c - 写一个 shell 。使用管道执行命令时为 "uninitialised byte(s)"

Question

我正在尝试用 C 语言编写自己的 shell。下面的代码适用于没有管道的命令，但不适用于其他情况。

使用 --trace-children=yes 和 --track-origins=yes 运行 valgrind 会给出“Syscall param execve(argv) point to uninitialized byte(s)”(请参阅​​下面的完整错误)。

在相关方法中(参见下面的 makeargs)，valgrind 告诉我“未初始化的值是由堆分配创建的”，在这一行“argv = (char *)malloc((count+1) * sizeof(char*));"

使用我的“ls | sort”测试输入，valgrind 表示“已分配大小为 12 的 block ”。我不明白这是怎么可能的，因为 ls 和 sort 每个都会​​调用 makeargs 并且都应该分配 8 个字节，因为应该有 4 个字节用于 char*，然后 4 个字节用于 execvp 最后需要的 (char*)NULL参数数组的。

执行此命令后程序挂起。

我不确定为什么会发生这种情况，因为如果只有一次 makeargs 调用(无管道)，它就会起作用。任何意见将不胜感激。

void execCommand(char** commandParts, int pipeCount)
{
  const int PIPE_READ = 0;
  const int PIPE_WRITE = 1;
  int numCommands = pipeCount + 1;
  int newfds[2];
  int oldfds[2];

  int k = 0;
  for(k; k < numCommands; k++)
  {

    //more commands exist
    if(k < pipeCount)
    {
      if (pipe(newfds) == -1) 
      {
        perror("new pipe error");
        exit(EXIT_FAILURE);
      }
    }

    if(fork() == 0) //child
    {
      //is prev command
      if(k > 0)
      {
        dup2(oldfds[PIPE_READ], STDIN_FILENO);
        close(oldfds[PIPE_READ]);
        close(oldfds[PIPE_WRITE]);
      }

      //more commands exist
      if(k < pipeCount)
      {
        close(newfds[PIPE_READ]);
        dup2(newfds[PIPE_WRITE], STDOUT_FILENO);
        close(newfds[PIPE_WRITE]);
      }
      char** args = NULL;
      int argcount = makeargs(commandParts[k], &args);

      if(execvp(args[0], args) == -1)
      {
        printf("%s: command not found \n", args[0]);
      }
    }
    else //parent
    {
      int status;
      waitpid(-1, &status, NULL);

      //is prev command
      if(k > 0)
      {
        close(oldfds[PIPE_READ]);
        close(oldfds[PIPE_WRITE]);
      }
      //more commands exist
      if(k < pipeCount)
      {
        oldfds[PIPE_READ] = newfds[PIPE_READ];
        oldfds[PIPE_WRITE] = newfds[PIPE_WRITE];
      }
    }
    //there are pipes
    if(pipeCount > 0 && k > 0)
    {
      close(newfds[PIPE_READ]);
      close(newfds[PIPE_WRITE]);
    } 
    //   if(argcount > 0)
    //    cleanArgs(argcount, args);   
  }
}

被调用的 make args 方法

int makeargs(char *s, char *** argv)
{
  stripLeadingAndTrailingSpaces(s);
  int k =0, count = 0;
  for(k; k < strlen(s); k++)
  {
    if(s[k] == ' ')
    count++;
  }
  count++;

  char* parts = strtok (s," ");
  strip(parts);

  *argv = (char **)malloc((count+1) * sizeof(char*));

  (*argv)[0] = (char *)malloc(strlen(parts)+1);
  strcpy((*argv)[0], parts);

  int i = 1;
  for(i; i < count; i++)
  {
    parts = strtok (NULL, " ");
    if(parts != NULL)
    {
      strip(parts);
      (*argv)[i] = (char *)malloc(strlen(parts)+1);
      strcpy((*argv)[i], parts);
    }
  }
  (*argv)[count] = NULL;

  return count;
}

valgrind 输出

==3603== Syscall param execve(argv) points to uninitialised byte(s)
==3603==    at 0x40E2CDF: execve (execve.c:60)
==3603==    by 0x40E314E: execvp (execvp.c:30)
==3603==    by 0x8049069: main (cscd340_s12_hw2.c:250)
==3603==  Address 0x41c617c is 4 bytes inside a block of size 12 alloc'd
==3603==    at 0x4028876: malloc (vg_replace_malloc.c:236)
==3603==    by 0x8049416: makeargs (ush.c:100)
==3603==    by 0x8048E61: execCommand (cscd340_s12_hw2.c:191)
==3603==    by 0x8049069: main (cscd340_s12_hw2.c:250)
==3603==  Uninitialised value was created by a heap allocation
==3603==    at 0x4028876: malloc (vg_replace_malloc.c:236)
==3603==    by 0x8049416: makeargs (ush.c:100)
==3603==    by 0x8048E61: execCommand (cscd340_s12_hw2.c:191)
==3603==    by 0x8049069: main (cscd340_s12_hw2.c:250)

Answer 1

你有一些错误的计数:count是空格的数量，而不是命令行中的单词的数量。这会影响复制代码:

for(i; i < count; i++)
{
  parts = strtok (NULL, " ");
  if(parts != NULL)
  {
    ...
  }
}

如果输入命令行中的单词太少，argv 中的一些尾随条目将无法初始化。您可能想做这样的事情:

for(i; i <= count; i++)
{
  parts = strtok (NULL, " ");
  if(parts != NULL)
  {
    ... // consider using strdup
  }
  else
  {
    (*argv)[i] = NULL;
    break;
  }
}