对指针施放结构

Question

现在我正在处理c上的数据包，我已经成功创建了icmp数据包发送，但是我没有办法将它自己放入套接字中，所以我发现了多个应执行类似操作的代码。所以他们都包含我完全不了解的部分，我找不到任何有关它的信息。他们正在铸造结构与数据包到一个字符指针，如下所示：

static char *packet; 
    ip= (struct iphdr*) packet; 
    icmp= (struct icmphdr*)(ip+1); 

当我试过这个时，我得到了分段错误。请向我解释铸造结构的目的，以及为什么同一段代码在我的情况下不起作用。谢谢。

EDITED

struct iphdr *ip; 
struct icmphdr *icmp; 

ip=malloc(sizeof(*ip)); 
icmp=malloc(sizeof(*icmp)); 

int sock; 

uint16_t psize = sizeof(*icmp)+sizeof(*ip); 
static char *packet; 
ip= (struct iphdr*) packet; 
icmp= (struct icmphdr*)(ip+1); 

unsigned int id = (unsigned int)rand(); 
unsigned int seq = (unsigned int)rand(); 

ip->version = 4; 
ip->ihl = 5; 
ip->tos = 0; 
ip->tot_len = htons(psize); 
ip->id = id; 
ip->frag_off = 0; 
ip->ttl = 255; 
ip->protocol = IPPROTO_ICMP; 
ip->saddr = subnet.ipDec; 
icmp->type = 8; 
icmp->code = 0; 
icmp->un.echo.id=id; 
icmp->un.echo.sequence=seq; 
icmp->checksum=0; 

sock=socket(AF_INET, SOCK_RAW, IPPROTO_ICMP); 
struct sockaddr_in destIP; 
destIP.sin_family=AF_INET; 
uint32_t destIPDec=0; 
ip->daddr = destIPDec; 
destIP.sin_addr.s_addr=destIPDec; 
sendto(sock, packet, psize, 0, (struct sockaddr*) &destIP, sizeof(destIP)); 

Answer 1

如果你想创建一个packet你必须一个块内存分配给它，而不是'子结构'。你有你有

ip=malloc(sizeof(*ip)); 
icmp=malloc(sizeof(*icmp)); 

int sock; 

uint16_t psize = sizeof(*icmp)+sizeof(*ip); 
static char *packet; 
ip= (struct iphdr*) packet;  // here you access undefined pointer packet; previous contents of ip is lost 
icmp= (struct icmphdr*)(ip+1); // same for icmp 

删除TE malloc() S和它更改为

---- 
static char *packet; 

packet = malloc(sizeof *ip + sizeof *icmp); 
ip= (struct iphdr*) packet; 
icmp= (struct icmphdr*)(ip+1); 

Answer 2

。在你的代码基本错误。

在这里，您分配内存两个变量

ip=malloc(sizeof(*ip)); 
icmp=malloc(sizeof(*icmp)); 

但你不久后

static char *packet; 
ip= (struct iphdr*) packet; 
icmp= (struct icmphdr*)(ip+1); 

所以ip和icmp不再指向分配的内存。相反，您已将它们分配给未初始化的变量的值，即packet。这是未定义的行为，并解释了seg故障。

Answer 3

，当你到ip和icmp分配值的问题发生。我推荐以下代码：

struct packet { 
    struct iphdr ip; 
    struct icmphdr icmp; 
} myPacket; 

myPacket = (struct packet*)malloc(sizeof(struct packet)); 

ip =(struct iphdr*) &packet.ip; 
icmp = (struct icmphdr*) &packet.icmp;