preload示例

参考csapp

preload_send.c

#define _GNU_SOURCE

#include <stdlib.h>
#include <stdio.h>
#include <dlfcn.h>
#include <fcntl.h>
// #include <sys/socket.h>

int send(int s, const void * msg, int len, unsigned int falgs) {
    // static int [1024];
    static int index = 0;
    // static psend = NULL;
    typedef int (*sendfunc_t)(int, const void *, int, unsigned int);
    // static int (*psend)(int, const void *, int, unsigned int) = NULL;
    static sendfunc_t psend = NULL;

    if(!psend) {
        psend = (sendfunc_t)dlsym(RTLD_NEXT, "send");
        if(!psend) {
            printf("dlsym send fail\n");
            exit(0);
        }

        int flags = fcntl(s, F_GETFL, 0);
        printf("flags&O_NONBLOCK=%x, O_NONBLOCK=%x, flags=%x, len=%d\n", flags&O_NONBLOCK, O_NONBLOCK, flags, len);

    }

    int ret = len;
    ret = psend(s, msg, len, falgs);
    printf("ret=%d, len=%d\n", ret, len);
    index++;
    return ret;
}

编译 （注：用g++编译会进行name mangling，符号名不是send）

gcc -o preload_send.so --shared -fPIC   preload_send.c

运行

LD_PRELOAD=./preload_send.so   ./program