Freeing a non-allocated pointer - c

Question

The following code throws the error:
mem(44582) malloc: * error for object 0x7f9f8a4000e0: pointer being freed was not allocated
* set a breakpoint in malloc_error_break to debug
Abort trap: 6
I'm not sure what's going on. I'm freeing an area of memory that I explicitly malloc'd, does it have something to do with passing in a pointer to another method?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFFERSIZE 10
void readstringbuffered(char *buffer);
int main(int argc, char const *argv[])
{
char *buffer = (char *)malloc(BUFFERSIZE);
readstringbuffered(buffer);
printf("%s",buffer);
free(buffer);
return EXIT_SUCCESS;
}
void readstringbuffered(char *buffer)
{
FILE *source;
source = fopen("hello.txt","r");
int current_size = BUFFERSIZE;
int len = 0;
int c;
while((c = fgetc(source)) != EOF)
{
if(len == current_size-1)
{
current_size *= 2;
char *temp = (char *)realloc(buffer,current_size);
if(!temp)
{
fprintf(stderr,"out of memory");
exit(1);
}
buffer = temp;
}
buffer[len] = c;
len++;
}
buffer[len] = 0;
}

Answer 1

C is a pass by value language. The modifications you make to buffer inside your readstringbuffered() function have no effect on the value of buffer in main(). If the realloc() there is ever executed, you've already freed the buffer that main knows about, and when you return - BAM - double free.
A possible solution is to pass a pointer to buffer, rather than buffer itself. Change the signature of readstringbuffered() to be:
void readstringbuffered(char **buffer)
And then use *buffer inside it. At the call site, you'd use readstringbuffered(&buffer) to pass the necessary pointer.

Answer 2

realloc() can release the previously allocated memory and allocate it again somewhere else, which makes your buffer pointer in main totally invalid. You can pass buffer by reference and have readstringbuffered() modify it accordingly, or return a pointer to buffer instead of void.

Answer 3

The buffer pointer in main() will not be modified according to the reallocations you do in readstringbuffered(), you may write back the buffer pointer using a pointer to the buffer pointer (pBuffer), like this:
void readstringbuffered(char** pBuffer,size_t* pSize)
{
char* buffer = *pBuffer;
size_t size = MIN_SIZE;
char* newBufferPtr = (char*) realloc(buffer,size);
if(newBufferPtr)
{
buffer = newBufferPtr;
}
else
{
//out of memory
free(buffer);
buffer = NULL;
size = 0;
}
if(buffer)
{
//fill the buffer
}
//must always execute
*pBuffer = buffer;
*pSize = size;
}
int main(int argc, char const *argv[])
{
char* buffer = NULL;
size_t size = 0;
readstringbuffered(&buffer,&size);
if(buffer)
{
printf("%s",buffer);
free(buffer);
}
else
{
//error
}
return EXIT_SUCCESS;
}