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Comments on Is partial allocation of an object Undefined Behavior?
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Is partial allocation of an object Undefined Behavior?
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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)
{
struct s *s;
s = malloc(50 * sizeof(int));
s->i[30] = 7;
printf("%d\n", s->i[30]);
free(s);
}
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?
2 comment threads