I'm designing a protocol (in C) to implement the layered OSI network structure, using cnet (http://www.csse.uwa.edu.au/cnet/). I'm getting a SIGSEGV error at runtime, however cnet compiles my source code files itself (I can't compile it through gcc) so I can't easily use any debugging tools such as gdb to find the error.

Here's the structures used, and the code in question:

typedef struct {
    char *data;
} DATA;

typedef struct {
    CnetAddr src_addr;
    CnetAddr dest_addr;
    PACKET_TYPE type;           
    DATA data;
} Packet;

typedef struct {
    int length;         
    int checksum;   
    Packet datagram;
} Frame;


static void keyboard(CnetEvent ev, CnetTimerID timer, CnetData data)
    {
    char line[80];
    int length;

    length = sizeof(line);
    CHECK(CNET_read_keyboard((void *)line, (unsigned int *)&length)); // Reads input from keyboard

    if(length > 1)
        {           /* not just a blank line */
        printf("\tsending %d bytes - \"%s\"\n", length, line);

        application_downto_transport(1, line, &length);
        }
    }

void application_downto_transport(int link, char *msg, int *length)
    {
    transport_downto_network(link, msg, length);
    }

void transport_downto_network(int link, char *msg, int *length)
    {
    Packet *p;
    DATA *d;

    p = (Packet *)malloc(sizeof(Packet));
    d = (DATA *)malloc(sizeof(DATA));

    d->data = msg;
    p->data = *d;

    network_downto_datalink(link, (void *)p, length);
    }

void network_downto_datalink(int link, Packet *p, int *length)
    {
    Frame *f;

    // Encapsulate datagram and checksum into a Frame.
    f = (Frame *)malloc(sizeof(Frame));

    f->checksum = CNET_crc32((unsigned char *)(p->data).data, *length); // Generate 32-bit CRC for the data.
    f->datagram = *p;
    f->length = sizeof(f);

    //Pass Frame to the CNET physical layer to send Frame to the require link.
    CHECK(CNET_write_physical(link, (void *)f, (size_t *)f->length));
    free(p->data);
    free(p);
    free(f);
    }

I managed to find that the line: CHECK(CNET_write_physical(link, (void *)f, (size_t *)f->length)); is causing the segfault but I can't work out why. Any help is greatly appreciated.