How to limit the API to be published in the C language shared library of Linux

** Trouble with API published in shared library **

I've been having a lot of trouble creating shared libraries. As the scale of the library grows, I think that naturally the inside will be divided into files and the API will be defined in the library for development.

At this time, I was always wondering if there was any way.

If you do it normally, it will be like this. ** The header API defined in the library is also included in the APIs available in the created shared library **

The public header is specified, but if you forcibly read the header used internally, that API will also be used. Especially when publishing a library on github, it's a bit crap to be able to use unintended APIs as you like.

So I checked it without expecting it.

** You can limit the API published in the shared library! ** **

There's nothing you can't do, yes with GNU! So, it seems that you can restrict publication with existing Linux functions.

Reference: http://0xcc.net/blog/archives/000108.html

For example, suppose you have a library public header like this: (Previously created time measurement library)

timetestlog.h

void * timetestlog_init(char *delimiter, size_t maxloglen, unsigned long maxstoresize);
int timetestlog_store_printf(void * handle, const char *format, ...);
void timetestlog_exit(void * handle);

As a library creator, I want you to use only this API.

Suppose you used this API when developing this library.

testfile.h

int testfunction(void);

** Normal compilation **

If you compile normally with this, you can use testfunction if you include testfile.h.

You can check the published API with the nm -D command or ʻobjdump -t`. Looking at the command execution result, you can see that the test function is included. Besides that, there are extra APIs here and there

Linux env$ nm -D .libs/libtimelog.so | grep " T "
0000000000000dec T _fini
00000000000007d8 T _init
0000000000000dc0 T testfunction
0000000000000d90 T timetestlog_exit
0000000000000b10 T timetestlog_init
0000000000000c90 T timetestlog_store_printf

Linux env$ objdump -t .libs/libtimelog.so | grep " g"
0000000000000dc0 g     F .text  000000000000002c              testfunction
0000000000202068 g       .data  0000000000000000              _edata
0000000000000dec g     F .fini  0000000000000000              _fini
0000000000000c90 g     F .text  00000000000000fc              timetestlog_store_printf
0000000000000b10 g     F .text  000000000000017f              timetestlog_init
0000000000202070 g       .bss   0000000000000000              _end
0000000000202068 g       .bss   0000000000000000              __bss_start
0000000000000d90 g     F .text  0000000000000022              timetestlog_exit
00000000000007d8 g     F .init  0000000000000000              _init

**--API restrictions by version-script **

Use "--version-script" to limit the public API. Specify the configuration file (libtimelog.map in this case) that defines version-script and public API in this way.

LDFLAGS+=-Wl,--version-script,libtimelog.map

In the configuration file, specify the API you want to expose to global, and specify other (*) in local.

libtimelog.map

{
  global:
    timetestlog_init;
    timetestlog_store_printf;
    timetestlog_exit;
  local: *;
};

Here is the public API of the library built by specifying the above configuration file. You can see that only the minimal API is exposed.

Linux env$ nm -D .libs/libtimelog.so | grep " T "
0000000000000cb0 T timetestlog_exit
0000000000000a30 T timetestlog_init
0000000000000bb0 T timetestlog_store_printf

Linux env$ objdump -t .libs/libtimelog.so | grep " g"
0000000000000bb0 g     F .text  00000000000000fc              timetestlog_store_printf
0000000000000a30 g     F .text  000000000000017f              timetestlog_init
0000000000000cb0 g     F .text  0000000000000022              timetestlog_exit

** Summary **

You can specify the public API by using ** --version-script **. Until now, I was concerned about the API to be published, and I actually put together the files I wanted to separate, but With this, you can create a library with your favorite file structure without hesitation.