C - Problems extracting data from buffer. Possibly endianess-related
c
Solution
Your pointer math is incorrect. You're using `&buf` where you should just be using `buf`. If this doesn't explain what is wrong, nothing else I can say will:
#include <stdio.h>
int main(int argc, char **argv)
{
char buff[100];
printf("buff : %p\nbuff+10 : %p\n&buff+10 : %p\n", buff, buff+10, &buff+10);
return 0;
}
Output (varies by platform, obviously)
buff : 0xbf87a8bc
buff+10 : 0xbf87a8c6
&buff+10 : 0xbf87aca4
See it live. The math you're doing is incrementing by type, which for `&buf` is a pointer to array of 100 `char`s; not a simple `char` address. Therefore, `&buff + 10` (in my sample) says "give me the 10th array of 100 chars from where I am now.". The subsequent write is invoking undefined behavior as a consequence.
Valgrind is your buddy here, btw. It would have caught this in a heartbeat.
Update
May as well fill in the entire gambit while I'm here. This is also wrong in `l1_send`:
memcpy(&id, &buf, sizeof(id));
// this------^
and the subsequent other areas you're using it in that function. You're taking the address of a parameter pointer, not the value within it. I'm confident you need `buf` there as well.
Problem
I'm having some difficulties extracting data from a buffer using memcpy. First, I memcpy some variables into a buffer: ``` int l1_connect(const char* hostname, int port) { // Variables to be stored in the buffer char *msg = "Hi, I'm a message"; // strlen(msg) == 17 uint16_t sender_id = htons(1); // sizeof(sender_id) == 2 uint16_t packet_size = htons(sizeof(packet_size)+sizeof(sender_id)+strlen(msg)); // sizeof(packet_size) == 2 // Checking values printf("l1_connect():\nsender_id: %d, packet_size: %d\n\n", ntohs(sender_id), ntohs(packet_size)); // sender_id == 1, packet_size == 21 // The buffer char buf[100]; // Copying everything memcpy(&buf, &sender_id, sizeof(sender_id)); memcpy(&buf+sizeof(sender_id), &packet_size, sizeof(packet_size)); memcpy(&buf+sizeof(sender_id)+sizeof(packet_size), &msg, strlen(msg)); // Passing buf to another function int bytes_sent = l1_send(1, buf, sizeof(buf)); } ``` I then try to extract that data (checking, before sending over UDP socket): ``` int l1_send( int device, const char* buf, int length ) { // Variables in which to store extracted data uint16_t id = 0; uint16_t size = 0; char msg[50]; memcpy(&id, &buf, sizeof(id)); memcpy(&size, &buf+sizeof(id), sizeof(size)); int remaining = ntohs(size) - (sizeof(id) + sizeof(size)); printf("l1_send():\nremaining: %d\n", remaining); // -37041 // memcpy-ing with correct(?) offset memcpy(&msg, &buf+sizeof(id)+sizeof(size), 50); msg[49] = '\0'; printf("id: %d\n", ntohs(id)); // 8372 printf("size: %d\n", ntohs(size)); // 37045 printf("msg: %s\n", msg); // ��$_� return 0; // For now } ``` As you can see, the values aren't quite what I'm expecting. Can anyone tell me what I'm doing wrong?