Auto merge of #3356 - magicant:fd_setsize_type, r=JohnTitor

Change `FD_SETSIZE` to `c_int`

`FD_SETSIZE` defines an upper bound for file descriptors that can be added to `fd_set`. It is common to check if a file descriptor is less than `FD_SETSIZE` before adding it to an `fd_set`. See the example below:

```rust
fn main() {
    let path = std::ffi::CStr::from_bytes_with_nul(b"/dev/null\0").unwrap();
    let fd = unsafe { libc::open(path.as_ptr(), libc::O_RDONLY) };
    if fd < 0 {
        eprint!("open failed\n");
        return;
    }

    let mut set = std::mem::MaybeUninit::<libc::fd_set>::uninit();
    unsafe { libc::FD_ZERO(set.as_mut_ptr()) }
    if fd < libc::FD_SETSIZE { // <- type mismatch!
        unsafe { libc::FD_SET(fd, set.as_mut_ptr()) }
    } else {
        eprint!("too large fd\n");
    }
}
```

Unfortunately, this example does not compile because of a type mismatch. `fd` is `c_int` while `FD_SETSIZE` is `usize` (or `size_t`).
Since `FD_SETSIZE` represents the max file descriptor + 1, I think it should have the same type as file descriptors. This pull request modifies the type to `c_int` and adds casts where needed.

Author: bors

src/fuchsia/mod.rs

@@ -380,7 +380,7 @@ s! {
     }
 
     pub struct fd_set {
-        fds_bits: [::c_ulong; FD_SETSIZE / ULONG_SIZE],
+        fds_bits: [::c_ulong; FD_SETSIZE as usize / ULONG_SIZE],
     }
 
     pub struct tm {
@@ -1827,7 +1827,7 @@ pub const SS_DISABLE: ::c_int = 2;
 
 pub const PATH_MAX: ::c_int = 4096;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const EPOLLIN: ::c_int = 0x1;
 pub const EPOLLPRI: ::c_int = 0x2;

src/unix/aix/mod.rs

@@ -2118,7 +2118,7 @@ pub const POWER_8: ::c_int = 0x10000;
 pub const POWER_9: ::c_int = 0x20000;
 
 // sys/time.h
-pub const FD_SETSIZE: usize = 65534;
+pub const FD_SETSIZE: ::c_int = 65534;
 pub const TIMEOFDAY: ::c_int = 9;
 pub const CLOCK_REALTIME: ::clockid_t = TIMEOFDAY as clockid_t;
 pub const CLOCK_MONOTONIC: ::clockid_t = 10;

src/unix/bsd/apple/mod.rs

@@ -4384,7 +4384,7 @@ pub const OS_SIGNPOST_INTERVAL_END: ::os_signpost_type_t = 0x02;
 pub const MINSIGSTKSZ: ::size_t = 32768;
 pub const SIGSTKSZ: ::size_t = 131072;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const ST_NOSUID: ::c_ulong = 2;
 

src/unix/bsd/freebsdlike/mod.rs

@@ -1193,7 +1193,7 @@ pub const SCHED_FIFO: ::c_int = 1;
 pub const SCHED_OTHER: ::c_int = 2;
 pub const SCHED_RR: ::c_int = 3;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const ST_NOSUID: ::c_ulong = 2;
 

src/unix/bsd/mod.rs

@@ -59,10 +59,10 @@ s! {
     pub struct fd_set {
         #[cfg(all(target_pointer_width = "64",
                   any(target_os = "freebsd", target_os = "dragonfly")))]
-        fds_bits: [i64; FD_SETSIZE / 64],
+        fds_bits: [i64; FD_SETSIZE as usize / 64],
         #[cfg(not(all(target_pointer_width = "64",
                       any(target_os = "freebsd", target_os = "dragonfly"))))]
-        fds_bits: [i32; FD_SETSIZE / 32],
+        fds_bits: [i32; FD_SETSIZE as usize / 32],
     }
 
     pub struct tm {

src/unix/bsd/netbsdlike/netbsd/mod.rs

@@ -1847,7 +1847,7 @@ pub const _SC_SCHED_RT_TS: ::c_int = 2001;
 pub const _SC_SCHED_PRI_MIN: ::c_int = 2002;
 pub const _SC_SCHED_PRI_MAX: ::c_int = 2003;
 
