Is there a way to determine a member offset at compile time?

c, gcc

Solution

Ok, answering my own question here: Note: I'm looking to determine the offset at compile time, that is, I don't want to have to run the code (I can also compile just the files I need, and not the whole system): The following can be cut and paste for those who are interested:

#include <stddef.h>

#define offsetof_ct(structname, membername) \
void ___offset_##membername ## ___(void) { \
        volatile char dummy[10000 + offsetof(structname, membername) ]; \
        dummy[0]=dummy[0]; \
}

struct x {
        int a[100];
        int b[20];
        int c[30];
};

offsetof_ct(struct x,a);
offsetof_ct(struct x,b);
offsetof_ct(struct x,c);

And then run:

~/tmp> gcc tst.c -Wframe-larger-than=1000
tst.c: In function ‘___offset_a___’:
tst.c:16:1: warning: the frame size of 10000 bytes is larger than 1000 bytes
tst.c: In function ‘___offset_b___’:
tst.c:17:1: warning: the frame size of 10400 bytes is larger than 1000 bytes
tst.c: In function ‘___offset_c___’:
tst.c:18:1: warning: the frame size of 10480 bytes is larger than 1000 bytes
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../lib64/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: ld returned 1 exit status

I then subtract 10000 to get the offset. Note: I tried adding `#pragma GCC diagnostic warning "-Wframe-larger-than=1000"` into the file, but it didn't like it, so it'll have to be specified on the command line.

John

Problem

I'm finding I'm spending a lot of time trying to determine member offsets of structures while debugging. I was wondering if there was a quick way to determine the offset of a member within a large structure at compile time. (Note: I know several ways of creating compile time assertions that an offset is in a given range, and I can do a binary search for the correct value, but I'm looking for something more efficient). I'm using a fairly recent version of `gcc` to compile C code.

Original source