bpgview.c的主要功能不是由gcc编译的

Question

BPG is a digital image format that is more compression-efficent than jpeg. I wanted to compile libbpg-0.9.6 on OS X 10.10.5 (14F1509). My Xcode version is 7.2.1 (7C1002).

$gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents 
/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin14.5.0
Thread model: posix

I have all the necessary libraries( see: github.com/mirrorer/libbpg) installed. When I run make I have the following error:

gcc -Os -Wall -MMD -fno-asynchronous-unwind-tables -fdata-sections -ffunction-sections -fno-math-errno -fno-signed-zeros -fno-tree-vectorize -fomit-frame-pointer -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_REENTRANT -I. -DCONFIG_BPG_VERSION=\"0.9.6\" -g -I./x265/source -I./x265.out/8bit -c -o x265_glue.o x265_glue.c
g++ -g -Wl,-dead_strip -o bpgenc bpgenc.o x265_glue.o x265.out/8bit/libx265.a x265.out/10bit/libx265.a x265.out/12bit/libx265.a  -lpng -ljpeg  -lm -lpthread
gcc -Os -Wall -MMD -fno-asynchronous-unwind-tables -fdata-sections -ffunction-sections -fno-math-errno -fno-signed-zeros -fno-tree-vectorize -fomit-frame-pointer -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_REENTRANT -I. -DCONFIG_BPG_VERSION=\"0.9.6\" -g -c -o bpgview.o bpgview.c
gcc -g -Wl,-dead_strip -o bpgview bpgview.o libbpg.a -lSDL_image -lSDL  -lm -lpthread
ld: entry point (_main) undefined. for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [bpgview] Error 1

I did check the symbols in bpgview.o. Not only didn't int main show up but also the static void help and static void set_caption in bpgview.c .

$nm bpgview.o
             U _IMG_Load
             U _SDL_AddTimer
             U _SDL_CreateRGBSurface
             U _SDL_EnableKeyRepeat
             U _SDL_FillRect
             U _SDL_Flip
             U _SDL_FreeSurface
             U _SDL_GetVideoInfo
             U _SDL_Init
             U _SDL_LockSurface
             U _SDL_MapRGB
             U _SDL_PushEvent
             U _SDL_RemoveTimer
             U _SDL_SetVideoMode
             U _SDL_UnlockSurface
             U _SDL_UpperBlit
             U _SDL_WM_SetCaption
             U _SDL_WaitEvent
 00000000000006f6 T _SDL_main
             U ___snprintf_chk
             U ___stack_chk_fail
             U ___stack_chk_guard
             U ___stderrp
             U _bpg_decoder_close
             U _bpg_decoder_decode
             U _bpg_decoder_get_frame_duration
             U _bpg_decoder_get_info
             U _bpg_decoder_get_info_from_buf
             U _bpg_decoder_get_line
             U _bpg_decoder_open
             U _bpg_decoder_start
0000000000000000 T _bpg_load
0000000000000404 T _center_image
0000000000000465 T _draw_image
             U _exit
             U _fclose
             U _fopen
             U _fprintf
             U _fread
             U _free
             U _fseek
             U _ftell
             U _fwrite
             U _getopt
000000000000021f T _load_image
             U _malloc
0000000000000c64 t _open_window
             U _optind
000000000000069a T _pan_image
             U _printf
             U _realloc
00000000000003c6 t _restart_frame_timer
0000000000000cbc t _timer_cb

The complete make output is on pastebin.

I find it very strange but there may be some gcc options I don't know about that let the compiler ignore functions. Thank you for your time.

Answer 1

I figured out that -e _SDL_main solved the issue of the missing entry point. This seems strange at first but it turned out the compiler used this symbol for jet unknown reasons instead of _main .

The disassembled object-file is proof as is the working binary: ( https://ptpb.pw/6kev.png )

The missing _help and _set_caption came from the -Os option which optimizes size.