使用带map容器的struct结构体指针引发的段错误

 近期在使用结构体指针时遇到段错误，虽然最后解决了，但是有必要在此记录各种坑和解决方案。

1. 结构体指针没有初始化：

#include <map>
#include <iostream>
#include <unistd.h>
#include <vector>


using namespace std;

struct node
{
	int id;
	float length;
	vector<int> passwords;
	struct node *next;
};

int main(int argc, char **argv)
{
    struct node *n;
    
    n->id =1;
    n->length=1.24;
    n->passwords.push_back(1);
    n->passwords.push_back(2);
    n->passwords.push_back(3);
    n->next = NULL;
    
    printf("sizeof(struct node) = %d\n",sizeof(struct node));
    printf("sizeof(int) = %d\n",sizeof(int));
    printf("sizeof(float) = %d\n",sizeof(float));
    printf("sizeof(vector<int>) = %d\n",sizeof(vector<int>));
    printf("sizeof(struct node*) = %d\n",sizeof(struct node*));
    
    printf("n addr = %p\n",n);
    printf("n->id addr = %p\n",&(n->id));
    printf("n->length addr = %p\n",&(n->length));
    printf("n->passwords addr = %p\n",&(n->passwords));
    printf("n->next addr = %p\n",&(n->next));
     
    
    return 0;
}

代码执行后，报段错误，什么输出都灭有!

这个好解决，是因为指针n为随机值，灭有进行初始化。增加n = (struct node *)malloc(sizeof(struct node));即可。

2.  struct中有vector这种结构

#include <map>
#include <iostream>
#include <unistd.h>
#include <vector>


using namespace std;

struct node
{
	int id;
	float length;
	vector<int> passwords;
	struct node *next;
};

int main(int argc, char **argv)
{
    struct node *n = ( struct node *)malloc(sizeof( struct node));
    
    n->id =1;
    n->length=1.24;
    n->passwords.push_back(1);
    n->passwords.push_back(2);
    n->passwords.push_back(3);
    n->next = NULL;
    
    printf("sizeof(struct node) = %d\n",sizeof(struct node));
    printf("sizeof(int) = %d\n",sizeof(int));
    printf("sizeof(float) = %d\n",sizeof(float));
    printf("sizeof(vector<int>) = %d\n",sizeof(vector<int>));
    printf("sizeof(struct node*) = %d\n",sizeof(struct node*));
    
    printf("n addr = %p\n",n);
    printf("n->id addr = %p\n",&(n->id));
    printf("n->length addr = %p\n",&(n->length));
    printf("n->passwords addr = %p\n",&(n->passwords));
    printf("n->next addr = %p\n",&(n->next));
     
    
    return 0;
}

 说明struct中带了vector这种容器时，我们使用malloc对结构体对象指针进行初始化是没有问题的。

更进一步，我们发现sizeof(vector<int>>)的大小为24，这个就有点意思了，实际上跟插入vector的数据量无关。我们打印一下vector的各项地址：

#include <map>
#include <iostream>
#include <unistd.h>
#include <vector>


using namespace std;

struct node
{
	int id;
	float length;
	vector<int> passwords;
	struct node *next;
};

int main(int argc, char **argv)
{
    struct node *n=(struct node*)malloc(sizeof(struct node));
    
    n->id =1;
    n->length=1.24;
    n->passwords.push_back(1);
    n->passwords.push_back(2);
    n->passwords.push_back(3);
    n->passwords.push_back(4);
    n->passwords.push_back(5);
    n->passwords.push_back(6);
    n->passwords.push_back(7);
    n->passwords.push_back(8);
    n->passwords.push_back(9);
    n->next = NULL;
    
    printf("sizeof(struct node) = %d\n",sizeof(struct node));
    printf("sizeof(int) = %d\n",sizeof(int));
    printf("sizeof(float) = %d\n",sizeof(float));
    printf("sizeof(vector<int>) = %d\n",sizeof(vector<int>));
    printf("sizeof(struct node*) = %d\n",sizeof(struct node*));
    
    printf("n addr = %p\n",n);
    printf("n->id addr = %p\n",&(n->id));
    printf("n->length addr = %p\n",&(n->length));
    printf("n->passwords addr = %p\n",&(n->passwords));
    printf("n->next addr = %p\n",&(n->next));
     
    for(int i=0;i<24;i++)
    {
      printf("%02X ",*(((unsigned char *)(&(n->passwords)))+i));
    }
    for(int i=0;i<9;i++)
    {
      printf("n->passwords[%d] addr = %p\n",i,&((n->passwords[i])));
    }
    return 0;
}

这里可以看到struct中给vector<int>申请的24个字节，存储了3个值。经过多次验证，可以确定分别为vector的首地址（第一个元素的地址），下一个插入的地址，和将来vector进行扩展时的地址（当vector的下一个插入地址达到了未来扩展地址，则vector会再申请一段空间来用）。

