How to print all symbols that a mach-o binary imports from dylibs?

Question

I am new to MacOS & MacOS programming. I would like to know how an application is working (by simply having a glimpse of the binary's imported system APIs for learning).

I tried otool, printed the help message, I saw the message says -L prints the shared libraries used. Then I ran otool -L <binary-file>, but it only printed the names of the used libraries, without any imported symbols.

Maybe I missed something in otool, or is there any other tool that can help get import symbols in a mach-o binary?

Answer 1

You can get a simple list of imported symbols with nm -u:

% nm -u /bin/echo
___mb_cur_max
___stdoutp
_err
_exit
_fflush
_getenv
_mbtowc
_putchar
_putwchar
_strcmp
_strlen
dyld_stub_binder

For more detailed information, including the library that each symbol is expected to come from, you can use dyldinfo. This requires Xcode though, and has to be invoked as xcrun dyldinfo. You'll want the union of -bind, -weak_bind and -lazy_bind:

% xcrun dyldinfo -bind /bin/echo 
bind information:
segment section          address        type    addend dylib            symbol
__DATA_CONST __got            0x100004000    pointer      0 libSystem        ___mb_cur_max
__DATA_CONST __got            0x100004008    pointer      0 libSystem        ___stdoutp
__DATA_CONST __got            0x100004010    pointer      0 libSystem        dyld_stub_binder
% xcrun dyldinfo -weak_bind /bin/echo
no weak binding
% xcrun dyldinfo -lazy_bind /bin/echo
lazy binding information (from lazy_bind part of dyld info):
segment section          address    index  dylib            symbol
__DATA  __la_symbol_ptr  0x100008000 0x0000 libSystem        _err
__DATA  __la_symbol_ptr  0x100008008 0x000B libSystem        _exit
__DATA  __la_symbol_ptr  0x100008010 0x0017 libSystem        _fflush
__DATA  __la_symbol_ptr  0x100008018 0x0025 libSystem        _getenv
__DATA  __la_symbol_ptr  0x100008020 0x0033 libSystem        _mbtowc
__DATA  __la_symbol_ptr  0x100008028 0x0041 libSystem        _putchar
__DATA  __la_symbol_ptr  0x100008030 0x0050 libSystem        _putwchar
__DATA  __la_symbol_ptr  0x100008038 0x0060 libSystem        _strcmp
__DATA  __la_symbol_ptr  0x100008040 0x006E libSystem        _strlen