安装valgrind¶

$ sudo apt-get install valgrind

使用valgrind¶

最典型的例子莫过于写一个简单的链表：

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

typedef struct Node
{   
    int data;
    struct Node * next;
}  Node;

Node *head = NULL;

//每次插入在链表的首部
void add(int num)
{
    Node *t = malloc(sizeof(Node));
    t -> data = num;
    t -> next = head;
    head = t;
}

void printList(Node *root)
{
    Node *t = root;
    while (t) {
        printf("%d ", t -> data);
        t = t -> next;
    }
    printf("\n");
}

int main()
{
    for (int i = 1; i < 11; ++i) {
        add(i);
    }
    printList(head);

    return 0;
}

注意到我们使用malloc动态分配了内存空间，但是却没有收回。如果用gcc编译发现是可以正常运行：

$ gcc -Werror -Wall -Wextra -O2 -std=c99 link.c -g -o link
$ ./link
10 9 8 7 6 5 4 3 2 1

但是没有报错信息并不意味着我们写的程序没有问题，很显然上面的程序存在内存泄漏。那这时候就需要用valgrind来进行检查。

valgrind的指令是：

$ valgrind --tool=memcheck --leak-check=full ./link
==261== Memcheck, a memory error detector
==261== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==261== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==261== Command: ./link
==261==
==261== error calling PR_SET_PTRACER, vgdb might block
10 9 8 7 6 5 4 3 2 1
==261==
==261== HEAP SUMMARY:
==261==     in use at exit: 160 bytes in 10 blocks
==261==   total heap usage: 11 allocs, 1 frees, 1,184 bytes allocated
==261==
==261== LEAK SUMMARY:
==261==    definitely lost: 0 bytes in 0 blocks
==261==    indirectly lost: 0 bytes in 0 blocks
==261==      possibly lost: 0 bytes in 0 blocks
==261==    still reachable: 160 bytes in 10 blocks
==261==         suppressed: 0 bytes in 0 blocks
==261== Reachable blocks (those to which a pointer was found) are not shown.
==261== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==261==
==261== For counts of detected and suppressed errors, rerun with: -v
==261== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

写的程序一共分配了10个节点，在结果里面有一句：

==261==     in use at exit: 160 bytes in 10 blocks
==261==   total heap usage: 11 allocs, 1 frees, 1,184 bytes allocated

代表程序结束后仍然存在于10blocks里，有10个allocs没有被free。

于是修改程序：

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

typedef struct Node
{   
    int data;
    struct Node * next;
}  Node;

Node *head = NULL;

//每次插入在链表的首部
void add(int num)
{
    Node *t = malloc(sizeof(Node));
    t -> data = num;
    t -> next = head;
    head = t;
}

void printList(Node *root)
{
    Node *t = root;
    while (t) {
        printf("%d ", t -> data);
        t = t -> next;
    }
    printf("\n");
}

void makeEmpty()
{
    while (head) {
        Node *tmp = head;
        head = head -> next;
        free(tmp);
        tmp = NULL;
    }
}

int main()
{
    for (int i = 1; i < 11; ++i) {
        add(i);
    }
    printList(head);
    makeEmpty();

    return 0;
}

然后再次运行valgrind检查：

$ gcc -Werror -Wall -Wextra -O2 -std=c99 link.c -g -o link
$ ./link
10 9 8 7 6 5 4 3 2 1
$ valgrind --tool=memcheck --leak-check=full ./link
==223== Memcheck, a memory error detector                                 
==223== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.   
==223== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==223== Command: ./link                                                   
==223==                                                                   
==223== error calling PR_SET_PTRACER, vgdb might block                    
10 9 8 7 6 5 4 3 2 1                                                      
==223==                                                                   
==223== HEAP SUMMARY:                                                     
==223==     in use at exit: 0 bytes in 0 blocks                           
==223==   total heap usage: 11 allocs, 11 frees, 1,184 bytes allocated    
==223==                                                                   
==223== All heap blocks were freed -- no leaks are possible               
==223==                                                                   
==223== For counts of detected and suppressed errors, rerun with: -v      
==223== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

加上makeEmpty()函数后检查发现11 allocs, 11 frees，不存在内存泄漏了。

==223==     in use at exit: 0 bytes in 0 blocks                           
==223==   total heap usage: 11 allocs, 11 frees, 1,184 bytes allocated