Is -ffunction-sections -fdata-sections and --gc-sections not working? - c

Question

I want to remove unused functions from code while compiling. Then I write some code (main.c):
#include <stdio.h>
const char *get1();
int main()
{
puts( get1() );
}
and getall.c:
const char *get1()
{
return "s97symmqdn-1";
}
const char *get2()
{
return "s97symmqdn-2";
}
const char *get3()
{
return "s97symmqdn-3";
}
Makefile
test1 :
rm -f a.out *.o *.a
gcc -ffunction-sections -fdata-sections -c main.c getall.c
ar cr libgetall.a getall.o
gcc -Wl,--gc-sections main.o -L. -lgetall
After run make test1 && objdump --sym a.out | grep get , I only find the next 2 lines output:
0000000000000000 l df *ABS* 0000000000000000 getall.c
0000000000400535 g F .text 000000000000000b get1
I guess the get2 and get3 was removed. But when I open the a.out by vim, I found s97symmqdn-1 s97symmqdn-2 s97symmqdn-3 exists.
Is the function get2 get3 removed really ? How I can remove the symbol s97symmqdn-2 s97symmqdn-3 ? Thank you for your reply.
My system is centos7 and gcc version is 4.8.5

Answer 1

The compilation options -ffunction-sections -fdata-sections and linkage option --gc-sections
are working correctly in your example. Your static library is superfluous, so it can
be simplified to:
$ gcc -ffunction-sections -fdata-sections -c main.c getall.c
$ gcc -Wl,--gc-sections main.o getall.o -Wl,-Map=mapfile
in which I'm also asking for the linker's mapfile.
The unused functions get2 and get3 are absent from the executable:
$ nm a.out | grep get
0000000000000657 T get1
and the mapfile shows that the unused function-sections .text.get2 and .text.get3 in which get2 and get3 are
respectively defined were discarded in the linkage:
mapfile (1)
...
Discarded input sections
...
.text.get2 0x0000000000000000 0xd getall.o
.text.get3 0x0000000000000000 0xd getall.o
...
Nevertheless, as you found, all three of the string literals "s97symmqdn-(1|2|3)"
are in the program:
$ strings a.out | egrep 's97symmqdn-(1|2|3)'
s97symmqdn-1
s97symmqdn-2
s97symmqdn-3
That is because -fdata-sections applies just to the same data objects that
__attribute__ ((__section__("name"))) applies to1, i.e. to the definitions
of variables that have static storage duration. It is not applied to anonymous string literals like your
"s97symmqdn-(1|2|3)". They are all just placed in the .rodata section as usual,
and there we find them:
$ objdump -s -j .rodata a.out
a.out: file format elf64-x86-64
Contents of section .rodata:
06ed 73393773 796d6d71 646e2d31 00733937 s97symmqdn-1.s97
06fd 73796d6d 71646e2d 32007339 3773796d symmqdn-2.s97sym
070d 6d71646e 2d3300 mqdn-3.
--gc-sections does not allow the linker to discard .rodata from the program
because it is not an unused section: it contains "s97symmqdn-1", referenced
in the program by get1 as well as the unreferenced strings "s97symmqdn-2"
and "s97symmqdn-3"
Fix
To get these three string literals separated into distinct data sections, you
need to assign them to distinct named objects, e.g.
getcall.c (2)
const char *get1()
{
static const char s[] = "s97symmqdn-1";
return s;
}
const char *get2()
{
static const char s[] = "s97symmqdn-2";
return s;
}
const char *get3()
{
static const char s[] = "s97symmqdn-3";
return s;
}
If we recompile and relink with that change, we see:
mapfile (2)
...
Discarded input sections
...
.text.get2 0x0000000000000000 0xd getall.o
.text.get3 0x0000000000000000 0xd getall.o
.rodata.s.1797
0x0000000000000000 0xd getall.o
.rodata.s.1800
0x0000000000000000 0xd getall.o
...
Now there are two new discarded data-sections, which contain
the two string literals we don't need, as we can see in the object file:
$ objdump -s -j .rodata.s.1797 getall.o
getall.o: file format elf64-x86-64
Contents of section .rodata.s.1797:
0000 73393773 796d6d71 646e2d32 00 s97symmqdn-2.
and:
$ objdump -s -j .rodata.s.1800 getall.o
getall.o: file format elf64-x86-64
Contents of section .rodata.s.1800:
0000 73393773 796d6d71 646e2d33 00 s97symmqdn-3.
Only the referenced string "s97symmqdn-1" now appears anywhere in the program:
$ strings a.out | egrep 's97symmqdn-(1|2|3)'
s97symmqdn-1
and it is the only string in the program's .rodata:
$ objdump -s -j .rodata a.out
a.out: file format elf64-x86-64
Contents of section .rodata:
06f0 73393773 796d6d71 646e2d31 00 s97symmqdn-1.
[1] Likewise, -function-sections has the same effect as qualifying the
definition of every function foo with __attribute__ ((__section__(".text.foo")))