分配内存双指针？ [英] Assigning memory to double pointer?

问题描述

我无法理解如何分配内存
到一个双指针。
我想读一个字符串数组和储存。

I am having trouble understanding how to assign memory to a double pointer. I want to read an array of strings and store it.

 char **ptr;
    fp=fopen("file.txt","r");
    ptr=(char**)malloc(sizeof(char*)*50);
    for(int i=0;i<20;i++)
    {
       ptr[i]=(char*)malloc(sizeof(char)*50);
       fgets(ptr[i],50,fp);
    }

而不是这个，我刚刚分配的内存块大和
存储字符串

instead of this of i just assign a large block of memory and store the string

char **ptr;
  ptr=(char**)malloc(sizeof(char)*50*50);

会是这样错了吗？如果是的话，为什么呢？

would that be wrong? And if so why is it?

推荐答案

您第二个例子是错误的，因为每个存储单元概念不会持有的char * 而是字符。如果你稍微改变你的思维，它可以帮助这一点：

Your second example is wrong because each memory location conceptually would not hold a char* but rather a char. If you slightly change your thinking, it can help with this:

char *x;  // Memory locations pointed to by x contain 'char'
char **y; // Memory locations pointed to by y contain 'char*'

x = (char*)malloc(sizeof(char) * 100);   // 100 'char'
y = (char**)malloc(sizeof(char*) * 100); // 100 'char*'

// below is incorrect:
y = (char**)malloc(sizeof(char) * 50 * 50);
// 2500 'char' not 50 'char*' pointing to 50 'char'

由于这一点，你的第一个循环将在C怎么办字符数组/指针数组。使用固定的内存块字符数组的数组是好的，但你可以使用一个的char * 而非的char ** ，因为你不会有任何指针在内存中，只字符秒。

Because of that, your first loop would be how you do in C an array of character arrays/pointers. Using a fixed block of memory for an array of character arrays is ok, but you would use a single char* rather than a char**, since you would not have any pointers in the memory, just chars.

char *x = calloc(50 * 50, sizeof(char));

for (ii = 0; ii < 50; ++ii) {
    // Note that each string is just an OFFSET into the memory block
    // You must be sensitive to this when using these 'strings'
    char *str = &x[ii * 50];
}

这篇关于分配内存双指针？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！