经过多次验证，确定在struct中使用vector并没有什么问题。

3.  struct中有map这种结构

#include <map>
#include <iostream>
#include <unistd.h>
#include <vector>


using namespace std;

struct node
{
	int id;
	float length;
	vector<int> passwords;
	map<std::string,int> amap;
	struct node *next;
};

int main(int argc, char **argv)
{
    struct node *n=(struct node*)malloc(sizeof(struct node));
    
    n->id =1;
    n->length=1.24;
    n->passwords.push_back(1);
    n->passwords.push_back(2);
    n->passwords.push_back(3);
    n->passwords.push_back(4);
    n->passwords.push_back(5);
    n->passwords.push_back(6);
    n->passwords.push_back(7);
    n->passwords.push_back(8);
    n->passwords.push_back(9);
    n->next = NULL;
    
    printf("sizeof(struct node) = %d\n",sizeof(struct node));
    printf("sizeof(int) = %d\n",sizeof(int));
    printf("sizeof(float) = %d\n",sizeof(float));
    printf("sizeof(vector<int>) = %d\n",sizeof(vector<int>));
    printf("sizeof(map<std::string,int>) = %d\n",sizeof(map<std::string,int>));
    printf("sizeof(struct node*) = %d\n",sizeof(struct node*));
    
    printf("n addr = %p\n",n);
    printf("n->id addr = %p\n",&(n->id));
    printf("n->length addr = %p\n",&(n->length));
    printf("n->passwords addr = %p\n",&(n->passwords));
    printf("n->next addr = %p\n",&(n->next));
    printf("n->amap addr = %p\n",&(n->amap));
     
    for(int i=0;i<24;i++)
    {
      printf("%02X ",*(((unsigned char *)(&(n->passwords)))+i));
    }
    for(int i=0;i<9;i++)
    {
      printf("n->passwords[%d] addr = %p\n",i,&((n->passwords[i])));
    }
    
    n->amap.insert(std::make_pair("123",1));
    printf("insert key/value finished!\n");
    return 0;
}

发现，sizeof(map<std::string,int>)为48字节，估计和vector的情形一致。但是后续的插入失败。我们想着前面我已经为node申请空间了，并且n->amap是有地址的，为什么还无法插入呢？我们把这48个字节打印出来：

#include <map>
#include <iostream>
#include <unistd.h>
#include <vector>


using namespace std;

struct node
{
	int id;
	float length;
	vector<int> passwords;
	map<std::string,int> amap;
	struct node *next;
};

int main(int argc, char **argv)
{
    struct node *n=(struct node*)malloc(sizeof(struct node));
    
    n->id =1;
    n->length=1.24;
    n->passwords.push_back(1);
    n->passwords.push_back(2);
    n->passwords.push_back(3);
    n->passwords.push_back(4);
    n->passwords.push_back(5);
    n->passwords.push_back(6);
    n->passwords.push_back(7);
    n->passwords.push_back(8);
    n->passwords.push_back(9);
    n->next = NULL;
    
    printf("sizeof(struct node) = %d\n",sizeof(struct node));
    printf("sizeof(int) = %d\n",sizeof(int));
    printf("sizeof(float) = %d\n",sizeof(float));
    printf("sizeof(vector<int>) = %d\n",sizeof(vector<int>));
    printf("sizeof(map<std::string,int>) = %d\n",sizeof(map<std::string,int>));
    printf("sizeof(struct node*) = %d\n",sizeof(struct node*));
    
    printf("n addr = %p\n",n);
    printf("n->id addr = %p\n",&(n->id));
    printf("n->length addr = %p\n",&(n->length));
    printf("n->passwords addr = %p\n",&(n->passwords));
    printf("n->next addr = %p\n",&(n->next));
    printf("n->amap addr = %p\n",&(n->amap));
    
    
    
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("*******************\n");
    
    for(int i=0;i<24;i++)
    {
      printf("%02X ",*(((unsigned char *)(&(n->passwords)))+i));
    }
    for(int i=0;i<9;i++)
    {
      printf("n->passwords[%d] addr = %p\n",i,&((n->passwords[i])));
    }
    
    n->amap.insert(std::make_pair("123",1));
    printf("insert key/value finished!\n");
    return 0;
}

