MojoELF/CategoryMojoELF

CategoryMojoELF

The latest version of MojoELF can be found at:

MojoELF is an ELF binary loader that runs in your application, instead of as part of the C runtime. Its most useful feature is that, unlike the standard dlopen(), it can load an ELF file from a place other than the filesystem. Notably, it can load one from a buffer in memory.

It can also be useful for overriding specific parts of the dependency management process, such as loading a replacement library or quietly replacing a function, etc.

To use MojoELF:

Add mojoelf.c to your build. MojoELF is intended to be compiled directly into your project as a single C file, not built as an external shared library (but you could probably do that if you like).

Compile with these #defines set as you please, or change the top of mojoelf.c:

  #define MOJOELF_SUPPORT_DLERROR 0  // remove MOJOELF_dlerror() + lots of strings.
  #define MOJOELF_SUPPORT_DLOPEN_FILE 0 // remove MOJOELF_dlopen_file()
  #define MOJOELF_REDUCE_LIBC_DEPENDENCIES 0  // use less libc calls. Scary!
  #define NDEBUG 1  // Turns off assert, which removes libc dependencies.

Your calling code should #include mojoelf.h - Put your ELF library in memory, and call MOJOELF_dlopen_mem() with the address of the memory buffer, the size of the buffer, and (optionally), callbacks that handle symbol resolution (they can be NULL). Refer to the documentation on MOJOELF_LoaderCallback, MOJOELF_ResolverCallback, and MOJOELF_UnloaderCallback for details.

If MOJOELF_dlopen_mem() returns non-NULL, the library is ready to use. If it returns NULL, there was a problem (MOJOELF_dlerror() can give you a human-readable error message). On success, you can free your buffer; we don't need it after MOJOELF_dlopen_mem() returns.

MOJOELF_dlopen_file() does the same thing, but takes a filename instead of a memory buffer. Internally, it just loads the file into a malloc()'d buffer and calls MOJOELF_dlopen_mem().

To request entry points into the library, use MOJOELF_dlsym():

typedef int (*my_fn_type)(int argument);
my_fn_type my_function = (my_fn_type) MOJOELF_dlsym(lib, "AwesomeFunc");
if (my_function == NULL) {
    printf("couldn't find AwesomeFunc!\n");
} else {
    printf("AwesomeFunc() returns %d\n", my_function(123));
}

When you are done with a library, call MOJOELF_dlclose() to free any resources. All pointers returned by MOJOELF_dlsym() for this library are invalid after this call. - Other fun stuff: MOJOELF_getentry() gets you the entry point for the ELF file (which isn't useful on shared libraries, but is how you eventually get to main() in an executable).

Thread safety:

MojoELF provides no locking mechanisms, and has a piece of global state that almost any function can set (the MOJOELF_dlerror() state). As such, the app is responsible for providing a serialization method if they plan to use MojoELF from multiple threads at once.

Other stuff:

Please see the file LICENSE.txt in the source's root directory for licensing and redistribution rights.

CategoryMojoELF

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