内存泄漏是coding中经常容易出现的问题， 而且很难查。 本文中总结了几个常见的内存泄漏问题， 分别举例实现， 并列出用代码分析工具——valgrind中memcheck检查的结果， 一 一对错误进行排查。
本文围绕工程valgrind-sample进行讲解。 先看下工程结构：
methods类写了几个可能存在内存操作问题的函数， main.cpp调用methods类函数：
methods.h:
#ifndef VALGRIND_METHODS_H
#define VALGRIND_METHODS_H
namespace sample{
void printx();
void access_violation();
void mem_overlap();
void nonfree();
}
#endif //VALGRIND_METHODS_Hmain.cpp:
#include <iostream>
#include "methods.h"
#include <string.h>
int main(int argc, char *argv[]){
sample::printx();
sample::access_violation();
sample::mem_overlap();
sample::nonfree();
} 下面分别看这几个函数可能会遇到什么问题。
1. sample::printx()
void printx(){
int x;
if (x == 0)
{
printf("X is zero");
}
} 问题显而易见， 变量赋值前引用， 但C编译器并不会报错。
valgrind ./valgrind-sample:
==17495== Conditional jump or move depends on uninitialised value(s)
==17495== at 0x400D10: sample::printx() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==17495== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
2. sample::access_violation()
void access_violation(){
int len = 5;
int *pt = (int*)malloc(len*sizeof(int)); //problem1: not freed
int *p = pt;
for (int i = 0; i < len; i++){
p++;
}
*p = 5; //problem2: heap block overrun
printf("%d\n", *p); //problem3: heap block overrun
} 问题如code所示，
line3: 指针pt申请了空间，但是没有释放;
line8: pt申请了5个int的空间，p经过4次循环（i=3时）已达到最后申请的p[4], 在i=4时p所指向的空间没有申请过; (下面valgrind报告中 Invalid write of size 4)
line9: 同line8 (下面valgrind报告中 Invalid read of size 4 )
valgrind ./valgrind-sample:
==21058== Invalid write of size 4
==21058== at 0x400D74: sample::access_violation() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==21058== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==21058== Address 0x4b32054 is 0 bytes after a block of size 20 alloc’d
==21058== at 0x490514E: malloc (vg_replace_malloc.c:195)
==21058== by 0x400D44: sample::access_violation() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==21058== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==21058==
==21058== Invalid read of size 4
==21058== at 0x400D7E: sample::access_violation() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==21058== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==21058== Address 0x4b32054 is 0 bytes after a block of size 20 alloc’d
==21058== at 0x490514E: malloc (vg_replace_malloc.c:195)
==21058== by 0x400D44: sample::access_violation() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==21058== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==21058==
3. sample::mem_overlap()
void mem_overlap(){
char str[11];
for (int i = 0; i < 11; i++){
str[i] = i;
}
memcpy(str + 1, str, 5);
char x[5] = "abcd";
strncpy(x + 2, x, 3);
} 问题出在memcpy上， 将str指针位置开始copy 5个char到str+1所指空间，会造成内存覆盖。strncpy也是同理。
valgrind ./valgrind-sample:
==27473== Source and destination overlap in memcpy(0x7feffedc1, 0x7feffedc0, 5)
==27473== at 0x4907566: memcpy (mc_replace_strmem.c:482)
==27473== by 0x400DD1: sample::mem_overlap() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==27473== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==27473==
==27473== Source and destination overlap in strncpy(0x7feffeda5, 0x7feffeda3, 3)
==27473== at 0x490737B: strncpy (mc_replace_strmem.c:329)
==27473== by 0x400DFA: sample::mem_overlap() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==27473== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
4. sample::nonfree()
void nonfree(){
char* str = (char*)malloc(5*sizeof(char));
char* ptr = str;
delete [] ptr; //problem<mismatch>: delete - new; malloc - free
free(str); //problem<invalid>: release freed memory
ptr[1] = 'a'; //problem<invalid>: use released memory
} 问题如code所示。
line 4: 用malloc申请空间的指针用free释放；用new申请的空间用delete释放 (valgrind中Mismatched free() / delete / delete [])；
line 5: 由于ptr=str, ptr已被释放，str无需再释放，此处释放了已经被释放的内存（valgrind中 Invalid free() / delete / delete[]）；
line 6: 用到了已经被释放的内存（valgrind中Invalid write of size 1）；
valgrind ./valgrind-sample:
==29210== Mismatched free() / delete / delete []
==29210== at 0x4906510: operator delete (vg_replace_malloc.c:368)
==29210== by 0x400E2B: sample::nonfree() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210== Address 0x4b32040 is 0 bytes inside a block of size 5 alloc’d
==29210== at 0x490514E: malloc (vg_replace_malloc.c:195)
==29210== by 0x400E0F: sample::nonfree() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210==
==29210== Invalid free() / delete / delete[]
==29210== at 0x4905E72: free (vg_replace_malloc.c:325)
==29210== by 0x400E34: sample::nonfree() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210== Address 0x4b32040 is 0 bytes inside a block of size 5 free’d
==29210== at 0x4906510: operator delete (vg_replace_malloc.c:368)
==29210== by 0x400E2B: sample::nonfree() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210==
==29210== Invalid write of size 1
==29210== at 0x400E3C: sample::nonfree() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210== Address 0x4b32041 is 1 bytes inside a block of size 5 free’d
==29210== at 0x4906510: operator delete (vg_replace_malloc.c:368)
==29210== by 0x400E2B: sample::nonfree() (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
==29210== by 0x400B4B: main (in /home/zhangruiqing01/disk2/study/code/valgrind-samples/valgrind-samples)
最后，整个methods.cpp文件如下：
methods.cpp:
#include <iostream>
#include <stdio.h>
#include <string.h>
#include "methods.h"
namespace sample{
void printx(){
int x;
if (x == 0)
{
printf("X is zero");
}
}
void access_violation(){
int len = 5;
int *pt = (int*)malloc(len*sizeof(int)); //problem1: not freed
int *p = pt;
for (int i = 0; i < len; i++){
p++;
}
*p = 5; //problem2: heap block overrun
printf("%d\n", *p); //problem3: heap block overrun
}
void mem_overlap(){
char str[11];
for (int i = 0; i < 11; i++){
str[i] = i;
}
memcpy(str + 1, str, 5);
char x[5] = "abcd";
strncpy(x + 2, x, 3);
}
void nonfree(){
char* str = (char*)malloc(5*sizeof(char));
char* ptr = str;
delete [] ptr; //problem<mismatch>: delete - new; malloc - free
free(str); //problem<invalid>: release freed memory
ptr[1] = 'a'; //problem<invalid>: use released memory
}
}