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Q&A Storing more bytes than a union member has, but less than the union size, with memcpy(3)

Let's say we have an object, we store it in a union (into some other narrower type, but with memcpy(3), so it's allowed --I guess--), and then read it from the union via it's original type (so no a...

2 answers · posted 11mo ago by alx‭ · last activity 10mo ago by Lundin‭

Question c strict-aliasing type-punning language-lawyer

#5: Post edited by

alx‭ · 2023-05-21T22:50:51Z (11 months ago)

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Let's say we have an object, we store it in a union (into some other narrower type, but with memcpy(3), so it's allowed --I guess--), and then read it from the union via it's original type (so no alignment issues or anything.
```c
$ cat union.c
#include <string.h>
struct s { int a; int b; };
struct t { int a; };
union u { struct s s; struct t t; };
int
main(void)
{
struct s x = {42, 53};
union u y;
int z;
~~memcpy(&y.t, &x, sizeof(x)); // y.t has declared type of 'struct t'~~
z = y.s.b; // Is this UB?
return z;
}
```
I would guess the above is undefined behavior, exactly at the point of the read of `y.s.b`.
The reason is that since we created an object of type `struct t` via memcpy(3), then the compiler is free to assume that the object is no wider than `sizeof(struct t)`, and so `y.s.b` (which is beyond that) would be "uninitialized" (even though we really wrote bytes to it).
Is it UB as I expect?
However, neither GCC and Clang complain about such program:
```sh
$ gcc-13 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fanalyzer -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
$ clang-16 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
```
---
BTW, does it change if I change and use allocated memory?
```c
int
main(void)
{
struct s x = {42, 53};
union u *y = xmalloc(sizeof(union u)); // No declared/effective type
int z;
memcpy(&y->t, &x, sizeof(x)); // This sets the effective type to 'struct s'
z = y->s.b; // No UB?
return z;
}
```

Let's say we have an object, we store it in a union (into some other narrower type, but with memcpy(3), so it's allowed --I guess--), and then read it from the union via it's original type (so no alignment issues or anything.
```c
$ cat union.c
#include <string.h>
struct s { int a; int b; };
struct t { int a; };
union u { struct s s; struct t t; };
int
main(void)
{
struct s x = {42, 53};
union u y;
int z;
memcpy(&y.t, &x, sizeof(x)); // y.t has declared/effective type of 'struct t'
z = y.s.b; // Is this UB?
return z;
}
```
I would guess the above is undefined behavior, exactly at the point of the read of `y.s.b`.
The reason is that since we created an object of type `struct t` via memcpy(3), then the compiler is free to assume that the object is no wider than `sizeof(struct t)`, and so `y.s.b` (which is beyond that) would be "uninitialized" (even though we really wrote bytes to it).
Is it UB as I expect?
However, neither GCC and Clang complain about such program:
```sh
$ gcc-13 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fanalyzer -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
$ clang-16 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
```
---
BTW, does it change if I change and use allocated memory?
```c
int
main(void)
{
struct s x = {42, 53};
union u *y = xmalloc(sizeof(union u)); // No declared/effective type
int z;
memcpy(&y->t, &x, sizeof(x)); // This sets the effective type to 'struct s'
z = y->s.b; // No UB?
return z;
}
```

#4: Post edited by

alx‭ · 2023-05-21T22:50:11Z (11 months ago)
What about allocated memory?

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Let's say we have an object, we store it in a union (into some other narrower type, but with memcpy(3), so it's allowed --I guess--), and then read it from the union via it's original type (so no alignment issues or anything.
```c
$ cat union.c
#include <string.h>
struct s { int a; int b; };
struct t { int a; };
union u { struct s s; struct t t; };
int
main(void)
{
struct s x = {42, 53};
union u y;
int z;
~~memcpy(&y.t, &x, sizeof(x));~~
z = y.s.b; // Is this UB?
return z;
}
```
I would guess the above is undefined behavior, exactly at the point of the read of `y.s.b`.
The reason is that since we created an object of type `struct t` via memcpy(3), then the compiler is free to assume that the object is no wider than `sizeof(struct t)`, and so `y.s.b` (which is beyond that) would be "uninitialized" (even though we really wrote bytes to it).
Is it UB as I expect?
However, neither GCC and Clang complain about such program:
```sh
$ gcc-13 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fanalyzer -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
$ clang-16 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
```

