dlwrap

When creating an application that supports multiple backends (for compression, cryptography, etc.), it is sometimes undesirable to link all supported libraries to the application at once.

With dlopen, it is possible to defer loading of a library until the first time a function from the library is called. Such mechanism is typically implemented through wrappers around the library functions, though writing the wrappers is cumbersome and error-prone.

dlwrap makes it easy for an application to implement the mechanism.

Usage

Let's consider a hypothetical application which calls ZSTD_versionNumber and ZSTD_versionString to retrieve the run-time version of the ZSTD library.

First create a source file zstdver.c with the following:

#include "zstdwrap.h"
#include <stdio.h>

int
main (void)
{
  unsigned v1 = ZSTDWRAP_FUNC(ZSTD_versionNumber)();
  printf ("ZSTD_versionNumber: %u\n", v1);
  const char *v2 = ZSTDWRAP_FUNC(ZSTD_versionString)();
  printf ("ZSTD_versionString: %s\n", v2);
  return 0;
}

A couple of things to note:

• Instead of the standard <zstd.h> header, "zstdwrap.h" is included
• Function symbols are wrapped with the ZSTDWRAP_FUNC macro

Now proceed to generate helper files:

$ cargo install dlwrap
$ dlwrap --input /usr/include/zstd.h \
         --output-dir out \
         --clang-resource-dir "$(clang -print-resource-dir)" \
         --loader-basename zstdwrap \
         --symbol-regex "^ZSTD_(versionNumber|versionString)$" \
         --prefix zstdwrap \
         --include "<zstd.h>"

This command will create 3 files under out/: zstdwrap.c, zstdwrap.h, and zstdwrapfuncs.h.

At this point the application can be compiled with:

$ gcc -pthread -I./out \
      -DZSTDWRAP_ENABLE_DLOPEN=1 \
      -DZSTDWRAP_SONAME='"libzstd.so.1"' \
      -DZSTDWRAP_ENABLE_PTHREAD=1 \
      -o zstdver examples/zstdver.c out/zstdwrap.c

The generated code provides a couple of configuration macros:

• <LIBRARY_PREFIX>_ENABLE_DLOPEN controls whether to enable this dlopen mechanism at all. If it is undefined or 0, the application needs to be linked to the required library at build time (see below). This is useful when conditionalizing builds based on platforms where dlopen is supported or not.

• <LIBRARY_PREFIX>_SONAME specifies the first argument to dlopen. If it is undefined, no library is loaded automatically and the application needs to call <library_prefix>_ensure_library function, which takes the library SONAME as the first argument. This is useful when the application determines the actual library at run time.

• <LIBRARY_PREFIX>_ENABLE_PTHREAD controls whether the automatic library loading is supposed to be thread safe; in that case pthread_once is used to protect library loading and symbol resolution.

When inspecting the zstdver executable with ldd, you will see libzstd.so.1 is not linked, but it works as if it is:

$ ldd zstdver
        linux-vdso.so.1 (0x00007ffc705bf000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f8a1199f000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f8a11ba3000)

$ ./zstdver
ZSTD_versionNumber: 10506
ZSTD_versionString: 1.5.6

$ ltrace -e dlopen ./zstdver
zstdver->dlopen("libzstd.so.1", 1)               = 0x13152c0
ZSTD_versionNumber: 10506
ZSTD_versionString: 1.5.6
+++ exited (status 0) +++

Without ZSTDWRAP_ENABLE_DLOPEN defined, the same application code can be compiled with the standard linkage (i.e., libzstd is linked at build time):

$ gcc -I./out \
      -o zstdver examples/zstdver.c out/zstdwrap.c \
      -lzstd

$ ldd ./zstdver
        linux-vdso.so.1 (0x00007ffcd43e4000)
        libzstd.so.1 => /lib64/libzstd.so.1 (0x00007f7323269000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f7323087000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f732334a000)

$ ltrace -e dlopen ./zstdver
ZSTD_versionNumber: 10506
ZSTD_versionString: 1.5.6
+++ exited (status 0) +++

License

Apache-2.0

The generated code can be distributed under FSFAP.