Can you convert a mut i8 into an i32? - unix

Question

I am attempting to build a small terminal emulator and am running into some interesting type conflicts with libc. When I am attempting to set up the slave portion of the pty connection I need to create the slave with a system call to ptsname() in order to get the name for the pts so I can access it. However, I get a type error saying that libc::ptsname() requires an i32 for the input. This is in direct conflict with the man page that says it should be passed a file descriptor. I'm just wondering if I can convert the libc::c_int that I have for a file descriptor into a i32 to pass into ptsname.
The code is as follows :
use libc::{self, c_int, grantpt, posix_openpt, ptsname, unlockpt, O_RDWR};
use std::os::unix::io::FromRawFd;
use std::process::{Child, Command, Stdio};
#[derive(Debug)]
pub struct Pty {
process: Child,
fd: i32,
}
fn create_pty(process: &str) -> Pty {
let master: c_int;
unsafe {
// create master/slave pair of fd
master = posix_openpt(O_RDWR);
if master == -1 {
panic!("Failed to posix_openpt");
}
// set slave ownership and mode as master
let mut result = grantpt(master);
if result == -1 {
panic!("Failed to grantpt");
}
// unlock slave
result = unlockpt(master);
if result == -1 {
panic!("Failed to unlockpt");
}
}
let slave: c_int = ptsname(master as i32);
slave = libc::open(slave);
let mut builder = Command::new(process);
match builder.spawn() {
Ok(process) => {
let pty = Pty {
process,
fd: master,
};
pty
}
Err(e) => {
panic!("Failed to create pty: {}", e);
}
}
}
fn main() {
let shell = "/bin/bish";
let pty = create_pty(shell);
println!("{:?}", pty);
}
and the console output(The second error can be ignored for now):
error[E0308]: mismatched types
--> src/main.rs:42:24
|
42 | let slave: c_int = ptsname(master as i32);
| ^^^^^^^^^^^^^^^^^^^^^^ expected i32, found *-ptr
|
= note: expected type `i32`
found type `*mut i8`
error[E0060]: this function takes at least 2 parameters but 1 parameter was supplied
--> src/main.rs:43:13
|
43 | slave = libc::open(slave);
| ^^^^^^^^^^^^^^^^^ expected at least 2 parameters
error: aborting due to 2 previous errors
Some errors have detailed explanations: E0060, E0308.
For more information about an error, try `rustc --explain E0060`.
error: could not compile `experiment`.

Answer 1

It's not saying that it requires an input of i32, but rather that you're asking that ptsname(master as i32); has the type i32. This might be a bit confusing because c_int is an alias for i32, so it sounds like it's asking for an unrelated type.
The problem is that you're giving slave the type c_int, when ptsname returns *mut c_char (c_char is also an alias, this time for i8).