How do I specify what function to export from .so library, when compiling the C code?

Question

I have a number of function in my "C" code. When I compile .so, I see all names in result .so file. How can I specify (in code or in make file) that only some functions should be exported, where as others are private just for internal use.

Answer 1

Since you mention .so files, it seems like a reasonable assumption that you're using gcc or a gcc-alike compiler.

By default all extern functions are visible in the linked object. You can hide functions (and global variables) on a case-by-case basis using the hidden attribute (while keeping them extern, which allows them to be used from other source files in the same library):

int __attribute__((visibility("hidden"))) foo(void)
{
    return 10;
}

Alternatively you can change the default to hidden by passing the -fvisibility=hidden option to gcc at compile time. You can then mark particular functions for export using:

__attribute__((visibility("default")))

Answer 2

In C, if you want a function to remain internal to the file (technically, the "compilation unit") that contains it, you declare it "static". For example,

static int privateAddOne(int x) { return x + 1; }