In the following example, the program should print "foo called\n"
:
// foo.c
#include <stdio.h>
__attribute__((constructor)) void foo()
{
printf("foo called\n");
}
// main.c
int main()
{
return 0;
}
If the program is compiled like this, it works:
gcc -o test main.c foo.c
However, if foo.c is compiled into a static library, the program prints nothing.
gcc -c main.c
gcc -c foo.c
as rcs foo.a foo.o
gcc -o test foo.a main.o
Why does this happen?
The linker does not include the code in foo.a in the final program because nothing in main.o references it. If main.c
is rewritten as follows, the program will work:
//main.c
void foo();
int main()
{
void (*f)() = foo;
return 0;
}
Also, when compiling with a static library, the order of the arguments to gcc (or the linker) is significant: the library must come after the objects that reference it.
gcc -o test main.o foo.a