Question

I have a struct that I am sending to a UDP socket:

typedef struct
{
    char field_id;
    short field_length;
    char* field;
} field_t, *field_p;

I am able to read the field_id and field_length once received on the UDP server-side, however the pointer to field is invalid as expected.

What is the best method to properly send and receive a dynamic char*?

I have a basic solution using memcpy on the client side:

char* data = 
    (char*)malloc(sizeof(field_t) + (sizeof(char) *  strlen(my_field->field)));
memcpy(data, my_field, sizeof(field_t));
memcpy(data+sizeof(field_t), my_field->field, strlen(my_field->field) + 1);

And on the server side:

field_p data = (field_p)buffer;
field_string = (char*)buffer+sizeof(field_t);

Is there a cleaner way of doing this or is this the only way?

Thanks.

Answer 1

You of course cannot send a pointer over a socket – get rid of the char* field; member. Instead, just append id and size pair with the data itself. Use writev(2) or sendmsg(2) to avoid moving data around from buffer to buffer.

Watch out for structure member alignment and padding and number endianness.