发现map所占的48个字节全是0，如果真的是和vector相似，那么后面往0这个地址插入内容，肯定出错。

#include <map>
#include <iostream>
#include <unistd.h>
#include <vector>


using namespace std;

struct node
{
	int id;
	float length;
	vector<int> passwords;
	map<std::string,int> amap;
	struct node *next;
};

int main(int argc, char **argv)
{
    struct node *n=(struct node*)malloc(sizeof(struct node));
    
    n->id =1;
    n->length=1.24;
    n->passwords.push_back(1);
    n->passwords.push_back(2);
    n->passwords.push_back(3);
    n->passwords.push_back(4);
    n->passwords.push_back(5);
    n->passwords.push_back(6);
    n->passwords.push_back(7);
    n->passwords.push_back(8);
    n->passwords.push_back(9);
    n->next = NULL;
    
    printf("sizeof(struct node) = %d\n",sizeof(struct node));
    printf("sizeof(int) = %d\n",sizeof(int));
    printf("sizeof(float) = %d\n",sizeof(float));
    printf("sizeof(vector<int>) = %d\n",sizeof(vector<int>));
    printf("sizeof(map<std::string,int>) = %d\n",sizeof(map<std::string,int>));
    printf("sizeof(struct node*) = %d\n",sizeof(struct node*));
    
    printf("n addr = %p\n",n);
    printf("n->id addr = %p\n",&(n->id));
    printf("n->length addr = %p\n",&(n->length));
    printf("n->passwords addr = %p\n",&(n->passwords));
    printf("n->next addr = %p\n",&(n->next));
    printf("n->amap addr = %p\n",&(n->amap));
    
    
    
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("*******************\n");
    
    for(int i=0;i<24;i++)
    {
      printf("%02X ",*(((unsigned char *)(&(n->passwords)))+i));
    }
    for(int i=0;i<9;i++)
    {
      printf("n->passwords[%d] addr = %p\n",i,&((n->passwords[i])));
    }
    
    std::map<std::string,int> tmpMap;
    
    tmpMap.insert(std::make_pair("123",1));
    
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(tmpMap)))+i));
    }
    printf("----------------------\n");
    
    n->amap = tmpMap;
    
    printf("insert key/value finished!\n");
     
     for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(tmpMap)))+i));
    }
    printf("+++++++++++++++++++++\n");
    
     for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("*******************\n");
    
    printf("n->amap[\"123\"]=%d\n",n->amap["123"]);
    n->amap["234"]=2;
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("*=========================\n");
    printf("n->amap[\"234\"]=%d\n",n->amap["234"]);
    
    n->amap.insert(std::make_pair("345",3));
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("||||||||||||||||||||||\n");
    
    printf("n->amap[\"345\"]=%d\n",n->amap["345"]);
    return 0;
}

通过上述结果，我们发现，只要将n->amap进行了初始化（先将一个临时map赋值，然后将内容给它），后面就可以使用各种方式进行插入。

#include <map>
#include <iostream>
#include <unistd.h>
#include <vector>


using namespace std;

struct node
{
	int id;
	float length;
	vector<int> passwords;
	map<std::string,int> amap;
	struct node *next;
};

int main(int argc, char **argv)
{
    struct node *n=(struct node*)malloc(sizeof(struct node));
    
    n->id =1;
    n->length=1.24;
    n->passwords.push_back(1);
    n->passwords.push_back(2);
    n->passwords.push_back(3);
    n->passwords.push_back(4);
    n->passwords.push_back(5);
    n->passwords.push_back(6);
    n->passwords.push_back(7);
    n->passwords.push_back(8);
    n->passwords.push_back(9);
    n->next = NULL;
    
    printf("sizeof(struct node) = %d\n",sizeof(struct node));
    printf("sizeof(int) = %d\n",sizeof(int));
    printf("sizeof(float) = %d\n",sizeof(float));
    printf("sizeof(vector<int>) = %d\n",sizeof(vector<int>));
    printf("sizeof(map<std::string,int>) = %d\n",sizeof(map<std::string,int>));
    printf("sizeof(struct node*) = %d\n",sizeof(struct node*));
    
    printf("n addr = %p\n",n);
    printf("n->id addr = %p\n",&(n->id));
    printf("n->length addr = %p\n",&(n->length));
    printf("n->passwords addr = %p\n",&(n->passwords));
    printf("n->next addr = %p\n",&(n->next));
    printf("n->amap addr = %p\n",&(n->amap));
    
    
    
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("*******************\n");
    
    for(int i=0;i<24;i++)
    {
      printf("%02X ",*(((unsigned char *)(&(n->passwords)))+i));
    }
    for(int i=0;i<9;i++)
    {
      printf("n->passwords[%d] addr = %p\n",i,&((n->passwords[i])));
    }
    
    std::map<std::string,int> tmpMap;
    std::map<std::string,int> *tmpMap2 =(std::map<std::string,int>*)malloc(sizeof(std::map<std::string,int>));
    printf("malloc a map!\n");
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)tmpMap2)+i));
    }
    printf("xxxxxxxxxxxxxxxxxxxx\n");
    tmpMap2->insert(std::make_pair("123",1));
    printf("malloc and insert to map!\n");
    
    tmpMap.insert(std::make_pair("123",1));
    
