c - 从函数返回二维数组

Question

我有一个加载数据并将它们放入数组的函数

char array_of_string() {
    char **array;
    size_t size = 5;
    size_t index = 0;
    size_t length = 0;
    int rd;
    char *line;
    array = (char **) malloc(size * sizeof(char*));
    while ((rd = getline(&line, &length, 0) != -1) {
      array[index] = malloc(strlen(line) + 1 * sizeof(char));
      strncpy(array[index], line);
      index++    
    }

   free(line);
   return array;
}

int main() { 
  char **here;
  here = array_of_strings();
}

但这不起作用，这是尝试的最佳结果，我怎样才能使它起作用？

Answer 1

正如评论中提到的，您有几个问题需要纠正。从“为什么 getline 不起作用？”开始getline 采用 FILE * 流指针而不是文件描述符作为其第三个参数。使用 stdin 而不是 0 将纠正该问题。剩下的问题涉及分配。这些已在评论中讨论，更正的示例如下所示。

您将遇到的最大问题是“我如何知道实际读取了多少行？”您已经硬编码了“5”，但是如果您读取的内容少于 5 怎么办？

要回答这个问题，您至少需要将指针作为参数传递给 array_of_string 函数，并使用 index 值更新它的值，以便读取的行数可在调用函数中获得(在本例中为 main())。这样，您始终知道读取的行数。例如:

char **array_of_string (size_t *n)
{
    ...
    free (line);

    *n = index;  /* update n with index, so available in main() */

    return array;
}

您的下一个挑战是“如何检查以确保我不会读取并尝试存储超过 5 行？”然后，“如果我要阅读的内容超过 5 行，我该怎么办？”、“然后呢？”。此时，您可以从读取循环中中断，或者更好的是，使用realloc 函数重新分配array 以增加可用指针的数量。 (您可以根据需要添加任意数量，但标准方法是将您当前拥有的指针数量加倍 - 该方法如下所示)。

#define MAXLN 5
...
char **array_of_string (size_t *n)
{
    ...
    size_t maxln = MAXLN;
    ...
    while ((rd = getline(&line, &length, stdin)) != -1)
    {
        ...
        array[index++] = strdup(line); /* alloc & copy  */

        if (index == maxln) {   /* check limit reached  - reallocate */
            char **tmp = realloc (array, maxln * sizeof *array * 2);
            if (!tmp) {
                fprintf (stderr, "error: realloc - memory exhausted.\n");
                *n = index;
                return array;
            }
            array = tmp;  /* set new pointers to NULL */
            memset (array + maxln, 0, maxln * sizeof *array);
            maxln *= 2;
        }
    }
    ...
}

将所有部分放在一起，您可以执行类似以下操作，它最初分配一些合理预期数量的指针来保存您期望读取的行数。如果在读取完成之前达到该限制，您只需重新分配当前2X的指针数量。 (如果 realloc 失败，则使用 tmp 指针进行 realloc 来防止 array 中的所有现有数据丢失。如果realloc 成功，您只需分配 array = tmp;)

请查看以下内容，如果您有任何疑问，请告诉我:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXLN 5

char **array_of_string (size_t *n);

int main (void) {

    char **here = NULL;
    size_t i, lines = 0;

    /* assign array_of_string to here, check return */
    if (!(here = array_of_string (&lines))) {
        fprintf (stderr, "error: array_of_string return NULL.\n");
        return 1;
    }

    for (i = 0; i < lines; i++) /* print lines */
        printf (" line[%3zu] : %s\n", i, here[i]);

    for (i = 0; i < lines; i++) /* free lines */
        if (here[i]) free (here[i]);
    free (here);  /* free here */

    return 0;
}

char **array_of_string (size_t *n)
{
    char **array = NULL;
    char *line = NULL;
    size_t index = 0, length = 0, maxln = MAXLN;
    ssize_t rd = 0;

    if (!(array = calloc (MAXLN, sizeof *array))) {
        fprintf (stderr, "error: virtual memory exhausted.\n");
        return NULL;
    }

    while ((rd = getline(&line, &length, stdin)) != -1)
    {
        while (rd > 0 && (line[rd-1] == '\r' || line[rd-1] == '\n'))
            line[--rd] = 0; /* strip carriage return or newline   */

        array[index++] = strdup(line); /* alloc & copy  */

        if (index == maxln) {   /* check limit reached  - reallocate */
            char **tmp = realloc (array, maxln * sizeof *array * 2);
            if (!tmp) {
                fprintf (stderr, "error: realloc - memory exhausted.\n");
                *n = index;
                return array;
            }
            array = tmp;  /* set new pointers to NULL */
            memset (array + maxln, 0, maxln * sizeof *array);
            maxln *= 2;
        }
    }
    free(line);  /* free memory allocated by getline */

