Question

I have been working on and off on an assignment for my C class for the last few days and encountered a curious crash concerning the realloc() function in C. Even the C/C++ programmers in house could not answer me right away what might be wrong with my code.

First i create the memory block in one function:

char *line = (char *)malloc( sizeof(char) * BUFSIZE);

Then i call getMoreBuf(start_of_block, end_of_block)

int getMoreBuf(char *start, char *end)
{
char *newBuf = 0;
int newSize = (end - start) + BUFSIZE;
    newBuf = (char *)realloc(start, sizeof(char) * newSize);
    if(NULL == newBuf) {
        printf("No virtual RAM available");
    }else{
        start = newBuf;
    }
    return newSize;
}

Depending on what i set the BUFSIZE to, it crashes after the 5th call (BUFSIZE = 1) or the 3rd call (BUFSIZE = 5) and replaces the read in characters with nonsense.

Could someone point me towards my error(s) and give suggestions where to read up on to fix them?
Any help is appreciated. 🙂

Bonus Question: I malloc a memory block with pointer 1 pointing to the start and later pointer 2 point2 towards a single block in the memory block. I realloc() the block and the block is moved due to size issues, does the pointer 2 still point to the old (now useless) block or does it “move” with the realloc to the new position of the memory block?

(Also for the future, should i put that extra question into a new question or can i leave it in here since it is strongly related to the first question?)

Thank you all for your input, it helped me a great deal to figure out what went wrong.
If i could i would mark each as the right answer since each helped my in some form to understand a bit more about all this damn pointer business. =)

Answer 1

I think your problem is that you’re only changing the local variable start… You need to use a pointer.

int getMoreBuf(char **start, char **end)
{
    char *newBuf = 0;
    int newSize = (*end - *start) + BUFSIZE;
    newBuf = (char *)realloc(*start, sizeof(char) * newSize);
    if(NULL == newBuf) {
        printf("No virtual RAM available");
    } else {
        *start = newBuf;
        *end = *start + newSize;
    }
    return newSize;
}

And then you’d call it like:

getMoreBuf(&start_of_block, &end_of_block)