    /*tmpMap2 = &tmpMap;
     for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)tmpMap2)+i));
    }
    printf("yyyyyyyyyyyyyyyyyyyy\n");
    */
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(tmpMap)))+i));
    }
    printf("----------------------\n");
    
    n->amap = tmpMap;
    
    printf("insert key/value finished!\n");
     
     for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(tmpMap)))+i));
    }
    printf("+++++++++++++++++++++\n");
    
     for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("*******************\n");
    
    printf("n->amap[\"123\"]=%d\n",n->amap["123"]);
    n->amap["234"]=2;
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("*=========================\n");
    printf("n->amap[\"234\"]=%d\n",n->amap["234"]);
    
    n->amap.insert(std::make_pair("345",3));
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("||||||||||||||||||||||\n");
    
    printf("n->amap[\"345\"]=%d\n",n->amap["345"]);
    return 0;
}

上述，我们将map进行了malloc，然后调用insert函数，也是出现段错误。

 这说明无论哪种情况，map的指针即使malloc申请了地址，也无法使用insert函数进行插入。如果把map指针指向一个具体的map，就可以使用。

而对于结构体中的map，我们只能把它当作一个没有初始化的map（或者说，并没有给它开辟空间）。

#include <map>
#include <iostream>
#include <unistd.h>
#include <vector>


using namespace std;

struct node
{
	int id;
	float length;
	vector<int> passwords;
	map<std::string,int> amap;
	struct node *next;
};

int main(int argc, char **argv)
{
    struct node *n=(struct node*)malloc(sizeof(struct node));
    
    n->id =1;
    n->length=1.24;
    n->passwords.push_back(1);
    n->passwords.push_back(2);
    n->passwords.push_back(3);
    n->passwords.push_back(4);
    n->passwords.push_back(5);
    n->passwords.push_back(6);
    n->passwords.push_back(7);
    n->passwords.push_back(8);
    n->passwords.push_back(9);
    n->next = NULL;
    
    printf("sizeof(struct node) = %d\n",sizeof(struct node));
    printf("sizeof(int) = %d\n",sizeof(int));
    printf("sizeof(float) = %d\n",sizeof(float));
    printf("sizeof(vector<int>) = %d\n",sizeof(vector<int>));
    printf("sizeof(map<std::string,int>) = %d\n",sizeof(map<std::string,int>));
    printf("sizeof(struct node*) = %d\n",sizeof(struct node*));
    
    printf("n addr = %p\n",n);
    printf("n->id addr = %p\n",&(n->id));
    printf("n->length addr = %p\n",&(n->length));
    printf("n->passwords addr = %p\n",&(n->passwords));
    printf("n->next addr = %p\n",&(n->next));
    printf("n->amap addr = %p\n",&(n->amap));
    
    
    
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("*******************\n");
    
    for(int i=0;i<24;i++)
    {
      printf("%02X ",*(((unsigned char *)(&(n->passwords)))+i));
    }
    for(int i=0;i<9;i++)
    {
      printf("n->passwords[%d] addr = %p\n",i,&((n->passwords[i])));
    }
    
    std::map<std::string,int> tmpMap;
    std::map<std::string,int> *tmpMap2 =(std::map<std::string,int>*)malloc(sizeof(std::map<std::string,int>));
    printf("malloc a map!\n");
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)tmpMap2)+i));
    }
    printf("xxxxxxxxxxxxxxxxxxxx\n");
   // tmpMap2->insert(std::make_pair("123",1));
    //printf("malloc and insert to map!\n");
    
    tmpMap.insert(std::make_pair("123",1));
    
    tmpMap2 = &tmpMap;
     for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)tmpMap2)+i));
    }
    printf("yyyyyyyyyyyyyyyyyyyy\n");
    
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(tmpMap)))+i));
    }
    printf("----------------------\n");
    
    n->amap = tmpMap;
    
    printf("insert key/value finished!\n");
     
     for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(tmpMap)))+i));
    }
    printf("+++++++++++++++++++++\n");
    
     for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("*******************\n");
    
    printf("n->amap[\"123\"]=%d\n",n->amap["123"]);
    n->amap["234"]=2;
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("*=========================\n");
    printf("n->amap[\"234\"]=%d\n",n->amap["234"]);
    
    n->amap.insert(std::make_pair("345",3));
    for(int i=0;i<48;i++)
    {
     printf("%02X ",*(((unsigned char *)(&(n->amap)))+i));
    }
    printf("||||||||||||||||||||||\n");
    
    printf("n->amap[\"345\"]=%d\n",n->amap["345"]);
    return 0;
}

即使把结构体中的amap改成指针，上述问题同样存在。

结论：struct中如果有map容器，则一定要用一个临时map对struct中的map赋值，然后才可以直接使用struct中的map。否则会出现段错误。具体深层次原因，待解！！！！