Copy structure to buffer - c

Question

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <string.h>
#include <math.h>
#include <time.h>
#define RAND(lower,upper) (rand()%(upper-lower+1))+lower
int power(int base, int exp)
{
int result=1;
while (exp != 0)
{
result *= base;
--exp;
}
return result;
}
int isKthBitSet(int n, int k)//from right, 1<=k<=n
{
int new_num = n >> (k - 1);
// if it results to '1' then bit is set,
// else it results to '0' bit is unset
return (new_num & 1);
}
struct Astructure{
uint16_t bitmap;
uint32_t a;
uint32_t b;
char str[10];
uint16_t d;
}__attribute__((packed));
int main()
{
struct Astructure abitmap;
abitmap.bitmap = 0;
char buffer[(sizeof(struct Astructure))];
abitmap.a = 52;
abitmap.b = 16;
char c[10]={"ramya"};
strcpy(abitmap.str, c);
abitmap.d = 59;
char ch;
srand(time(0));
for(uint8_t position =1;position<5;position++)
{
int random10 = RAND(0,1);
if(random10==1)
{
int value = power(2,position-1);
abitmap.bitmap = abitmap.bitmap | value;
}
}
memcpy(buffer, (char *)&abitmap.bitmap, sizeof(abitmap.bitmap)+1);
uint8_t temp4 = isKthBitSet(abitmap.bitmap,4);
uint8_t temp3 = isKthBitSet(abitmap.bitmap,3);
uint8_t temp2 = isKthBitSet(abitmap.bitmap,2);
uint8_t temp1 = isKthBitSet(abitmap.bitmap,1);
uint8_t previousLength = sizeof(abitmap.bitmap);
if(temp4){
//add a to buffer
memcpy(buffer + previousLength, (void *)&abitmap.a, sizeof(abitmap.a)+1);
previousLength += sizeof(abitmap.a);
}
if(temp3){
//add b to buffer
memcpy(buffer + previousLength, (void *)&abitmap.b, sizeof(abitmap.b)+1);
previousLength += sizeof(abitmap.b);
}
if(temp2){
//add c to buffer
memcpy(buffer + previousLength, (void *)&abitmap.str, sizeof(abitmap.str)+1);
previousLength += sizeof(abitmap.str);
}
if(temp1){
//add d to buffer
memcpy(buffer + previousLength, (char *)&abitmap.d, sizeof(abitmap.d)+1);
previousLength += sizeof(abitmap.d);
}
//SHOW BUFFER
previousLength = sizeof(abitmap.bitmap);
uint32_t a;
uint32_t b;
char str[10];
uint16_t d;
if(temp4){
memcpy(&a , (void *) buffer+previousLength , sizeof(abitmap.a)+1);//
printf("a = %d\t",a);
previousLength += sizeof(a);
}
if(temp3){
memcpy(&b , (void *) buffer+previousLength , sizeof(abitmap.b)+1);
printf("b = %d\t",b);
previousLength += sizeof(b);
}
if(temp2){
memcpy(str , (void *) buffer+previousLength , sizeof(abitmap.str)+1);//memccpy
printf("string = %s\t",str);
previousLength += sizeof(str);
}
if(temp1){
memcpy(&d , (void *) buffer+previousLength , sizeof(abitmap.d)+1);
printf("d = %d\t",d);
previousLength += sizeof(d);
}
printf("\n");
}
I trying to copy some variables to my buffer. The variables I want to add in the buffer is picked at random. Here I gave the size of my buffer before hand to the size of my structure. But as per my program I may not be adding all my variables in the buffer. So its a waste.
So How am I supposed to declare my buffer here.
if my replace this
char buffer[(sizeof(struct Astructure))];
to
char *buffer;
this
I get below error.
a = 52 string = ramya
*** stack smashing detected ***: terminated
Aborted (core dumped)
I dont want to waste the size of my buffer in here, giving the whole size of structure. so what do i do.

Answer 1

If you want to allocate only the required memory:
typedef struct
{
size_t length;
unsigned char data[];
}buff_t;
buff_t *addtobuffer(buff_t *buff, const void *data, size_t size)
{
size_t pos = buff ? buff -> length : 0;
buff = realloc(buff, pos + size + sizeof(*buff));
if(buff)
{
memcpy(buff -> data + pos, data, size);
buff -> length = pos + size;
}
return buff;
}
and example usage:
buff_t *buffer = NULL;
/* ... */
if(temp4)
{
buff_t *tmp = addtobuffer(buffer, &abitmap.a, sizeof(abitmap.a)+1);
if(!tmp) { /* error handling */}
buffer = tmp;
/*...*/
}