linker - c program involving linking get strange output -

in s1.c:

#include <stdio.h> int foo(); struct t1 {   int j; int k; int i; } a;  int main() {   foo();   printf("%d %d %d\n", a.i, a.j, a.k); } 

in s.c:

struct t1 {   int i; int j; int k; } a; int foo() {   a.i = 5; a.j = 7; a.k = 9;   return 0; } 

compile gcc s.c s1.c -o ee

run ./ee, got following output: 9 5 7.

i know has linking, what's reason in deed?

this program exhibit of undefined behavior: when declaration of struct in 1 translation unit not match definition in other translation unit, program ill-defined, , can anything.

in case, got lucky, , program printed numbers in opposite order. although did on system, there no requirement program same (or run!) on standard-compliant system. because types of fields match pairwise (j <==> i, k <===> j, i , <===> k). if change types of i, j, , k char, double, , int, program print garbage values or crash.

from c99, 6.2.7.1:

two structure, union, or enumerated types declared in separate translation units compatible if tags , members satisfy following requirements: if 1 declared tag, other shall declared same tag. if both complete types, following additional requirements apply: there shall one-to-one correspondence between members such each pair of corresponding members declared compatible types, , such if 1 member of corresponding pair declared name, other member declared same name. 2 structures, corresponding members shall declared in same order. 2 structures or unions, corresponding bit-fields shall have same widths. 2 enumerations, corresponding members shall have same values. (emphasis added)