CODEWITHSUNDEEP
c
Adrian
Adrian

Reputation: 20078

What happens when a buffer is to small to hold some data

My question is why the following code snippet doesn't crash ?

int main(int argc, const char * argv[])
{
    const char* source = "SomeText";
    char* dest = (char*)malloc(sizeof(char)); // isn't this wrong ?

    strcpy(dest, source);

    printf("%s\n", dest);

    free(dest);


    return 0;
}

Upvotes: 0

Views: 112

Answers (4)

Marian
Marian

Reputation: 7482

Behavior in such a case is undefined. In fact a memory outside the reserved place is overwritten. This memory can hold some important information or not. It may be mapped into address space of the process or not. If it is not mapped a system error happens. If it is mapped, then the most probably this memory contains values required for correct working of malloc and free, so you may expect crash on the next malloc or free. If you overwrite a larger part of memory you may write into some other dynamically allocated buffer.

In your case you are overwriting only 8 bytes outside the reserved place. The most probable is that those 8 bytes are mapped into address space and are not used at all. This is why nothing happened. The exact behavior depends on the particular implementation of malloc and free..

Upvotes: 1

ajay
ajay

Reputation: 9680

strcpy will overrun the buffer when it is too small to hold the string being copied. This will cause undefined behaviour or even program crash (segfault) because strcpy will try to write into memory which is not allocated to the buffer. You can't rely on what happens. Next time you run your program, it may very well crash. Here are some fixes I suggest. 

// in main

const char *source = "SomeText";
char *dest = malloc(strlen(source) + 1); // +1 for the null byte at the end
if(dest != NULL) { // check for NULL
    strcpy(dest, source);
    printf("%s\n", dest);
    free(dest);
}
else {
    printf("not enough memory\n");
}

strlen doesn't count the null byte in a string, so you have to allocate one extra byte for it to be copied by strcpy from the source string. You should check the result of malloc for null. Also, you should not cast the result of malloc. There's no benefit and it can lead to bugs if you forget to include the stdlib.h header file.

Upvotes: 2

Ed Heal
Ed Heal

Reputation: 60027

Luck?

But usually malloc grabs memory that is convenient for the OS. So maybe it is 1K

Upvotes: 1

Marco A.
Marco A.

Reputation: 43662

Because it is undefined behavior and can happen to work or not. Writing to unallocated space is a risky operation which is not guaranteed to succeed.

Upvotes: 1

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