Let's say we have an object, we store it in a union (into some other narrower type, but with memcpy(3), so it's allowed --I guess--), and then read it from the union via it's original type (so no alignment issues or anything.
```c
$ cat union.c
#include <string.h>
struct s { int a; int b; };
struct t { int a; };
union u { struct s s; struct t t; };
int
main(void)
{
struct s x = {42, 53};
union u y;
int z;
memcpy(&y.t, &x, sizeof(x)); // y.t has declared type of 'struct t'
z = y.s.b; // Is this UB?
return z;
}
```
I would guess the above is undefined behavior, exactly at the point of the read of `y.s.b`.
The reason is that since we created an object of type `struct t` via memcpy(3), then the compiler is free to assume that the object is no wider than `sizeof(struct t)`, and so `y.s.b` (which is beyond that) would be "uninitialized" (even though we really wrote bytes to it).
Is it UB as I expect?
However, neither GCC and Clang complain about such program:
```sh
$ gcc-13 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fanalyzer -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
$ clang-16 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
```
---
BTW, does it change if I change and use allocated memory?
```c
int
main(void)
{
struct s x = {42, 53};
union u *y = xmalloc(sizeof(union u)); // No declared/effective type
int z;
memcpy(&y->t, &x, sizeof(x)); // This sets the effective type to 'struct s'
z = y->s.b; // No UB?
return z;
}
```

#3: Post edited by

alx‭ · 2023-05-21T15:18:42Z (11 months ago)

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~~Storing more bytes than a union member has, but less than the union size~~

Storing more bytes than a union member has, but less than the union size, with memcpy(3)

#2: Post edited by

alx‭ · 2023-05-21T15:18:28Z (11 months ago)

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Let's say we have an object, we store it in a union (via some other narrower type, but with memcpy(3), so it's allowed --I guess--), and then read it from the union via it's original type (so no alignment issues or anything.
```c
$ cat union.c
#include <string.h>
struct s { int a; int b; };
struct t { int a; };
union u { struct s s; struct t t; };
int
main(void)
{
struct s x = {42, 53};
union u y;
int z;
memcpy(&y.t, &x, sizeof(x));
z = y.s.b; // Is this UB?
return z;
}
```
I would guess the above is undefined behavior, exactly at the point of the read of `y.s.b`.
The reason is that since we created an object of type `struct t` via memcpy(3), then the compiler is free to assume that the object is no wider than `sizeof(struct t)`, and so `y.s.b` (which is beyond that) would be "uninitialized" (even though we really wrote bytes to it).
Is it UB as I expect?
However, neither GCC and Clang complain about such program:
```sh
$ gcc-13 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fanalyzer -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
$ clang-16 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
```

Let's say we have an object, we store it in a union (into some other narrower type, but with memcpy(3), so it's allowed --I guess--), and then read it from the union via it's original type (so no alignment issues or anything.
```c
$ cat union.c
#include <string.h>
struct s { int a; int b; };
struct t { int a; };
union u { struct s s; struct t t; };
int
main(void)
{
struct s x = {42, 53};
union u y;
int z;
memcpy(&y.t, &x, sizeof(x));
z = y.s.b; // Is this UB?
return z;
}
```
I would guess the above is undefined behavior, exactly at the point of the read of `y.s.b`.
The reason is that since we created an object of type `struct t` via memcpy(3), then the compiler is free to assume that the object is no wider than `sizeof(struct t)`, and so `y.s.b` (which is beyond that) would be "uninitialized" (even though we really wrote bytes to it).
Is it UB as I expect?
However, neither GCC and Clang complain about such program:
```sh
$ gcc-13 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fanalyzer -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
$ clang-16 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
```

#1: Initial revision by

alx‭ · 2023-05-21T15:16:51Z (11 months ago)

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Storing more bytes than a union member has, but less than the union size

Let's say we have an object, we store it in a union (via some other narrower type, but with memcpy(3), so it's allowed --I guess--), and then read it from the union via it's original type (so no alignment issues or anything.

```c
$ cat union.c 
#include <string.h>

struct s { int       a;  int       b; };
struct t { int       a;               };
union u  { struct s  s;  struct t  t; };

int
main(void)
{
	struct s  x = {42, 53};
	union u   y;
	int       z;

	memcpy(&y.t, &x, sizeof(x));
	z = y.s.b;  // Is this UB?

	return z;
}
```

I would guess the above is undefined behavior, exactly at the point of the read of `y.s.b`.

The reason is that since we created an object of type `struct t` via memcpy(3), then the compiler is free to assume that the object is no wider than `sizeof(struct t)`, and so `y.s.b` (which is beyond that) would be "uninitialized" (even though we really wrote bytes to it).

Is it UB as I expect?

However, neither GCC and Clang complain about such program:

```sh
$ gcc-13 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fanalyzer -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
$ clang-16 -Wall -Wextra -Wpedantic -pedantic-errors union.c -O3 -fsanitize=undefined -fsanitize=address
$ ./a.out; echo $?
53
```

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