什么时候应该使用malloc C和没有什么时候？ [英] When should I use malloc in C and when don't I?

问题描述

我理解的malloc（）是如何工作的。我的问题是，我会看到这样的事情：

 的#define A_MEGABYTE（1024 * 1024）字符* some_memory;
为size_t size_to_allocate = A_MEGABYTE;
some_memory =（字符*）malloc的（size_to_allocate）;
sprintf的（some_memory，的Hello World）;
的printf（％S \\ n，some_memory）;
免费（some_memory）;
 

我省略错误检查简便起见。我的问题是，你不能只是做上述通过初始化一个指向内存中的一些静态存储？或许是：

 的char * some_memory =Hello World的;
 

在什么时候你真的需要分配自己的内存而不是声明/初始化你需要保留价值？

解决方案

 的char * some_memory =Hello World的;
 

是创建一个指向字符串常量。这意味着字符串Hello World将在内存的只读部分地方，你只是有一个指针。您可以使用该字符串为只读。您不能进行更改。例如：

  some_memory [0] ='H';
 

是自寻烦恼。

在另一方面

  some_memory =（字符*）malloc的（size_to_allocate）;
 

被分配一个字符数组（变量）和some_memory指向分配的存储器。现在，这个数组是读写。现在你可以这样做：

  some_memory [0] ='H';
 

和数组内容更改为Hello World

I understand how malloc() works. My question is, I'll see things like this:

#define A_MEGABYTE (1024 * 1024)

char *some_memory;
size_t size_to_allocate = A_MEGABYTE;
some_memory = (char *)malloc(size_to_allocate);
sprintf(some_memory, "Hello World");
printf("%s\n", some_memory);
free(some_memory);

I omitted error checking for the sake of brevity. My question is, can't you just do the above by initializing a pointer to some static storage in memory? perhaps:

char *some_memory = "Hello World";

At what point do you actually need to allocate the memory yourself instead of declaring/initializing the values you need to retain?

解决方案

char *some_memory = "Hello World";

is creating a pointer to a string constant. That means the string "Hello World" will be somewhere in the read-only part of the memory and you just have a pointer to it. You can use the string as read-only. You cannot make changes to it. Example:

some_memory[0] = 'h';

Is asking for trouble.

On the other hand

some_memory = (char *)malloc(size_to_allocate);

is allocating a char array ( a variable) and some_memory points to that allocated memory. Now this array is both read and write. You can now do:

some_memory[0] = 'h';

and the array contents change to "hello World"

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