How to check if a memory address is writable or not at runtime?
How to check if a memory address is writable or not at runtime?
For example, I want to implement is_writable_address in the following code. Is it possible?
#include <stdio.h>
int is_writable_address(void *p) {
// TODO
}
void func(char *s) {
if (is_writable_address(s)) {
*s = 'x';
}
}
int main() {
char *s1 = "foo";
char s2[] = "bar";
func(s1);
func(s2);
printf("%s, %s\n", s1, s2);
return 0;
}
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I generally agree, which suggests that this is a bad idea.
However, given that the question has a tag UNIX, the classic way to do it on UNIX-like operating systems is to use read()
from /dev/zero
:
#include <fcntl.h>
#include <unistd.h>
int is_writeable(void *p)
{
int fd = open("/dev/zero", O_RDONLY);
int writeable;
if (fd < 0)
return -1; /* Should not happen */
writeable = read(fd, p, 1) == 1;
close(fd);
return writeable;
}
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It may be technically possible, but there is no portable way to do it, and it will never be necessary for it. If you manage to lose information about whether a pointer is writable or not, there is even more important information that you do not know either, for example, what it points to, and whether your write or not.
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