Function does not see field of struct inside struct

Question

So, I have to do library sending IPv4 packets (without libraries with prepared headers) and problem is that I can't access field in struct inside struct while using function, though eclipse does not see error, hint does not show that is possible.

Structs:

struct packet{
    struct ipv4hdr * iphdr;
    struct icmphdr * icmphdr;
    char * data;
};
struct ipv4hdr{
    u_int8_t version_length;
    u_int8_t type;
    u_int16_t total_length;
    u_int16_t id;
    u_int16_t frag_off;
    u_int8_t ttl;
    u_int8_t protocol;
    u_int16_t checksum;
    u_int32_t source;
    u_int32_t destination;
};

Pointers to function (that is requirement, both work):

struct packet *(*packetCreate)() = dlsym(lib, "createPacket");
void (*setIP)(struct packet *, u_int8_t, u_int8_t, u_int16_t, char*, u_int8_t, char*) = dlsym(lib, "prepareIP");

Creation of struct packet:

struct packet * createPacket(){
    struct packet * pack= malloc(sizeof(struct packet));
    return pack;
}

Now, calling fields inside main does show posssible fields:

struct packet * pack = packetCreate();
pack->iphdr->(CTRL+space displays all the fields)   //and eclipse automatically corrects all the . to ->

Meanwhile calling created pointer on function does not display fields while using hint and that causes segmentation fault (core dumped):

void prepareIP(struct packet * packet, u_int8_t length, u_int8_t type, u_int16_t id, char* destination, u_int8_t ttl, char *data){

    packet->iphdr->type = type;     //it is not treated by error, though any field causes crash
    //meanwhile hint
    packet->iphdr->(CTRL+space gets)ipv4hdr;

}

Question again is why I can't call specific field inside function that points to the original struct and how to do this inside that function

Didn't do that, switched the code like 1st answer, packetCreate() was (and still is) all the allocating — Myszsoda

UniversE UniversE · Accepted Answer · 2017-11-07T10:58:08

It looks like you forgot to allocate space for struct ipv4hdr * iphdr;.

But maybe this isn't even what you intended. Do you really want iphdr to be a pointer in your struct? It makes mucht more sense, if it was part of the struct, like this:

struct packet{
    struct ipv4hdr iphdr;  // notice that we do not use pointers here!
    struct icmphdr icmphdr; // this looks curious to me - we have either an IP or an ICMP package, why both headers in the same package?
    char * data;
};

Function does not see field of struct inside struct

1 Answers