Mac OS X for Unix Geeks

Dynamically Loading Libraries

You can turn answer.o into a bundle, which can be dynamically loaded using the commands shown in Example 5-6.

Example 5-6. Commands for converting answer.o into a bundle

cc -flat_namespace -bundle -undefined suppress \
  -o libanswer.bundle answer.o

You do not need to specify the bundle at link time. Instead, use the dyld functions NSCreateObjectFileImageFromFile and NSLinkModule to load the library. Then, you can use NSLookupSymbolInModule and NSAddressOfSymbol to access the symbols that the library exports. Example 5-7 loads libanswer.bundle and invokes the get_answer function. Example 5-7 is similar to Example 5-4, but many lines (shown in bold) have been added.

Example 5-7. Dynamically loading a bundle and invoking a function

/*
 * deep_thought_dyld.c: Obtain the answer to life, the universe,
 * and everything, and act startled when you actually hear it.
 */
#include <stdio.h>
#import <mach-o/dyld.h>

int main(  )
{
  int the_answer;
  int rc;                // Success or failure result value
  NSObjectFileImage img; // Represents the bundle's object file
  NSModule handle;       // Handle to the loaded bundle
  NSSymbol sym;          // Represents a symbol in the bundle
int (*get_answer) (void);  // Function pointer for get_answer

  /* Get an object file for the bundle. */
  rc = NSCreateObjectFileImageFromFile("libanswer.bundle", &img);
  if (rc != NSObjectFileImageSuccess) {
    fprintf(stderr, "Could not load libanswer.bundle.\n");
    exit(-1);
  }

  /* Get a handle for the bundle. */
  handle = NSLinkModule(img, ...