From 14b74b1ab4081138d360bb7402c66712fa60a726 Mon Sep 17 00:00:00 2001
From: Yuki Okushi <huyuumi.dev@gmail.com>
Date: Tue, 28 Jul 2026 07:22:47 +0900
Subject: [PATCH] std: make positioned I/O unsupported on VxWorks

---
 library/std/src/sys/fd/unix.rs | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/library/std/src/sys/fd/unix.rs b/library/std/src/sys/fd/unix.rs
index a96df3972be6a..28115c1b1743c 100644
--- a/library/std/src/sys/fd/unix.rs
+++ b/library/std/src/sys/fd/unix.rs
@@ -8,6 +8,7 @@ mod tests;
     target_os = "l4re",
     target_os = "android",
     target_os = "hurd",
+    target_os = "vxworks",
 )))]
 use libc::off_t as off64_t;
 #[cfg(any(
@@ -18,6 +19,7 @@ use libc::off_t as off64_t;
 ))]
 use libc::off64_t;
 
+#[cfg(not(target_os = "vxworks"))]
 cfg_select! {
     any(
         all(target_os = "linux", not(target_env = "musl")),
@@ -45,6 +47,12 @@ use crate::sys::pal::weak::syscall;
 use crate::sys::pal::weak::weak;
 use crate::sys::{AsInner, FromInner, IntoInner, cvt};
 
+// VxWorks does not have pread/pwrite, so we here define an error to tell it's unsupported.
+// ref. <https://github.com/rust-lang/libc/issues/5328>
+#[cfg(target_os = "vxworks")]
+const POSITIONED_IO_UNSUPPORTED: io::Error =
+    io::const_error!(io::ErrorKind::Unsupported, "positioned I/O is not supported on VxWorks",);
+
 #[derive(Debug)]
 pub struct FileDesc(OwnedFd);
 
@@ -166,6 +174,7 @@ impl FileDesc {
         (&mut me).read_to_end(buf)
     }
 
+    #[cfg(not(target_os = "vxworks"))]
     pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
         cvt(unsafe {
             pread64(
@@ -178,6 +187,11 @@ impl FileDesc {
         .map(|n| n as usize)
     }
 
+    #[cfg(target_os = "vxworks")]
+    pub fn read_at(&self, _buf: &mut [u8], _offset: u64) -> io::Result<usize> {
+        Err(POSITIONED_IO_UNSUPPORTED)
+    }
+
     pub fn read_buf(&self, mut cursor: BorrowedCursor<'_, u8>) -> io::Result<()> {
         // SAFETY: `cursor.as_mut()` starts with `cursor.capacity()` writable bytes
         let ret = cvt(unsafe {
@@ -195,6 +209,7 @@ impl FileDesc {
         Ok(())
     }
 
+    #[cfg(not(target_os = "vxworks"))]
     pub fn read_buf_at(&self, mut cursor: BorrowedCursor<'_, u8>, offset: u64) -> io::Result<()> {
         // SAFETY: `cursor.as_mut()` starts with `cursor.capacity()` writable bytes
         let ret = cvt(unsafe {
@@ -213,6 +228,11 @@ impl FileDesc {
         Ok(())
     }
 
+    #[cfg(target_os = "vxworks")]
+    pub fn read_buf_at(&self, _cursor: BorrowedCursor<'_, u8>, _offset: u64) -> io::Result<()> {
+        Err(POSITIONED_IO_UNSUPPORTED)
+    }
+
     #[cfg(any(
         target_os = "aix",
         target_os = "dragonfly", // DragonFly 1.5
@@ -397,6 +417,7 @@ impl FileDesc {
         )))
     }
 
+    #[cfg(not(target_os = "vxworks"))]
     pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
         #[cfg(not(any(
             all(target_os = "linux", not(target_env = "musl")),
@@ -422,6 +443,11 @@ impl FileDesc {
         }
     }
 
+    #[cfg(target_os = "vxworks")]
+    pub fn write_at(&self, _buf: &[u8], _offset: u64) -> io::Result<usize> {
+        Err(POSITIONED_IO_UNSUPPORTED)
+    }
+
     #[cfg(any(
         target_os = "aix",
         target_os = "dragonfly", // DragonFly 1.5