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Is this undefined behavior in C to use a pointer to typedef as a pointer to a primitive type?

Let's assume you have the following code:

#include <stdio.h>

typedef int myIntType;

int main()
{
    int a;
    myIntType *ptr = &a;

    *ptr = 1;

    printf("%d\n", a);

    return 0;
}

Is this causing undefined behavior?

With any reasonable interpretation, I would expect this to just set the value a

to, 1

and therefore print the string 1

to stdout

.

Compiling with gcc -Wall -Wextra -pedantic main.c -o main

on Mac OS X produces no errors or warnings and produces the expected result when executed.

My question is purely theoretical, and I do not intend to use such a construction in real code.

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2 answers


From cppreference.com about the typedef specifier:

The typedef ... specifier can declare array and function types, pointers and references, class types, etc. Each identifier entered in this declaration becomes a typedef-name, which is a synonym for an object or function type, as if the typedef keyword was removed.

In other words, it is exactly the same as if you removed your own type and replaced it with int

. So this is clear behavior. int

and myIntType

are 100% interchangeable.



This is actually a C ++ reference. From K&R, chapter 6.7 to typedef

:

C provides a named object typedef

for creating new data type names. For example, a declaration typedef int Length;

makes the name Length

synonymous int

. The type Length

can be used in ads, casts, etc. Just like the type int

.

Remember, K&R is not the most current standard. Another answer refers to the current standard. As far as I know, typedef

it hasn't changed.

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In your example, is the myIntType *

same as int *

.

Section 6.7.8 of the C standard provides an example:



4 EXAMPLE 1 After

typedef int MILES, KLICKSP();
typedef struct { double hi, lo; } range;

constructions

MILES distance;
extern KLICKSP *metricp;
range x;
range z, *zp;

- all valid ads. The type distance

isint

, and the type metricp

is "a pointer to a function without a specification of the return parameter int

", and x

and z

is the given structure; zp

is a pointer to such a structure. The object distance

is of a type compatible with any other object int

.

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