Using malloc(0) and memcpy
c, malloc, undefined-behavior
Solution
When you pass `0` as an argument to `malloc`, then it `free` the allocated memory to the pointer which `malloc` returns to.
Result is implementation defined.
C11: 7.22.3 Memory management functions:
[...] If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.
Also standard says:
The `free` function causes the space pointed to by `ptr` to be deallocated, that is, made available for further allocation. If `ptr` is a null pointer, no action occurs.
So, in either case of implementation defined behavior, freeing will not invoke undefined behavior.
Now moving to another part of the question.
7.1.4 Use of library functions:
If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer, or a pointer to non-modifiable storage when the corresponding parameter is not const-qualified) or a type (after promotion) not expected by a function with variable number of arguments, the behavior is undefined.
C11: 7.24.1 p(2):
Where an argument declared as `size_t n` specifies the length of the array for a function, n can have the value zero on a call to that function. Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call shall still have valid values, as described in 7.1.4. On such a call, a function that locates a character finds no occurrence, a function that compares two character sequences returns zero, and a function that copies characters copies zero characters.
Problem
I read somewhere: Dereferencing a pointer returned by a "new" allocation of size zero is UB. Is it same in C? If yes, is the following code UB? (assuming size = 0) ``` a->object[index].data = malloc(size); memcpy(a->object[index].data, bytes, size); ``` To my understanding: NO. Just wanted to double check.