c-将结构复制到缓冲区

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 = %dt",a);
previousLength += sizeof(a);

}
if(temp3){
memcpy(&b , (void *) buffer+previousLength , sizeof(abitmap.b)+1);
printf("b = %dt",b);
previousLength += sizeof(b);
}
if(temp2){
memcpy(str , (void *) buffer+previousLength , sizeof(abitmap.str)+1);//memccpy
printf("string = %st",str);
previousLength += sizeof(str);

}
if(temp1){
memcpy(&d , (void *) buffer+previousLength , sizeof(abitmap.d)+1);
printf("d = %dt",d);
previousLength += sizeof(d);

}
printf("n");
}

我试图将一些变量复制到我的缓冲区。我想添加到缓冲区中的变量是随机选取的。在这里，我给出了我的缓冲区的大小，在手之前，我的结构的大小。但根据我的程序，我可能不会在缓冲区中添加所有变量。所以这是浪费。

那么我该如何在这里申报我的缓冲区呢。

如果我更换这个

char buffer[(sizeof(struct Astructure))];

至

char *buffer;

这个

我得到以下错误。

a = 52  string = ramya  
*** stack smashing detected ***: terminated
Aborted (core dumped)

我不想在这里浪费缓冲区的大小，给出整个结构的大小。那我该怎么办。

Answer 1

如果只想分配所需的内存：

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;
}

和示例用法：

buff_t *buffer = NULL;  
/* ... */

if(temp4)
{
buff_t *tmp = addtobuffer(buffer, &abitmap.a, sizeof(abitmap.a)+1);
if(!tmp) { /* error handling */}
buffer = tmp;
/*...*/
}