具体读取 13 行时出现分段错误

发布于 2024-12-07 22:16:23 字数 641 浏览 0 评论 0原文

char **getLines(FILE *pFile)
{
  int nLines = 0;
  size_t memRes = 0;
  char **lines = malloc(memRes);
  char *current = (char*) malloc(20);
  while(fgets(current, 20, pFile) != NULL)
    {
      memRes += sizeof(char*);
      lines = realloc(lines, memRes);
      lines[nLines] = malloc(20);
      strcpy(lines[nLines], current);
      nLines++;
    }
  free(current);
  return lines;
}

void printLines(char **lines)
{
  int lineI = 0;
  while(lines[lineI] != NULL)
    {
      printf("%s", lines[lineI]);
      lineI++;
    }
}

首先我得到线条然后打印它们。奇怪的是，当我的代码读取了 13 行代码并打印它们时，它就失败了。打印最后一行后出现分段错误。它适用于 12 行和 14 行。

原文

char **getLines(FILE *pFile)
{
  int nLines = 0;
  size_t memRes = 0;
  char **lines = malloc(memRes);
  char *current = (char*) malloc(20);
  while(fgets(current, 20, pFile) != NULL)
    {
      memRes += sizeof(char*);
      lines = realloc(lines, memRes);
      lines[nLines] = malloc(20);
      strcpy(lines[nLines], current);
      nLines++;
    }
  free(current);
  return lines;
}

void printLines(char **lines)
{
  int lineI = 0;
  while(lines[lineI] != NULL)
    {
      printf("%s", lines[lineI]);
      lineI++;
    }
}

First I get the lines then I print them. The odd thing about it is that my code fails when it has read exactly 13 lines of code and then prints them. I get a segmentation fault after printing the last line. It works with 12 lines and with 14 lines perfectly.

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夜血缘 2024-12-14 22:16:23

在 printLines 函数中，“while(lines[lineI] != NULL)”

发生的情况是，lines[lineI] 不是 NULL，只是最后还没有分配。因为 lineI 大于 nLines，所以它会出现段错误。您必须将 nLines 传递到函数中并使用它来检查边界。 “同时（lineI < nLines）”

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静若繁花 2024-12-14 22:16:23

size_t memRes = 0;
char **lines = malloc(memRes);

线条可能有点太小了。

size_t memRes = 0;
char **lines = malloc(memRes);

Lines might be a bit too small.

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回忆凄美了谁 2024-12-14 22:16:23

您假设行尾之后的内存将包含 NULL，它可能会也可能不会 - 它未初始化，因此它可以容纳任何内容。您对一些小数据集感到“幸运”，并且它恰好包含零，所以它有效。对于更大的数据集，您会遇到其中有其他内容的内存区域，并且它会失败。如果您希望超过保存 fgets() 调用数据的指针集的行为 NULL，则需要分配足够大的空间以容纳额外的指针，并将该空间设置为 NULL。

char **getLines(FILE *pFile)
{
  int nLines = 0;
  /* Start lines big enough to hold at least the NULL */
  size_t memRes = sizeof(char *);
  char **lines = malloc(memRes);
  lines[0] = NULL; /* the memory returned by malloc must be initialized */
  char *current = (char*) malloc(20);
  while(fgets(current, 20, pFile) != NULL)
  {
    memRes += sizeof(char*);
    lines = realloc(lines, memRes);
    lines[nLines + 1] = NULL;  /* Prepare the NULL to terminate display loop */
    lines[nLines] = malloc(20);
    strcpy(lines[nLines], current);
    nLines++;
  }
  free(current);
  return lines;
}

void printLines(char **lines)
{
  int lineI = 0;
  while(lines[lineI] != NULL)
  {
    printf("%s", lines[lineI]);
    lineI++;
  }
}

You are assuming that the memory past the end of lines will contain a NULL, which it may or may not - it is uninitialized, so it could hold anything. You are getting 'lucky' for some small data sets, and it happens to contain zeros, so it works. For bigger data sets, you are running into an area of memory that has something else in it, and it is failing. If you want lines to be NULL past the set of pointers that hold the data from the fgets() calls, you need to allocate it big enough to hold an extra pointer, and set that space to NULL.

char **getLines(FILE *pFile)
{
  int nLines = 0;
  /* Start lines big enough to hold at least the NULL */
  size_t memRes = sizeof(char *);
  char **lines = malloc(memRes);
  lines[0] = NULL; /* the memory returned by malloc must be initialized */
  char *current = (char*) malloc(20);
  while(fgets(current, 20, pFile) != NULL)
  {
    memRes += sizeof(char*);
    lines = realloc(lines, memRes);
    lines[nLines + 1] = NULL;  /* Prepare the NULL to terminate display loop */
    lines[nLines] = malloc(20);
    strcpy(lines[nLines], current);
    nLines++;
  }
  free(current);
  return lines;
}

void printLines(char **lines)
{
  int lineI = 0;
  while(lines[lineI] != NULL)
  {
    printf("%s", lines[lineI]);
    lineI++;
  }
}

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