Resetting a char buffer in C - c

Question

I'm working on a homework assignment and I need to basically create a character buffer. One of the functions I need to create is called "b_reset". It's purpose is to reinitialize the given buffer so that it will point to the first position in the char buffer. This is needed because later on, when a new char is added to the buffer, it needs to be added to the first position in the buffer.
This is the code I have thus far:
The struct:
typedef struct BufferDescriptor {
char * ca_head ;
int capacity ;
char inc_factor;
int addc_offset ;
int mark_offset ;
char r_flag;
char mode;
} Buffer ;
The code:
int b_reset ( Buffer *pB )
{
Buffer *temp = NULL;
int i = 0;
int j = 1;
if (pB == NULL)
{
return R_FAIL_1;
}
else
{
temp = (Buffer*)malloc(sizeof(Buffer*));
if (temp == NULL)
{
return R_FAIL_1;
}
temp->ca_head = (char*)malloc(pB->capacity);
if (!temp->ca_head)
{
temp = NULL;
return R_FAIL_1;
}
for(i = 0;i < ca_getsize(pB);++i)
{
temp->ca_head[j] = pB->ca_head[i];
j++;
}
pB->ca_head = temp->ca_head;
//free(temp->ca_head);
//free(temp);
return 0;
}
}
My goal in this code was to create a temporary buffer that would basically shift over everything 1 time based on the actual given buffer. This would make the first position empty so another char could be added.
The problem I'm running into is that the original buffer doesn't seem to be returning the right values after I reset it.
When I do this for example:
temp->ca_head[0] = 'a';
temp->ca_head[1] = 'b';
temp->ca_head[2] = 'c';
temp->ca_head[3] = 'd';
temp->ca_head[4] = 'e';
b_reset(temp); //this will return the size as 0, when it's actually 5
//temp->ca_head[0] = 'i'; //if this is executed, it returns the size as 6
//and prints out the right values, but if it's not,
//it will not print out anything
printf("%d", ca_getsize(temp));
for(i = 0;i < ca_getsize(temp);++i)
{
printf("%c", temp->ca_head[i]);
}
I know something is going wrong here, but I'm not too sure what. Any suggestions would be greatly appreciated.

Answer 1

This code is based on your followup comment:
well I'm not trying to resize the buffer, I just want to create an
empty space in the first position, so basically shifting everything to
the right 1 time. The assumption is that there is a enough space in
the buffer to handle this process.
I don't think you need to do any malloc() ing beyond the initial one. You can just shift everything up in a loop:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#define R_FAIL_1 1
#define BUFFER_SIZE 10
typedef struct BufferDescriptor {
char * ca_head ;
int capacity ;
char inc_factor;
int addc_offset ;
int mark_offset ;
char r_flag;
char mode;
} Buffer ;
void allocate_buffer(Buffer *pB, int size)
{
pB->ca_head = malloc(size);
assert(pB->ca_head);
pB->capacity = size;
}
int ca_getsize( Buffer *pB)
{
return pB->capacity;
}
int b_reset ( Buffer *pB )
{
int i = 0;
if (pB == NULL)
{
return R_FAIL_1;
}
else
{
if ( ca_getsize(pB) <= 0 || pB->ca_head == NULL )
return R_FAIL_1;
}
// shift data up by 1 byte
for( i = ca_getsize(pB) - 1 ; i > 0;i-- )
{
pB->ca_head[i] = pB->ca_head[i-1];
}
pB->ca_head[0] = '\0';
return 0;
}
void print_buffer(Buffer *pB)
{
printf("capacity: %d \n", ca_getsize(pB));
for (int i = 0;i < ca_getsize(pB);++i)
{
printf("buffer(%d): [%d] ",i, pB->ca_head[i]);
}
printf("\n");
}
int main(void)
{
Buffer a_buffer;
allocate_buffer(&a_buffer,BUFFER_SIZE);
strcpy(a_buffer.ca_head,"abcdefgh");
print_buffer(&a_buffer);
int ret = b_reset(&a_buffer);
assert(ret == 0);
print_buffer(&a_buffer);
}

Answer 2

temp = (Buffer*)malloc(sizeof(Buffer*));
You need to allocate enough space to hold a Buffer, but you only allocate enough space to hold a pointer to a buffer. This should be:
temp = (Buffer*)malloc(sizeof(Buffer));

Answer 3

You are managing your memory incorrectly. You are allocating memory for a new Buffer struct when actually you only need to handle the memory of the ca_head member (if my interpretation of your homework problem is correct).
Each time you invoke b_reset, you will allocate memory for this struct that will not be released. If you don't handle your memory correctly, you will experience unexpected results as the one you are reporting in your question.
I suggest you to make a research on the function realloc and use it properly in your b_reset function.
Good luck with your homework.