Destructor Destructor - 1 year ago 58
C Question

Is fetching the value of an invalid pointer undefined or implementation defined behaviour in C?

Fetching the value of an invalid pointer is an implementation defined behavior in C++ according to this. Now consider the following C program:

#include <stdio.h>
#include <stdlib.h>
int main(void)
int* p=(int*)malloc(sizeof(int));
printf("%p\n",(void*)p); // Is this undefined or implementation defined in behaviour C?

But is the behaviour same in C also? Is the behaviour of the above C program undefined or implementation defined? What does the C99/C11 standard say about this?
Please tell me if the behaviour is different in C99 & C11.

Answer Source

Expanding on Andrew Henle's answer:

From the C99 Standard, 6.2.4:

An object has a storage duration that determines its lifetime. There are three storage durations: static, automatic, and allocated. Allocated storage is described in 7.20.3. […] The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the end of its lifetime.

Then in the standard goes on describing malloc(), calloc() and free(), mentioning that

The free function causes the space pointed to by ptr to be deallocated.

In 3.17.2:

indeterminate value

either an unspecified value or a trap representation


Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. […] Such a representation is called a trap representation.

Since the pointer becomes indeterminate, and an indeterminate value can be a trap representation, and you have a variable which is an lvalue, and reading an lvalue trap representation is undefined, therefore yes, the behavior may be undefined.

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