anything else is an error.

Better to define

int main()
{ 
    /* code */ 
    return 0;
}

as your main function. But this is a digression - see diciu's answer for a potential explanation of your problem