《软件调试的艺术》学习笔记——GDB使用技巧摘要(4)——Pthread线程调试

Unix下最普遍的线程包是POSIX标准的Pthreads。Pthreads使用的抢占式线程管理策略，程序中的一个线程可能在任何时候被另一个线程中断。所以，使用Pthreads开发的应用程序有些错误不太容易重现。

GDB线程相关命令汇总

• info threads 给出关于当前所有线程的信息
• thread n 改为线程n，或者说是进入线程n的栈中进行观察
• break line_num thread n 表示当线程n到达源码行line_num时停止执行
• break line_num thread n if expression 上一命令增加条件断点而已

加入怀疑线程之间有死锁，可以用gdb进行调试定位。流程大致是：

1. 用gdb启动或者插入待调试程序
2. 当程序挂起时候，通过按下Ctrl+C组合键中断它；
3. 这个时候用info threads查看所有线程都在干嘛，然后找到自己的工作线程（注意排除main线程和pthreads的管理线程）
4. 分别查看自己的工作线程在干嘛，用bt(backtrace)查看对应的帧，记得用thread n切换进入对应线程的帧
5. 关注像__pthread_wait_for_restart_signal()和lock等函数，如果有源码的话，会比较方便地定位到具体的问题代码位置

下面是一个简单的例子。如果在worker线程里面，上锁和解锁没有匹配，则会发生死锁

// finds the primes between 2 and n; uses the Sieve of Eratosthenes,// deleting all multiples of 2, all multiples of 3, all multiples of 5,// etc.; not efficient, e.g. each thread should do deleting for a whole// block of values of base before going to nextbase for more // usage:  sieve nthreads n// where nthreads is the number of worker threads#include      <stdio.h>#include      <math.h>#include      <pthread.h>#define MAX_N     100000000#define MAX_THREADS  100// shared variablesint nthreads,  // number of threads (not counting main())    n,  // upper bound of range in which to find primes    prime[MAX_N+1],  // in the end, prime[i] = 1 if i prime, else 0    nextbase;  // next sieve multiplier to be usedint work[MAX_THREADS];  // to measure how much work each thread does,                        // in terms of number of sieve multipliers checked// lock index for the shared variable nextbasepthread_mutex_t nextbaselock = PTHREAD_MUTEX_INITIALIZER;// ID structs for the threadspthread_t id[MAX_THREADS];// "crosses out" all multiples of k, from k*k onvoid crossout(int k){  int i;   for (i = k; i*k <= n; i++)  {      prime[i*k] = 0;   }}// worker thread routinevoid *worker(int tn)  // tn is the thread number (0,1,...){  int lim,base;   // no need to check multipliers bigger than sqrt(n)   lim = sqrt(n);   do  {      // get next sieve multiplier, avoiding duplication across threads      pthread_mutex_lock(&nextbaselock);      base = nextbase += 2;       pthread_mutex_unlock(&nextbaselock);          if (base <= lim)  {         work[tn]++;  // log work done by this thread         // don't bother with crossing out if base is known to be         // composite         if (prime[base])            crossout(base);      }      else return;   } while (1);}main(int argc, char **argv){  int nprimes,  // number of primes found        totwork,  // number of base values checked       i;   void *p;   n = atoi(argv[1]);   nthreads = atoi(argv[2]);   for (i = 2; i <= n; i++)      prime[i] = 1;   crossout(2);   nextbase = 1;   // get threads started   for (i = 0; i < nthreads; i++)  {      pthread_create(&id[i],NULL,(void *) worker,(void *) i);   }   // wait for all done   totwork = 0;   for (i = 0; i < nthreads; i++)  {      pthread_join(id[i],&p);      printf(" %d           values of base done          /n          ",work[i]);      totwork += work[i];   }   printf("          %d           total values of base done          /n          ",totwork);   // report results   nprimes = 0;   for (i = 2; i <= n; i++)      if (prime[i]) nprimes++;   printf("the number of primes found was %d          /n          ",nprimes);}