如何在C / C ++中“使用write（）系统调用从页面读取（写入虚拟管道（）文件描述符）”？

Question

I need to test if memory address is readable, so I searched and found this question: How to test if an address is readable in linux userspace app As user @caf stated:

The canonical way is to use the write() system call to read from the page (writing to a dummy pipe() file descriptor).

Unfortunatelly he didn't provide any code example, and I can't comment (low reputation) nor send him a message (at least I don't know how).

EDIT

Can't I just omit creating the pipe by doing something like that?

int result = write(0, addr, 1);

@Mats Petersson: I'm trying to analyze and change a function after already loading it to memory using function pointer (I'm incrementing the pointer and copying its value byte by byte), so to determine whether it's still this function i need to know if the memory is writeable and readable (it's a little stupid thing to do, I know). Anyhow I'd like to not change my program's memory reading/writing privileges, so what you've said might be a problem.

EDIT 2 @Damon: so is there any other sure way to do this?

Answer 1

First, create a pipe pair to write the data to:

 int fd[2];
 pipe(fd);

Then try to write some data from your address to the write side of the pipe:

 int result = write(fd[1], addr, 1);

If result is 1, the write succeeded, so the address was readable. If it's zero and errno == EFAULT , the address is not readable. If errno is something else, something else went wrong.

Make sure to close the pipes once you're done, of course:

 close(fd[0]);
 close(fd[1]);

Answer 2

I've never used that method myself, but I think that yo have to create a pipe (using the pipe system call), when write to that pipe providing the address you want to check as the buffer.

So lets say you want to check address 0x1000000 , then you do eg

int res = write(dummypipe[1], 0x1000000, 1);

If res is -1 then you check if errno == EFAULT which indicates that the address is not valid.