In memcpy how to handle memory overflow?

Question

int main ()
{
    char *destination;
    char source[10] = "jigarpatel";
    destination = (char*) malloc(5);
    memcpy(destination, source, 10);
    printf("%s and size is %d", destination, strlen(destination));
    free(destination);
    return 0;
}

Output:

 jigarpatel and size is 10

Question:

Here I have allocated just 5 bytes to destination, but the destination length is 10, why is this?

Where are the other bytes stored?

Is it safe in embedded system? Any chances of a crash or a segmentation fault?

How can I detect this type of mistake?

Another Question :

see i am writing one library where user asks for needed memory and library says allocate 10 bytes & then user malloc 10 bytes & pass its pointer to library. now library store some data there...now see if library said to allocate 10 bytes but user has allocated only 5 bytes & give that pointer to library then how can i detect that user hasnt malloc suffecient memory.

Note: in C you should not cast the result of malloc() - you have to do this in C++ but it is redundant and potentially dangerous in C. — Paul R
Another note: quite aside from the fluke that writing past the end of the buffer, you have another fluke that printf and strlen have found a 0 byte after the l, but you had no right to expect that either. — Steve Jessop

Alok Save Alok Save · Accepted Answer · 2011-08-16T08:01:41

Here i have just allocated only 5 bytes to destination then still why destination length is 10?

printf("%s and size is %d",destination,strlen(destination));

The strlen() considers \0 as the end of the string, So it continues counting until it enounters a \0. This does not mean destination has that much memory allocated.

You are writing beyond the bounds of the allocated memory and luckily it does not crash, but sure is an Undefined Behavior to do so. An Undefined Behavior means anything can happen & the behavior cannot be explained, luckily Your program does not crash.

where other bytes are stored?
The other bytes overwrite some other memory allocation beyond the 5 bytes allocated to destination.

Is it safe in embedded system? any chances to crash or segmention fault?
It is NOT safe. It causes an Undefined Behavior and If you are lucky that it works.

how can i detect this type of mistakes?
Each platform has certain Memory profiling tools like Valgrind for Linux/Unix, You can use them and they will point out such memory overrites.

In memcpy how to handle memory overflow?

4 Answers