    *n = index;  /* update n with index, so available in main() */

    return array;
}

编译 - 启用警告

gcc -Wall -Wextra -o bin/stdintst stdintst.c

(或为您的编译器使用类似的选项)

输入文件

cat ../dat/damages.txt
Personal injury damage awards are unliquidated
and are not capable of certain measurement; thus, the
jury has broad discretion in assessing the amount of
damages in a personal injury case. Yet, at the same
time, a factual sufficiency review insures that the
evidence supports the jury's award; and, although
difficult, the law requires appellate courts to conduct
factual sufficiency reviews on damage awards in
personal injury cases. Thus, while a jury has latitude in
assessing intangible damages in personal injury cases,
a jury's damage award does not escape the scrutiny of
appellate review.

Because Texas law applies no physical manifestation
rule to restrict wrongful death recoveries, a
trial court in a death case is prudent when it chooses
to submit the issues of mental anguish and loss of
society and companionship. While there is a
presumption of mental anguish for the wrongful death
beneficiary, the Texas Supreme Court has not indicated
that reviewing courts should presume that the mental
anguish is sufficient to support a large award. Testimony
that proves the beneficiary suffered severe mental
anguish or severe grief should be a significant and
sometimes determining factor in a factual sufficiency
analysis of large non-pecuniary damage awards.

使用/输出

$ ./bin/stdintst <../dat/damages.txt
 line[  0] : Personal injury damage awards are unliquidated
 line[  1] : and are not capable of certain measurement; thus, the
 line[  2] : jury has broad discretion in assessing the amount of
 line[  3] : damages in a personal injury case. Yet, at the same
 line[  4] : time, a factual sufficiency review insures that the
 line[  5] : evidence supports the jury's award; and, although
 line[  6] : difficult, the law requires appellate courts to conduct
 line[  7] : factual sufficiency reviews on damage awards in
 line[  8] : personal injury cases. Thus, while a jury has latitude in
 line[  9] : assessing intangible damages in personal injury cases,
 line[ 10] : a jury's damage award does not escape the scrutiny of
 line[ 11] : appellate review.
 line[ 12] :
 line[ 13] : Because Texas law applies no physical manifestation
 line[ 14] : rule to restrict wrongful death recoveries, a
 line[ 15] : trial court in a death case is prudent when it chooses
 line[ 16] : to submit the issues of mental anguish and loss of
 line[ 17] : society and companionship. While there is a
 line[ 18] : presumption of mental anguish for the wrongful death
 line[ 19] : beneficiary, the Texas Supreme Court has not indicated
 line[ 20] : that reviewing courts should presume that the mental
 line[ 21] : anguish is sufficient to support a large award. Testimony
 line[ 22] : that proves the beneficiary suffered severe mental
 line[ 23] : anguish or severe grief should be a significant and
 line[ 24] : sometimes determining factor in a factual sufficiency
 line[ 25] : analysis of large non-pecuniary damage awards.

内存错误检查

在您编写的动态分配内存的任何代码中，对于分配的任何内存块，您都有两个责任:(1)始终保留指向内存块起始地址的指针，因此，(2)可以在以下情况下释放它:不再需要它了。

您必须使用内存错误检查程序来确保您没有在分配的内存块之外进行写入，尝试读取或基于未初始化的值进行跳转，最后确认您已释放所有内存您分配的内存。

对于 Linux，valgrind 是正常选择。有许多微妙的方法可以滥用新的内存块。使用内存错误检查器可以让您识别任何问题并验证您分配的内存的正确使用，而不是通过段错误找出存在的问题。每个平台都有类似的内存检查器。它们使用起来都很简单，只需通过它运行您的程序即可。

$ valgrind ./bin/stdintst <../dat/damages.txt
==12233== Memcheck, a memory error detector
==12233== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==12233== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==12233== Command: ./bin/stdintst
==12233==
 line[  0] : Personal injury damage awards are unliquidated
 line[  1] : and are not capable of certain measurement; thus, the
<snip>
 line[ 24] : sometimes determining factor in a factual sufficiency
 line[ 25] : analysis of large non-pecuniary damage awards.
==12233==
==12233== HEAP SUMMARY:
==12233==     in use at exit: 0 bytes in 0 blocks
==12233==   total heap usage: 31 allocs, 31 frees, 1,989 bytes allocated
==12233==
==12233== All heap blocks were freed -- no leaks are possible
==12233==
==12233== For counts of detected and suppressed errors, rerun with: -v
==12233== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)

始终确认所有堆 block 均已释放 - 不可能发生泄漏并且同样重要错误摘要:0 个上下文中出现 0 个错误。

如果您还有任何其他问题，请告诉我。