如何增加函数参数指向的块的大小？

发布于 2024-10-19 20:29:21 字数 430 浏览 0 评论 0原文

void foo(char *p)  
{  
    int i;
    int len = strlen(p);
    p = malloc(sizeof(char)*len+2);
    p[0] = '1';
    for(i=1; i<len+1; i++)
        p[i] = '0';
    p[i] = '\0';
}  

int main()  
{
    char p[2] = "1";

    foo(p);
    printf("%s\n", p);  // "10" expected

    return 0;
}

我意识到当我在foo中调用malloc时，p的值已经改变，所以数组p在主要不会有影响。但我不知道如何纠正它。

原文

void foo(char *p)  
{  
    int i;
    int len = strlen(p);
    p = malloc(sizeof(char)*len+2);
    p[0] = '1';
    for(i=1; i<len+1; i++)
        p[i] = '0';
    p[i] = '\0';
}  

int main()  
{
    char p[2] = "1";

    foo(p);
    printf("%s\n", p);  // "10" expected

    return 0;
}

I realized that when I call malloc in foo, p's value has been changed, so array p in main will not be influence. But I don't know how to correct it.

分享到QQ

分享到微博

如果你对这篇内容有疑问，欢迎到本站社区发帖提问参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。

发布评论

需要登录才能够评论，你可以免费注册一个本站的账号。

恋竹姑娘 2024-10-26 20:29:21

您的代码有一些错误：

在 main 中，您将 p 声明为驻留在堆栈上的数组。堆栈上的内容以后无法调整大小。
然后在 foo 中，您希望将 p 更改为指向堆中的内存，而不是指向您声明的数组。

可以通过首先使用 malloc 分配 p，然后使用 realloc 再次重新分配该内存来完成：

void foo (char **p) {
  *p = realloc(*p, 4);
}

int main (void) {
  char *p = malloc(2);
  foo(&p);
  ...
}

您想要实现的目标请注意， foo 是一个双指针。这是因为 realloc 不仅可以调整内存块的大小，而且实际上还可以移动内存块，因此它会影响指针的值。

另请注意，realloc 的第二个参数不是要递增的大小，而是块的当前大小加上要递增的大小。

You have a few wrong things with your code:

In main, you declare p as an array that resides on the stack. Things that are on the stack cannot later be resized.
Then in foo you want to change p to point to memory from the heap rather than to the array you have declared.

What you want to achieve can be done by initially allocating p with malloc, and then reallocating that memory again with realloc:

void foo (char **p) {
  *p = realloc(*p, 4);
}

int main (void) {
  char *p = malloc(2);
  foo(&p);
  ...
}

Notice that the argument of foo is a double pointer. That's because realloc may not only resize the memory block, but it may actually move it as well, so it will affect the value of the pointer.

Also note that the second argument of realloc is not the size you want to increment with, but rather the current size of the block plus the size to increment with.

回复收藏 0 原文