If my GNU ld linker script has OUTPUT_FORMAT(binary) the ld --gc-sections command-line flag seems to be ignored:
$ cat gcs.c
extern void magic(void);
void callmagic(void) { magic(); }
int uanswer = 42;
int main(void) { return 0; }
$ cat ofbin.scr
OUTPUT_FORMAT(binary)
OUTPUT_ARCH(i386)
ENTRY(main)
SECTIONS {
. = 0x10000;
.text : { *(.text*) }
.data : { *(.data*) *(.rodata*) }
.bss : { *(.bss*); }
}
$ cat ofelf.scr
OUTPUT_FORMAT(elf32-i386)
OUTPUT_ARCH(i386)
ENTRY(main)
SECTIONS {
. = 0x10000;
.text : { *(.text*) }
.data : { *(.data*) *(.rodata*) }
.bss : { *(.bss*); }
}
$ gcc -m32 -nostdlib -nostartfiles -nodefaultlibs gcs.c -ffunction-sections -fdata-sections -Wl,--gc-sections,--print-gc-sections -Wl,-T,ofelf.scr
/usr/bin/ld: Removing unused section '.text.callmagic' in file '/tmp/ccSFoPYg.o'
/usr/bin/ld: Removing unused section '.data.uanswer' in file '/tmp/ccSFoPYg.o'
/usr/bin/ld: Removing unused section '.eh_frame' in file '/tmp/ccSFoPYg.o'
$ gcc -m32 -nostdlib -nostartfiles -nodefaultlibs gcs.c -ffunction-sections -fdata-sections -Wl,--gc-sections,--print-gc-sections -Wl,-T,ofbin.scr
/tmp/cceXCw2b.o: In function `callmagic':
gcs.c:(.text.callmagic+0x7): undefined reference to `magic'
collect2: error: ld returned 1 exit status
I'm using GNU ld 2.24 and GCC 4.8.4 on Ubuntu 14.04.
How can I make this .c file compile and link with OUTPUT_FORMAT(binary) and with the code of callmagic eliminated by --gc-sections?
It looks like this is impossible, and GNU ld ignores --gc-sections with OUTPUT_FORMAT(binary).
A possible workaround is using OUTPUT_FORMAT(elf32-i386) and post-processing the output ELF file, e.g. with objcopy -O binary a.out a.out.bin.