【发布时间】：2012-09-02 18:08:06
【问题描述】：
我正在尝试使用 LD_PRELOAD 机制插入对 pthread_cond_broadcast 的调用。我插入的 pthread_cond_broadcast 函数只是调用了原始的 pthread_cond_broadcast。但是，对于一个同时调用 pthread_cond_wait 和 pthread_cond_broadcast 的非常简单的 pthread 代码，我要么在 glibc 中出现段错误（对于 glibc 2.11.1），要么程序挂起（对于 glibc 2.15）。有什么线索吗？
插入代码（编译为共享库）：
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <dlfcn.h>
static int (*orig_pthread_cond_broadcast)(pthread_cond_t *cond) = NULL;
__attribute__((constructor))
static void start() {
orig_pthread_cond_broadcast =
(int (*)()) dlsym(RTLD_NEXT, "pthread_cond_broadcast");
if (orig_pthread_cond_broadcast == NULL) {
printf("pthread_cond_broadcast not found!!!\n");
exit(1);
}
}
__attribute__((__visibility__("default")))
int pthread_cond_broadcast(pthread_cond_t *cond) {
return orig_pthread_cond_broadcast(cond);
}
简单的 pthread 程序：
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
pthread_mutex_t cond_mutex;
pthread_cond_t cond_var;
int condition;
void *thread0_work(void *arg) {
pthread_mutex_lock(&cond_mutex);
printf("Signal\n");
condition = 1;
pthread_cond_broadcast(&cond_var);
pthread_mutex_unlock(&cond_mutex);
return NULL;
}
void *thread1_work(void *arg) {
pthread_mutex_lock(&cond_mutex);
while (condition == 0) {
printf("Wait\n");
pthread_cond_wait(&cond_var, &cond_mutex);
printf("Done waiting\n");
}
pthread_mutex_unlock(&cond_mutex);
return NULL;
}
int main() {
pthread_t thread1;
pthread_mutex_init(&cond_mutex, NULL);
pthread_cond_init(&cond_var, NULL);
pthread_create(&thread1, NULL, thread1_work, NULL);
// Slowdown this thread, so the thread 1 does pthread_cond_wait.
usleep(1000);
thread0_work(NULL);
pthread_join(thread1, NULL);
return 0;
}
编辑：
对于 glibc 2.11.1，gdb bt 给出：
(gdb) set environment LD_PRELOAD=./libintercept.so
(gdb) run
Starting program: /home/seguljac/intercept/main
[Thread debugging using libthread_db enabled]
[New Thread 0x7ffff7436700 (LWP 19165)]
Wait
Signal
Before pthread_cond_broadcast
Program received signal SIGSEGV, Segmentation fault.
0x00007ffff79ca0e7 in pthread_cond_broadcast@@GLIBC_2.3.2 () from /lib/libpthread.so.0
(gdb) bt
#0 0x00007ffff79ca0e7 in pthread_cond_broadcast@@GLIBC_2.3.2 () from /lib/libpthread.so.0
#1 0x00007ffff7bdb769 in pthread_cond_broadcast () from ./libintercept.so
#2 0x00000000004008e8 in thread0_work ()
#3 0x00000000004009a4 in main ()
编辑 2：
（已解决） 正如 R.. 所建议的（谢谢！），问题是在我的平台上 pthread_cond_broadcast 是一个版本符号，而 dlsym 给出了错误的版本。这篇博客非常详细地解释了这种情况：http://blog.fesnel.com/blog/2009/08/25/preloading-with-multiple-symbol-versions/
【问题讨论】：
-
如果你在调试器下运行它，回溯是什么样的？您是否尝试过单步执行代码？
-
我注意到这是一个版本符号。
dlsym会不会给你错误的版本？
标签： c pthreads segmentation-fault glibc dlsym