C 编程：函数内部的 malloc()

Question

我想在函数中分配内存，然后在 main() 中使用该空间。
函数中一切正常，但我永远无法访问 main() 中的数据。
有两条内存分配指令用于模拟二维字符数组

这是我的代码：

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

int getNetworks(char **networks) {

  networks = malloc(5 * sizeof(char*));
  printf("networks in function:%p\n", networks);

  networks[0] = (char*) malloc(4);
  strcpy(networks[0], "aaa");
  networks[1] = (char*) malloc(3);
  strcpy(networks[1], "bb");
  networks[2] = (char*) malloc(5);
  strcpy(networks[2], "cccc");
  networks[3] = (char*) malloc(6);
  strcpy(networks[3], "ddddd");
  networks[4] = (char*) malloc(2);
  strcpy(networks[4], "e");
  return 5;
}

int main ()
{
  char **networks = NULL;
  int nbNetworks;

  printf("networks before call:%p\n", networks);
  nbNetworks = getNetworks(&*networks);
  printf("networks after call:%p\n", networks);
  for (int i=0; i<=nbNetworks-1; i++) {
    printf ("%s\n", networks[i]);
  }
  return 0;
}

输出是

networks before call:0x0
networks in function:0x7feb65c02af0
networks after call:0x0
Segmentation fault: 11

Answer 1

以下需要修复：

int getNetworks(char ***networks)

因为它是指向字符指针数组的指针。 （您也可以写成
char **networks[]）。

在函数中，您必须首先取消引用指针才能处理父变量：

    *networks = malloc(5 * sizeof(char*));
    (*networks)[0] = (char*) malloc(4);

在 main 中，您的声明是正确的，但是，您必须调用为：

    getNetworks(&networks);

所以函数可以作用于父变量。

Answer 2

你的代码有很多问题，我就不评论了，建议你这个解决方案：

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

char** getNetworks(int *n)
{
  int count = 5;
  char ** networks = malloc(count * sizeof(char*));
  printf("networks in function:%p\n", networks);

  networks[0] = (char*) malloc(4);
  strcpy(networks[0], "aaa");
  networks[1] = (char*) malloc(3);
  strcpy(networks[1], "bb");
  networks[2] = (char*) malloc(5);
  strcpy(networks[2], "cccc");
  networks[3] = (char*) malloc(6);
  strcpy(networks[3], "ddddd");
  networks[4] = (char*) malloc(2);
  strcpy(networks[4], "e");
  *n = count;
  return networks;
}

int main ()
{
  char **networks = NULL;
  int nbNetworks;

  printf("networks before call:%p\n", networks);
  networks = getNetworks(&nbNetworks);
  printf("networks after call:%p\n", networks);
  for (int i=0; i < nbNetworks; i++) {
    printf ("%s\n", networks[i]);
  }
  return 0;
}