-pub const FD_SETSIZE: usize = 0x100;
+pub const FD_SETSIZE: ::c_int = 0x100;
 
 pub const ST_NOSUID: ::c_ulong = 8;
 

src/unix/bsd/netbsdlike/openbsd/mod.rs

@@ -1422,7 +1422,7 @@ pub const _SC_AVPHYS_PAGES: ::c_int = 501;
 pub const _SC_NPROCESSORS_CONF: ::c_int = 502;
 pub const _SC_NPROCESSORS_ONLN: ::c_int = 503;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const SCHED_FIFO: ::c_int = 1;
 pub const SCHED_OTHER: ::c_int = 2;

src/unix/haiku/mod.rs

@@ -1100,7 +1100,7 @@ pub const SA_ONESHOT: ::c_int = SA_RESETHAND;
 pub const SS_ONSTACK: ::c_int = 0x1;
 pub const SS_DISABLE: ::c_int = 0x2;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const RTLD_LOCAL: ::c_int = 0x0;
 pub const RTLD_NOW: ::c_int = 0x1;

src/unix/hurd/mod.rs

@@ -1211,7 +1211,7 @@ pub const SHM_UNLOCK: ::c_int = 12;
 pub const STDIN_FILENO: ::c_int = 0;
 pub const STDOUT_FILENO: ::c_int = 1;
 pub const STDERR_FILENO: ::c_int = 2;
-pub const __FD_SETSIZE: usize = 256;
+pub const __FD_SETSIZE: ::c_int = 256;
 pub const R_OK: ::c_int = 4;
 pub const W_OK: ::c_int = 2;
 pub const X_OK: ::c_int = 1;
@@ -1256,7 +1256,7 @@ pub const PDP_ENDIAN: usize = 3412;
 pub const BYTE_ORDER: usize = 1234;
 
 // sys/select.h
-pub const FD_SETSIZE: usize = 256;
+pub const FD_SETSIZE: ::c_int = 256;
 pub const __SIZEOF_PTHREAD_MUTEX_T: usize = 32;
 pub const __SIZEOF_PTHREAD_ATTR_T: usize = 32;
 pub const __SIZEOF_PTHREAD_RWLOCK_T: usize = 28;
@@ -1588,8 +1588,8 @@ pub const _POSIX_MQ_OPEN_MAX: usize = 8;
 pub const _POSIX_MQ_PRIO_MAX: usize = 32;
 pub const _POSIX_NAME_MAX: usize = 14;
 pub const _POSIX_NGROUPS_MAX: usize = 8;
-pub const _POSIX_OPEN_MAX: usize = 20;
-pub const _POSIX_FD_SETSIZE: usize = 20;
+pub const _POSIX_OPEN_MAX: ::c_int = 20;
+pub const _POSIX_FD_SETSIZE: ::c_int = 20;
 pub const _POSIX_PATH_MAX: usize = 256;
 pub const _POSIX_PIPE_BUF: usize = 512;
 pub const _POSIX_RE_DUP_MAX: usize = 255;

src/unix/linux_like/mod.rs

@@ -85,7 +85,7 @@ s! {
     }
 
     pub struct fd_set {
-        fds_bits: [::c_ulong; FD_SETSIZE / ULONG_SIZE],
+        fds_bits: [::c_ulong; FD_SETSIZE as usize / ULONG_SIZE],
     }
 
     pub struct tm {
@@ -1065,7 +1065,7 @@ pub const PATH_MAX: ::c_int = 4096;
 
 pub const UIO_MAXIOV: ::c_int = 1024;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const EPOLLIN: u32 = 0x1;
 pub const EPOLLPRI: u32 = 0x2;