[Sorry! Meant to send this to the list, not just to Ian!]
On Dec 17, 2010, at 11:36 AM, Ian Lance Taylor wrote:
> Amittai Aviram <amittai.aviram@xxxxxxxx> writes:
>
>> Thank you very much! But I just tried linking with the -v option, and
>> the output looks as if the linker does in fact put head.o immediately
>> before hello.o and tail.o immediately after it. (Output below. The
>> relevant bit is near the bottom.) Why doesn't that give me the
>> results I was expecting in the finished executable binary?
>
> I don't know, you would have to provide more details about exactly what
> input you used, what you expected to see, exactly what you saw, and why
> you expected it to be different.
>
> One specific issue is that common symbols don't match your scheme, since
> they are not defined in the .bss section in a .o file but are placed
> there later by the linker.
>
> Ian
On Dec 17, 2010, at 11:36 AM, Ian Lance Taylor wrote:
> Amittai Aviram <amittai.aviram@xxxxxxxx> writes:
>
>> Thank you very much! But I just tried linking with the -v option, and
>> the output looks as if the linker does in fact put head.o immediately
>> before hello.o and tail.o immediately after it. (Output below. The
>> relevant bit is near the bottom.) Why doesn't that give me the
>> results I was expecting in the finished executable binary?
>
> I don't know, you would have to provide more details about exactly what
> input you used, what you expected to see, exactly what you saw, and why
> you expected it to be different.
>
> One specific issue is that common symbols don't match your scheme, since
> they are not defined in the .bss section in a .o file but are placed
> there later by the linker.
>
> Ian
I start with three source code files, head.s, hello.c, and tail.s. Here is their source code in that order.
// head.s
.section .rodata
.globl my_data_start
my_data_start:
.section .bss
.global my_bss_start
my_bss_start:
// end head.s
// hello.c
#include <stdio.h>
#include <stdlib.h>
int global_a, global_b;
int main(void) {
global_a = 3;
global_b = 4;
printf("Hello! %d %d\n", global_a, global_b);
return EXIT_SUCCESS;
}
// end hello.c
// tail.s
.section .bss
.globl my_bss_end
my_bss_end:
// end tail.s
And here is my Makefile:
all: l1
l1: head.o hello.o tail.o
gcc -Wall -static -g -v -o $@ $^
head.o: head.s
gcc -c -o $@ $<
tail.o: tail.s
gcc -c -o $@ $<
hello.o: hello.c
gcc -c -o $@ $<
clean:
rm -f l1 *.o
Here is what I was expecting to see in the output from objdump -D l1, somewhere after "Disassembly of section .bss" (where the three dots actually appear in the output, but any [text] is my shorthand notation):
[Some address] <my_data_start>:
...
[Next address 1] <global_b>:
[Next address 1]: 00 00 add %al,(%rax)
...
[Next address 2] <global_a>:
[Next address 2]: 00 00 add %al,(%rax)
...
[Next address 3] <my_data_end>:
...
But here is what I get instead:
Disassembly of section .bss:
00000000006a1cc0 <completed.6885>:
...
00000000006a1cc8 <dtor_idx.6887>:
...
00000000006a1ce0 <object.6911>:
...
00000000006a1d10 <my_bss_end>:
...
00000000006a1d20 <static_dtv>:
...
00000000006a2120 <static_slotinfo>:
...
[... snip ...]
00000000006a49e0 <frame_hdr_cache_head>:
...
00000000006a49e8 <global_b>:
6a49e8: 00 00 add %al,(%rax)
...
00000000006a49ec <global_a>:
6a49ec: 00 00 add %al,(%rax)
...
00000000006a49f0 <_dl_tls_static_used>:
...
[...snip...]
So note two things:
(a) my_data_start is missing, only my_data_end appears. (If, however, I only include head.o in my Makefile,rule, then my_data_start appears in the location where my_data_end appears in the above extract.)
(b) my_data_end is separated from global_a and global_b and seems to be in an arbitrary location in relation to them.
Thanks!
Amittai
Amittai Aviram
PhD Student in Computer Science
Yale University
646 483 2639
amittai.aviram@xxxxxxxx
http://www.amittai.com
