Linux下coredump调试2：实例

前面文章只是给出简单演示，实际的程序运行中会遇到这样或那样的问题。所以，本文结合笔者实际编程经历，给出一些曾经遇到过的实际例子。

笔者遇到的大多数程序崩溃原因，基本上都是段错误：非法内存使用，越界。这就要在程序编码中注意代码的质量了。比如使用指针前必须先判断其合法性，释放指针后及时将指针置为NULL，使用数组注意不能超出其范围，等等。

指针非法

下面的例子是笔者前段时间进行的onvif程序的片段。调试过程如下：

GNU gdb (Ubuntu 7.7-0ubuntu3.1) 7.7
Copyright (C) 2014 Free Software Foundation, Inc.
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Type "show configuration" for configuration details.
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Reading symbols from a.out...done.
[New LWP 22255]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
Core was generated by `./a.out'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x083e95b2 in WsddProxyImpl::discoverDevices (this=0xa617628, ip=0x0) at src/onvifwsddProxyImpl.cpp:131
131 matchDevice.Scopes = resp.wsdd__ProbeMatches->ProbeMatch->Scopes->__item;
(gdb) bt
#0 0x083e95b2 in WsddProxyImpl::discoverDevices (this=0xa617628, ip=0x0) at src/onvifwsddProxyImpl.cpp:131
#1 0x083b3d41 in OnvifClient::test (this=0xa5eb008, ip=0x0) at src/onvifClient.cpp:65
#2 0x083b3a0f in main_cpp (argc=1, argv=0xbfcc7ec4) at main.cpp:27
#3 0x083b39c8 in main (argc=1, argv=0xbfcc7ec4) at main.cpp:17

从gdb信息中看到出现问题地方为resp.wsdd__ProbeMatches->ProbeMatch->Scopes->__item，这么多级的指针，可能是中间某个指针非法，所以应该在代码中逐级判断。
实际原因：resp.wsdd__ProbeMatches->ProbeMatch->Scopes指针为空。

注：笔者在实际工作中就遇到一个RTSP模块使用多级指针但不做判断的情况，由于接手的程序庞大又不熟悉架构，而且还是在特定方案中出现，所以排查起来很麻烦，所幸用coredump还是能定位到问题所在。

vector使用方式

下面的例子同样是onvif程序，是vector使用方式不恰当导致。

Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x083deb46 in DeviceBindingProxyImpl::getServices (this=0xa383ff8, services=...) at src/onvifDeviceBindingProxyImpl.cpp:68
68 services_[i].VersionMajor = resp.Service[i]->Version->Major;
(gdb) bt
#0 0x083deb46 in DeviceBindingProxyImpl::getServices (this=0xa383ff8, services=...) at src/onvifDeviceBindingProxyImpl.cpp:68
#1 0x083af56a in OnvifClient::test (this=0xa33a008, ip=0xbf88d91e "172.18.45.16") at onvifClient.cpp:101
#2 0x083af024 in main_cpp (argc=2, argv=0xbf88bdd4) at main.cpp:24
#3 0x083aefc4 in main (argc=2, argv=0xbf88bdd4) at main.cpp:10

代码片段：
for (unsigned int i = 0; i < resp.Service.size(); i++)
{
odt__Service tmp;
tmp.Namespace = resp.Service[i]->Namespace; // 先给临时变量存储
tmp.XAddr = resp.Service[i]->XAddr; // 先给临时变量存储
if (resp.Service[i]->Version)
{

// 此处出现错误，在vector没有确定容器大小时，不能这样搞。。。
services_[i].VersionMajor = resp.Service[i]->Version->Major;
services_[i].VersionMinor = resp.Service[i]->Version->Minor;
}
services_.push_back(tmp); // push_back，vector会自动增长
}

注：std::vector为空情况下，不能直接用[i].XXX的方法来赋值，可以调用resize()预先设置大小。不过最好的是使用push_back。

后续可能也许会不定时更新本文。

李迟 2016.5.31 周二晚