elf - Creating a relocatable shared library with binutils -

i have custom toolchain generates relocatable shared libraries. works equally custom elf loader loads these memory, fixing them up. trying persuade gcc , binutils produce elf files compatible loader.

unfortunately, appear binutils refusing generate relocatable shared objects. it'll generate pic shared objects, don't want due overhead of got/plt (and besides, custom elf loader doesn't support it). , it'll generate relocatable objects, they're not dynamic objects , don't have appropriate sections elf loader wants able load them.

i'm not clear why gnu ld refuses allow me specify --relocatable , -shared on same command line. can enlighten me? , know incantation make ld generate object files i'm looking for?

i see nobody has answered question, think i'll try.

i'll try show how works means of practical example. here c code, has artificial purposeful mix of external global functions , data -- bread , butter of relocations.

/* hello.c */ char* hello = "hello";  /* say.c */ #include "stdio.h" extern char* hello; void say(void){     printf(hello); }  /* main.c */ extern void say(void); void please_say(void){     say(); } int main(void){     please_say();     return 0; } 

now usual way shared object/library compile each c file -fpic, , link lot -shared. after that, can use readelf examine relocations. this:

gcc -fpic -c *.c gcc -shared -o libtemp.so *.o  readelf -r libtemp.so 

the relocation data follows:

 relocation section '.rel.dyn' @ offset 0x34c contains 6 entries:  offset     info    type            sym.value  sym. name 000016dc  00000008 r_386_relative 000016e0  00000008 r_386_relative 000016ac  00000106 r_386_glob_dat    00000000   __gmon_start__ 000016b0  00000206 r_386_glob_dat    00000000   _jv_registerclasses 000016b4  00000d06 r_386_glob_dat    000016e0   hello 000016b8  00000406 r_386_glob_dat    00000000   __cxa_finalize  relocation section '.rel.plt' @ offset 0x37c contains 5 entries:  offset     info    type            sym.value  sym. name 000016c8  00000107 r_386_jump_slot   00000000   __gmon_start__ 000016cc  00000507 r_386_jump_slot   000004fc   please_say 000016d0  00000807 r_386_jump_slot   00000540   say 000016d4  00000307 r_386_jump_slot   00000000   printf 000016d8  00000407 r_386_jump_slot   00000000   __cxa_finalize 

the r_386_glob_dat item hello got entry. similarly, r_386_jump_slot items say, please_say , printf plt entries. come use of position independent code, not fact have made shared object.

after doing same build process without -fpic, different relocations. so

gcc -c *.c gcc -shared -o libtemp.so *.o  readelf -r libtemp.so 

gives us

 relocation section '.rel.dyn' @ offset 0x34c contains 9 entries:  offset     info    type            sym.value  sym. name 00001674  00000008 r_386_relative 00001678  00000008 r_386_relative 000004d3  00000802 r_386_pc32        000004f0   say 000004e0  00000502 r_386_pc32        000004cc   please_say 000004f7  00000d01 r_386_32          00001678   hello 000004ff  00000302 r_386_pc32        00000000   printf 00001654  00000106 r_386_glob_dat    00000000   __gmon_start__ 00001658  00000206 r_386_glob_dat    00000000   _jv_registerclasses 0000165c  00000406 r_386_glob_dat    00000000   __cxa_finalize  relocation section '.rel.plt' @ offset 0x394 contains 2 entries:  offset     info    type            sym.value  sym. name 0000166c  00000107 r_386_jump_slot   00000000   __gmon_start__ 00001670  00000407 r_386_jump_slot   00000000   __cxa_finalize 

now shared object has familiar relocations definitions. there absolute relocation of hello, , pc relative relocations functions.

what mean? got , plt tables still there. there 2 important things note. first there no got or plt entries compiled code. second got , plt tables still needed. being used initialisation , cleanup (possibly standard library). since using custom elf loader, advisable implement basic support got , plt entries, if main application standard relocations instead.

your application pay price of relocation, not of position independence.