ライブラリ関数のhook

hook.c

/*
    compile:
    $ gcc -shared -fPIC -o hook.so hook.c -ldl
*/

#define _GNU_SOURCE
#include <dlfcn.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>

static int (*getaddrinfo_0)(const char *node, const char *service, const struct addrinfo *hints, struct addrinfo **res);

extern int h_errno;
static struct hostent *(*gethostbyname_0)(const char *name);

void __attribute__((constructor))
hook_init()
{
    getaddrinfo_0 = dlsym(RTLD_NEXT, "getaddrinfo");
    gethostbyname_0 = dlsym(RTLD_NEXT, "gethostbyname");
}

int getaddrinfo(const char *node, const char *service, const struct addrinfo *hints, struct addrinfo **res)
{
    fprintf(stderr, "getaddrinfo: node=%s, service=%s\n", node, service);
    return (*getaddrinfo_0)(node, service, hints, res);
}

struct hostent *gethostbyname(const char *name)
{
    fprintf(stderr, "gethostbyname: name=%s\n", name);
    return (*gethostbyname_0)(name);
}