Call trampoline for GCC/Clang, similar to Microsoft C++ import libraries, permitting loading failures for shared objects without immediately aborting the program

generate.sh

#!/bin/sh
LIB_NAME="$1"
shift 1

# The arguments should be ./generate.sh LIB_NAME [FUNC_NAME_1 [FUNC_NAME_2 [...]]]

cat > trampoline_"${LIB_NAME}".h <<EOF
#include <stdbool.h>

#ifdef __cplusplus
extern "C" {
#endif

bool init_lib${LIB_NAME}(const char* filename, int flags);

#ifdef __cplusplus
}
#endif
EOF

cat > trampoline_"${LIB_NAME}".c <<EOF
#define _GNU_SOURCE

#include <dlfcn.h>
#include <stdbool.h>
#include <stddef.h>

#include "trampoline_${LIB_NAME}.h"

static void* handle = NULL;
EOF

for func in "$@"; do
  echo "__attribute__((used)) static void* ${func}_ptr = NULL;" >> trampoline_"${LIB_NAME}".c
done

cat >> trampoline_"${LIB_NAME}".c <<EOF

bool init_lib${LIB_NAME}(const char* filename, int flags) {
  if (handle != NULL) return true;
  if (filename == NULL) filename = "lib${LIB_NAME}.so";
  if (flags == 0) flags = RTLD_LAZY;

  handle = dlopen(filename, flags);
  if (handle == NULL) goto cleanup;

EOF

for func in "$@"; do
cat >> trampoline_"${LIB_NAME}".c <<EOF
  ${func}_ptr = dlsym(handle, "${func}");
  if (${func}_ptr == NULL) goto cleanup;

EOF
done

cat >> trampoline_"${LIB_NAME}".c <<EOF
  return true;

cleanup:
  if (handle != NULL) dlclose(handle);
  handle = NULL;

EOF

for func in "$@"; do
  echo "  ${func}_ptr = NULL;" >> trampoline_"${LIB_NAME}".c
done

cat >> trampoline_"${LIB_NAME}".c <<EOF

  return false;
}

EOF

for func in "$@"; do
  # Works best on x86_64; better to use higher levels than -O0 to avoid overwriting %rax
  echo "__attribute__((noreturn, naked, optimize(3))) void ${func}() { goto *${func}_ptr; }" >> trampoline_"${LIB_NAME}".c
done

main.c

#include <curl/curl.h>
#include <stddef.h>
#include <stdio.h>

#include "trampoline_curl.h"

int main() {
  if (!init_libcurl(NULL, 0)) {
    fprintf(stderr, "init_libcurl() failed\n");
    return 1;
  }

  CURL* curl = curl_easy_init();
  if (!curl) {
    fprintf(stderr, "curl_easy_init() failed\n");
    return 1;
  }

  curl_easy_setopt(curl, CURLOPT_URL, "https://httpbin.org/get");
  curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);

  CURLcode res = curl_easy_perform(curl);
  if (res != CURLE_OK) {
    fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
  }

  curl_easy_cleanup(curl);
  return 0;
}

Makefile

.PHONY: clean
.DEFAULT: main

main: main.o trampoline_curl.o
	gcc $^ -ldl -o $@

main.o: main.c trampoline_curl.h
	gcc -c -O3 -fPIC $< -o $@

trampoline_curl.o: trampoline_curl.c trampoline_curl.h
	gcc -c -O3 -fPIC $< -o $@

trampoline_curl.c trampoline_curl.h: generate.sh
	./generate.sh curl curl_easy_init curl_easy_setopt curl_easy_perform curl_easy_strerror curl_easy_cleanup

clean:
	rm -f main *.o trampoline_*