copy a struct with dynamic strings to a buffer c

Question

i have this struct:

typedef struct
{
   auth_header header; // another struct
   uint8_t action;
   uint8_t priv_lvl;
   uint8_t authen_type;
   uint8_t service;
   uint8_t user_len;
   uint8_t port_len;
   uint8_t rem_addr_len;
   uint8_t data_len;
   char *user;
   char *port;
   char *rem_addr;
   char *data;

} auth_start;

and i want to create a buffer and insert the data in it so i can send this data to my server.

im not sure of what type the buffer needs to be (uint8_t/char/auth_start).
im not sure how should i do it, should i copy every field 1 by 1 with memcpy() ? or is there another way?

thanks ! :)

Sergey Kalinichenko · Accepted Answer · 2016-12-03 16:41:09Z

1

I'm not sure of what type the buffer needs to be

Any byte type will do. uint8_t is a good choice, considering the types of non-pointer fields in your struct are all uint8_t.

I'm not sure how should i do it, should i copy every field one by one with memcpy()

First, you need to figure out how much memory your buffer needs. The you allocate that memory, and copy non-pointer portion with assignments. Finally, you memcpy the data fron the four pointers:

auth_start s = ...
// Add 8 for the initial 8 members
uint8_t *buf = malloc(8+s.user_len+s.port_len+s.rem_addr_len+s.data_len);
uint8_t p = buf;
// Copy the initial fields
*p++ = s.action;
*p++ = s.priv_lvl;
... // And so on for the remaining members
// Copy pointer-based members
memcpy(p, s.user, s.user_len);
p += s.user_len;
memcpy(p, s.port, s.port_len);
p += s.port_len;
... // And so on for the remaining pointer members

answered Dec 3, 2016 at 16:41

Sergey Kalinichenko

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2 Comments

phantttom Over a year ago

thanks alot! and how do i check if the buffer is good ? i need to reset p to the start of auth and print it in a loop ?

Sergey Kalinichenko Over a year ago

@phantttom the actual pointer is buf, not p. Reset p back to buf if you want to process the next struct.

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