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Is it valid to partly allocate an object, as long as you only use the allocated part of it? #include <stdio.h> #include <stdlib.h> struct s { int i[100]; }; int main(void) {...
#3: Nominated for promotion
by
Alexei
·
2023-01-22T08:03:54Z (almost 2 years ago)
#2: Post edited
by
alx
·
2023-01-21T03:54:57Z (almost 2 years ago)
- Is it valid to partly allocate an object, as long as you only use the allocated part of it?
- ```c
- #include <stdio.h>
- #include <stdlib.h>
- struct s {
- int i[100];
- };
- int main(void)
- {
- struct s *s;
- s = malloc(50 * sizeof(int));
- s->i[30] = 7;
- printf("%d\n", s->i[30]);
- free(s);
- }
- ```
- ```sh
- alx@debian$ gcc --version | head -n1
- gcc (Debian 12.2.0-13) 12.2.0
- alx@debian$ clang --version | head -n1
- Debian clang version 14.0.6
alx@debian$ clang -Weverything -Wall -Wextra malloc.c -O3alx@debian$ clang -Weverything -Wall -Wextra malloc.c- alx@debian$ gcc -Wall -Wextra malloc.c -fanalyzer
- alx@debian$ gcc -Wall -Wextra malloc.c -fanalyzer -O3
- alx@debian$ ./a.out
- 7
- ```
- The compiler don't seem to complain. It seems to work.
- I didn't find anything in the standard that makes this code Undefined Behavior. However, I still suspect of it: the last half of the array within the structure is not allocated. Is it well-defined?
- Is it valid to partly allocate an object, as long as you only use the allocated part of it?
- ```c
- #include <stdio.h>
- #include <stdlib.h>
- struct s {
- int i[100];
- };
- int main(void)
- {
- struct s *s;
- s = malloc(50 * sizeof(int));
- s->i[30] = 7;
- printf("%d\n", s->i[30]);
- free(s);
- }
- ```
- ```sh
- alx@debian$ gcc --version | head -n1
- gcc (Debian 12.2.0-13) 12.2.0
- alx@debian$ clang --version | head -n1
- Debian clang version 14.0.6
- alx@debian$ clang -Weverything malloc.c -O3
- alx@debian$ clang -Weverything malloc.c
- alx@debian$ gcc -Wall -Wextra malloc.c -fanalyzer
- alx@debian$ gcc -Wall -Wextra malloc.c -fanalyzer -O3
- alx@debian$ ./a.out
- 7
- ```
- The compiler don't seem to complain. It seems to work.
- I didn't find anything in the standard that makes this code Undefined Behavior. However, I still suspect of it: the last half of the array within the structure is not allocated. Is it well-defined?
#1: Initial revision
by
alx
·
2023-01-21T03:49:28Z (almost 2 years ago)
Is partial allocation of an object Undefined Behavior?
Is it valid to partly allocate an object, as long as you only use the allocated part of it?
```c
#include <stdio.h>
#include <stdlib.h>
struct s {
int i[100];
};
int main(void)
{
struct s *s;
s = malloc(50 * sizeof(int));
s->i[30] = 7;
printf("%d\n", s->i[30]);
free(s);
}
```
```sh
alx@debian$ gcc --version | head -n1
gcc (Debian 12.2.0-13) 12.2.0
alx@debian$ clang --version | head -n1
Debian clang version 14.0.6
alx@debian$ clang -Weverything -Wall -Wextra malloc.c -O3
alx@debian$ clang -Weverything -Wall -Wextra malloc.c
alx@debian$ gcc -Wall -Wextra malloc.c -fanalyzer
alx@debian$ gcc -Wall -Wextra malloc.c -fanalyzer -O3
alx@debian$ ./a.out
7
```
The compiler don't seem to complain. It seems to work.
I didn't find anything in the standard that makes this code Undefined Behavior. However, I still suspect of it: the last half of the array within the structure is not allocated. Is it well-defined?