I'm trying to integrate some third-party library to my project. I compile all my files with no problems, but when i try to add custom library, all constants in this library becomes garbage.
Here is my linker command (split over lines for readability):
arm-none-eabi-gcc
-Wall
-Wextra
-Wno-unused-but-set-variable
-mthumb
-mfloat-abi=soft
-mcpu=cortex-m4
-ffunction-sections
-fdata-sections
-ffreestanding
-g -ggdb -gdwarf-2 -g3
-Ibuild/ -Iinc/ -ICMSIS/ -I../lib/ -I.
-fverbose-asm
-DSTM32F407xx
-Wl,--gc-sections
-Wl,-Map=build/output.map
--specs=nosys.specs
-TLinkerScript.ld
-o build/main.elf
build/main.o {tons of o-files} ../lib/sdk.a
Here is the objdump for one function, first in library and then in resulting ELF:
arm-none-eabi-objdump -S ../lib/sdk.a build/main.elf | grep "<some_function>:" -A 9
00000000 <some_function>:
0: 4b02 ldr r3, [pc, #8] ; (c <some_function+0xc>)
2: 4a03 ldr r2, [pc, #12] ; (10 <some_function+0x10>)
4: 447b add r3, pc
6: 589b ldr r3, [r3, r2]
8: 6018 str r0, [r3, #0]
a: 4770 bx lr
c: 00000004 .word 0x00000004
10: 00000000 .word 0x00000000
08004b28 <some_function>:
8004b28: 4b02 ldr r3, [pc, #8] ; (8004b34 <some_function+0xc>)
8004b2a: 4a03 ldr r2, [pc, #12] ; (8004b38 <some_function+0x10>)
8004b2c: 447b add r3, pc
8004b2e: 589b ldr r3, [r3, r2]
8004b30: 6018 str r0, [r3, #0]
8004b32: 4770 bx lr
8004b34: 17ffc880 .word 0x17ffc880
8004b38: 00000164 .word 0x00000164
Notice:
.word 0x00000004
replaced by
.word 0x17ffc880
What can cause such transformation an how can i avoid that?
The 0x00000004 in library is not a real value - it's just an offset which needs to be added to some external symbol. The name of the symbol is stored in so called relocation section and can be displayed by running objdump -Sr.
The value of the external symbol is unknown until link-time but once the code is fully linked, the offset is replaced with final (so called "absolute") address which happens to be 0x17ffc880 in your case.