再次调用相同源的strncpy（）失败 [英] strncpy() fails on second call for same source

问题描述

我是c的新手，想将字符串分为两部分。这是我的代码：

I'm new with c and want to separate string in two parts. Here is my code:

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

void test(char** a, char** b)
{
  const char * c = "abcdef";

  *a = (char *)malloc(4* sizeof(char));
  *b = (char *)malloc(4* sizeof(char));


  strncpy(*a, c, 3);

  *a[3] = '\0';

  fprintf(stderr, "a -> %s\n", *a);



  strncpy(*b, c+3, 3);

  *b[3] = '\0';

  fprintf(stderr, "b -> %s\n", *b);

}

int main()
{
setvbuf (stderr, NULL, _IONBF, 0);
char *a = NULL;
char *b  = NULL;

test(&a, &b);

fprintf(stderr, "a -> %s\n", *a);
fprintf(stderr, "b -> %s\n", *b);
}

我想拥有 abc a 和变量 b 中的 def 。但是我的问题是它失败并出现 Segmentation Fault 。运行此命令后，将得到以下输出：

I want to have abc on a variable and def in variable b. But my problem is that it fails with Segmentation Fault. After I run this I get this output:

a -> abc
Segmentation fault

我不明白为什么。我正在使用 cygwin 并使用命令

I can't understand why. I'm using cygwin and build it with command

gcc test.cpp -o进行构建test.exe

对不起，如果问题听起来很傻。谢谢。

Sorry if question sounds silly. Thank you.

推荐答案

array-subscript-operator [] 有

The array-subscript-operator [] has higher precedence then the dereferencing operator *.

所以您要更改

*a[3] = ...

成为

(*a)[3] = ...

与 b 相同。

已将编译器的警告级别设置得足够高，它应该已经对此发出警告。或至少告诉过您，它们是带有

Having set the compiler's warning level high enough, it should have warned you about this. Or at least told you that their is something fishy with

*a[3] = '\0';

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