I wrote a toy program in C that looked like this:
#include <stdio.h>
int a, b;
int main() {
a = 3;
b = 5;
printf("a = %d\n", a);
printf("b = %d\n", b);
a = a + b;
b = a - b;
a = a - b;
printf("a = %d\n", a);
printf("b = %d\n", b);
return (0);
}
I tried compiling it:
$ clang varswap.c -o varswap
In file included from varswap.c:1:
In file included from /usr/include/stdio.h:28:
/usr/include/features.h:323:10: fatal error: 'bits/predefs.h' file not found
#include <bits/predefs.h>
^
1 diagnostic generated.
So . . . no compiled program resulted. Here's the same program again, with a critical line removed:
int a, b;
int main() {
a = 3;
b = 5;
printf("a = %d\n", a);
printf("b = %d\n", b);
a = a + b;
b = a - b;
a = a - b;
printf("a = %d\n", a);
printf("b = %d\n", b);
return (0);
}
I tried compiling that:
$ clang varswap.c -o varswap
varswap.c:7:3: warning: implicitly declaring C library function 'printf' with type 'int (char const *, ...)' [-pedantic]
printf("a = %d\n", a);
^
varswap.c:7:3: note: please include the header <stdio.h> or explicitly provide a declaration for 'printf'
2 diagnostics generated.
Hmm. Different error message. The really interesting part, though, is that I then had a varswap file:
$ ./varswap
a = 3
b = 5
a = 5
b = 3
As mentioned in Reminding Myself How To Write Simple C, I do not have these problems on a FreeBSD laptop. It compiles with the #include <stdio.h> line without complaints, and gives me a program that Just Works.