Shim layer C++

shim layer for library

Simple replacement layer that replace gnu library

Application

// test.cpp
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char** argv) {
    for (int i=0; i<5; i++) {
        printf("%d: %d\n", i, rand());
    }
    return 0;
}

Shim layer for rand()

// shim.cpp
extern "C" {
int rand();
}
int rand() {
    return 888;
}

Makefile

CC=g++
CFLAGS=-Wall -g
BINS=test.elf shim.so
all: $(BINS)
%.so: %.cpp
 $(CC) $(CFLAGS) -fPIC -shared -o $@ $^
%.elf: %.cpp
 $(CC) $(CFLAGS) -o $@ $^
clean:
 rm $(BINS)
 rm -r *.so

Run

$ make
g++ -Wall -g -o test.elf test.cpp
g++ -Wall -g -fPIC -shared -o shim.so shim.cpp
$ LD_PRELOAD=./shim.so ./test.elf
0: 888
1: 888
2: 888
3: 888
4: 888

Shim layer that adds more to gnu rand()

Application

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char** argv) {
    for (int i=0; i<5; i++) {
        printf("%d: %d\n", i, rand());
    }
    return 0;
}

Shim layer

#define _GNU_SOURCE
#include <unistd.h>
#include <dlfcn.h>
extern "C" {
int rand(); // this is needed for C++ shared library because C++ complier mangles function name
}
int rand() {
    typedef int (*func)(void);
    func f;
    f = (func)dlsym(RTLD_NEXT, "rand");
    return f() % 1000;
}

Makefile

CC=g++
CFLAGS=-Wall -g
BINS=test.elf shim.so
all: $(BINS)
%.so: %.cpp
 $(CC) $(CFLAGS) -fPIC -shared -Wl,--no-as-needed -ldl -o $@ $^
%.elf: %.cpp
 $(CC) $(CFLAGS) -o $@ $^
clean:
 rm $(BINS)
 rm -r *.so

Execution

$ make
g++ -Wall -g -fPIC -shared -Wl,--no-as-needed -ldl -o shim.so shim.cpp
shim.cpp:1:0: warning: "_GNU_SOURCE" redefined
 #define _GNU_SOURCE
 
<command-line>:0:0: note: this is the location of the previous definition
$ LD_PRELOAD=./shim.so ./test.elf
0: 383
1: 886
2: 777
3: 915
4: 793