C中的多线程错误分段错误

Question

我正在尝试使用多线程将两个矩阵相乘。这里我在linux中使用gcc编译程序，输入线程数运行。

gcc multiThread.c -o test -lpthread
./test 4

在这里，我运行 N*N 矩阵的乘法，其中 N 从 10 到 1000，间隔为 10，并计算每次迭代的执行时间。当我运行程序时，它会出现分段错误。请帮忙。

#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include<time.h>

int SIZE = 10;   // Size by SIZE matrices
int num_thrd;   // number of threads

int A[2000][2000], B[2000][2000], C[2000][2000];

// initialize a matrix
void init_matrix(int m[SIZE][SIZE])
{
  int i, j;
  for (i = 0; i < SIZE; i++)
    for (j = 0; j < SIZE; j++)
      m[i][j] = rand() % 100 + 1;
}

// thread function: taking "slice" as its argument
void* multiply(void* slice)
{
  int s = (int)slice;   // retrive the slice info
  int from = (s * SIZE)/num_thrd; // note that this 'slicing' works fine
  int to = ((s+1) * SIZE)/num_thrd; // even if SIZE is not divisible by num_thrd
  int i,j,k;

  printf("computing slice %d (from row %d to %d)\n", s, from, to-1);
  for (i = from; i < to; i++)
  {  
    for (j = 0; j < SIZE; j++)
    {
      C[i][j] = 0;
      for ( k = 0; k < SIZE; k++)
    C[i][j] += A[i][k]*B[k][j];
    }
  }
  printf("finished slice %d\n", s);
}

int main(int argc, char* argv[])
{
  FILE *outFile;
  outFile = fopen("Algorithm3_Times.txt", "r");
  pthread_t* thread;  // pointer to a group of threads
for(int ini=0; ini<100; ini++)
{


  int i;

  if (argc!=2)
  {
    printf("Usage: %s number_of_threads\n",argv[0]);
    exit(-1);
  }

  num_thrd = atoi(argv[1]);
  init_matrix(A);
  init_matrix(B);
  clock_t start = clock();
  thread = (pthread_t*) malloc(num_thrd*sizeof(pthread_t));
  // this for loop not entered if threadd number is specified as 1
  for (i = 1; i < num_thrd; i++)
  {
    // creates each thread working on its own slice of i
    if (pthread_create (&thread[i], NULL, multiply, (void*)i) != 0 )
    {
      perror("Can't create thread");
      free(thread);
      exit(-1);
    }
  }

  // main thread works on slice 0
  // so everybody is busy
  // main thread does everything if threadd number is specified as 1
  multiply(0);

  // main thead waiting for other thread to complete
  for (i = 1; i < num_thrd; i++)
    pthread_join (thread[i], NULL);

  clock_t end = clock();

  float time = (end - start)*1000 / CLOCKS_PER_SEC;
  fprintf(outFile,"time taken for Multiplication using %d", num_thrd);
  fprintf(outFile," threads =  %f", time);
  fprintf(outFile," milliseconds \n");
  if (thread != NULL)
  {
      free(thread);
      thread = NULL;
  }
  SIZE += 10;

 }

  printf("calculation completed.\n\n");
  return 0;

}

Answer 1

C 是一门硬语言，特别是因为默认情况下运行时错误不包含有用的调试信息。这就是调试器的用武之地。

我如何使用 docker/alpine 进行调试：

1. 将代码放入~/gcc/t.c，分享给我的docker
2. docker run --rm -it -vls -d ~/gcc:/code alpine
3. apk add build-base gdb musl-dbg 安装 gcc 和朋友、gdb 和 musl-dbg，我需要在标准库中调试 segfault。
4. cd /code
5. gcc -g t.c -o t
6. gdb t

现在这是我的调试器会话：

GNU gdb (GDB) 8.0.1

[ ... preamble removed for brevity ... ]

Reading symbols from t...done.
(gdb) run 1
Starting program: /code/t 1
warning: Error disabling address space randomization: Operation not permitted
computing slice 0 (from row 0 to 9)
finished slice 0

Program received signal SIGSEGV, Segmentation fault.
vfprintf (f=0x0, fmt=0x560896553020 "time taken for Multiplication using %d",
    ap=ap@entry=0x7ffcb18b29f8) at src/stdio/vfprintf.c:671
671 src/stdio/vfprintf.c: No such file or directory.
(gdb) bt
#0  vfprintf (f=0x0,
    fmt=0x560896553020 "time taken for Multiplication using %d",
    ap=ap@entry=0x7ffcb18b29f8) at src/stdio/vfprintf.c:671
#1  0x00007fa6ef0c056f in fprintf (f=<optimized out>, fmt=<optimized out>)
    at src/stdio/fprintf.c:9
#2  0x0000560896552ee2 in main (argc=2, argv=0x7ffcb18b2b68) at t.c:87

太好了，现在我们有一个行号可以查看。第 87 行就是这一行：

fprintf(outFile,"time taken for Multiplication using %d", num_thrd);

我们可以看一下线路，稍微思考一下，最终来看看outFile的定义：

outFile = fopen("Algorithm3_Times.txt", "r");

啊哈！我们希望写入 outFile，但我们打开它是为了阅读！我改为开写：

  outFile = fopen("Algorithm3_Times.txt", "w");

你的程序运行（注意下面我只显示前 10 行）

/code # ./t 10  |head
computing slice 1 (from row 1 to 1)
finished slice 1
computing slice 2 (from row 2 to 2)
computing slice 3 (from row 3 to 3)
finished slice 3
finished slice 2
computing slice 4 (from row 4 to 4)
finished slice 4
computing slice 5 (from row 5 to 5)
finished slice 5

现在您已经了解了gdb，您可以开始在特定行中添加 gdb break 语句并解决任何剩余的错误。我的程序似乎没有完成，但我没有给它太多时间。