malloc 内部函数 / 引用调用 / valgrind

Question

我在 C 方面不是很专业，所以我遇到了 valgrind 的问题。

我想从命令行读取输入。

如果我以这种方式尝试它，它工作正常。

//call the function
char *command = getUserInput();


//function
char *getUserInput()
{ 
  char *buffer = NULL;
  char *temp = NULL;
  unsigned int count = 0;
  unsigned int lenght = 10;
  char character = 0;

  buffer = malloc((lenght+1)*sizeof(char));
  if(buffer == NULL)
  {
    // printf(ERROR_OUT_OF_MEM);
    // return EXIT_OUT_OF_MEM;
  }

  while((character = getchar()) != '\n')
  {
    if(count == lenght)
    {
      lenght += 10;
      temp = realloc(buffer,lenght*sizeof(char));
      if(temp != NULL)
        {
          buffer = temp;
        }
      else
        {
          free (buffer);
          // printf(ERROR_OUT_OF_MEM);
          // return EXIT_OUT_OF_MEM; 
        }
    }

    buffer[count] = character;
    count++;

  }
  buffer[count] = '\0';

  return buffer;
}

但我无法获得错误返回值。

如果我通过引用调用的方式尝试它，我会得到一些我不明白的 valgrind 错误。 而且我知道，在这个示例函数调用中，我不请求返回值。

//call function
char *command = NULL;
getUserInput(command);

//function
int getUserInput(char *name)
{ 
  char *temp = NULL;
  unsigned int count = 0;
  unsigned int lenght = 10;
  char character = 0;

  name = malloc((lenght+1)*sizeof(char));
  checkMemory(name);

  while((character = getchar()) != '\n')
  {
    if(count == lenght)
    {
      lenght += 10;
      temp = realloc(name,lenght*sizeof(char));
      if(temp != NULL)
        {
          name = temp;
        }
      else
        {
          free (name);
          printf(ERROR_OUT_OF_MEMORY_MESSAGE);
          return ERROR_OUT_OF_MEMORY; 
        }
    }

    name[count] = character;
    count++;

  }
  name[count] = '\0';

  return RETURN_SUCCESS;
}

Commandhandler-Line 1199 是 if (strcmp(command, "thing_to_compare") == 0)

瓦尔格林：

==23886== Use of uninitialised value of size 4
==23886==    at 0x40256BB: strcmp (mc_replace_strmem.c:426)
==23886==    by 0x8049B39: commandHandler (assa.c:1199)
==23886==    by 0x8049D6C: main (assa.c:1295)
==23886==  Uninitialised value was created by a stack allocation
==23886==    at 0x8049B00: commandHandler (assa.c:1189)
==23886== 
==23886== Invalid read of size 1
==23886==    at 0x40256BB: strcmp (mc_replace_strmem.c:426)
==23886==    by 0x8049B39: commandHandler (assa.c:1199)
==23886==    by 0x8049D6C: main (assa.c:1295)
==23886==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

我完全不知道问题出在哪里。

亲切的问候 菲利普

Answer 1

getUserInput 无法更改 command 的值，因为所有函数都是 C 中的值传递。因此，当您访问所有 strcmp 函数时，您将取消引用 NULL导致各种坏事的指针。

如果您希望getUserInput 能够更改command 的值，则必须将command 的地址传递给函数（这意味着函数必须接受char** 参数）。