引子

我一直以来都很喜欢C语言——纯粹的C语言，不加任何C++。这是一种纯粹的开发过程，我是内存的巫师。

我在YouTube关注了一个开发者，叫做Tsoding。他用C语言实现了很多有意思的东西——音乐可视化器、BPE算法等等。

然而，许久不用，我已经快忘记了如何进行基本的内存管理，这篇文章就来记叙一下我写Side Project过程中遇到的一些问题。

这次我的Side Project是动态表(Dynamic Table, Dynamic Array)，这是《算法导论》一书中所遇到的第一个数据结构（或许）。

嵌套结构体的malloc

嵌套结构体需要一层一层分配空间，比如：

typedef struct {
    int actual_value;
    char description;
} Node;

typedef struct {
    Node* node;
    int node_size;
} NodeList;

这个时候，不仅要给NodeList分配空间，也要给Node分配空间，即：

NodeList* nl = (NodeList*)malloc(sizeof(NodeList));
Node* nd = (Node*)malloc(sizeof(Node));
nl->node = nd;

否则就会造成段错误，比如下面的代码：

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

typedef struct {
        int a;
        char d;
} Node;

typedef struct {
        Node* nd;
        int size;
} NodeList;

int main(int argc, char** argv) {
        NodeList* nl = (NodeList*)malloc(sizeof(NodeList));
        nl->size = 1;
        nl->nd->a = 2; // ERROR
        return 0;
}

因为，nl->nd并没有被开辟空间，访问这个地址是错误的。下面的代码就是正确的：

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

typedef struct {
        int a;
        char d;
} Node;

typedef struct {
        Node* nd;
        int size;
} NodeList;

int main(int argc, char** argv) {
        NodeList* nl = (NodeList*)malloc(sizeof(NodeList));
        nl->size = 1;
        Node* nd = (Node*)malloc(sizeof(Node));
        nl->nd = nd;
        nl->nd->a = 2;
        return 0;
}

malloc必须对应free

free()函数是用来销毁malloc()申请的空间的，二者是一个对应关系，哪里有malloc，在它后面就必须有free，否则会造成内存泄露。

比如下面的代码：

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

int main(int argc, char** argv) {
        int* a = (int*)malloc(sizeof(int));
        return 0;
}

使用valgrind进行检查：

gcc test.c -o test; valgrind ./test

输出如下：

==30767== Memcheck, a memory error detector
==30767== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==30767== Using Valgrind-3.25.1 and LibVEX; rerun with -h for copyright info
==30767== Command: ./a.out
==30767==
==30767==
==30767== HEAP SUMMARY:
==30767==     in use at exit: 4 bytes in 1 blocks
==30767==   total heap usage: 1 allocs, 0 frees, 4 bytes allocated
==30767==
==30767== LEAK SUMMARY:
==30767==    definitely lost: 4 bytes in 1 blocks
==30767==    indirectly lost: 0 bytes in 0 blocks
==30767==      possibly lost: 0 bytes in 0 blocks
==30767==    still reachable: 0 bytes in 0 blocks
==30767==         suppressed: 0 bytes in 0 blocks
==30767== Rerun with --leak-check=full to see details of leaked memory
==30767==
==30767== For lists of detected and suppressed errors, rerun with: -s
==30767== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

这里的definitely lost的内存，就是我们malloc但是没有free掉的内存。我们如果将代码改为：

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

int main(int argc, char** argv) {
        int* a = (int*)malloc(sizeof(int));
        free(a);
        return 0;
}

再去检查，输出为：

==31045== Memcheck, a memory error detector
==31045== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==31045== Using Valgrind-3.25.1 and LibVEX; rerun with -h for copyright info
==31045== Command: ./a.out
==31045==
==31045==
==31045== HEAP SUMMARY:
==31045==     in use at exit: 0 bytes in 0 blocks
==31045==   total heap usage: 1 allocs, 1 frees, 4 bytes allocated
==31045==
==31045== All heap blocks were freed -- no leaks are possible
==31045==
==31045== For lists of detected and suppressed errors, rerun with: -s
==31045== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

问题解决。

这表示，申请开辟的空间必须要去销毁。

更进一步，如果结构体出现了嵌套情况，则必须一层一层进行free。