Question

I’m having difficulty learning C language’s malloc and pointer:

What I learned so far:

• Pointer is memory address pointer.

• malloc() allocate memory locations and returns the memory address.

I’m trying to create a program to test malloc and pointer, here’s what I have:

#include<stdio.h>

main()
{
char *x;
x = malloc(sizeof(char) * 5);

strcpy(*x, "123456");
printf("%s",*x); //Prints 123456

}

I’m expecting an error since the size I provided to malloc is 5, where I put 6 characters (123456) to the memory location my pointer points to. What is happening here? Please help me.

Update

Where to learn malloc and pointer? I’m confused by the asterisk thing, like when to use asterisk etc. I will not rest till I learn this thing! Thanks!

Answer 1

You are invoking undefined behaviour because you are writing (or trying to write) beyond the bounds of allocated memory.

Other nitpicks:

• Because you are using strcpy(), you are copying 7 bytes, not 6 as you claim in the question.
• Your call to strcpy() is flawed – you are passing a char instead of a pointer to char as the first argument.
• If your compiler is not complaining, you are not using enough warning options. If you’re using GCC, you need at least -Wall in your compiler command line.
• You need to include both <stdlib.h> for malloc() and <string.h> for strcpy().
• You should also explicitly specify int main() (or, better, int main(void)).
• Personally, I’m old school enough that I prefer to see an explicit return(0); at the end of main(), even though C99 follows C++98 and allows you to omit it.

You may be unlucky and get away with invoking undefined behaviour for a while, but a tool like valgrind should point out the error of your ways. In practice, many implementations of malloc() allocate a multiple of 8 bytes (and some a multiple of 16 bytes), and given that you delicately do not step over the 8 byte allocation, you may actually get away with it. But a good debugging malloc() or valgrind will point out that you are doing it wrong.

Note that since you don’t free() your allocated space before you return from main(), you (relatively harmlessly in this context) leak it. Note too that if your copied string was longer (say as long as the alphabet), and especially if you tried to free() your allocated memory, or tried to allocate other memory chunks after scribbling beyond the end of the first one, then you are more likely to see your code crash.

Undefined behaviour is unconditionally bad. Anything could happen. No system is required to diagnose it. Avoid it!