Add current_pid function

Fixes rust-lang#44971

src/libstd/process.rs

@@ -1167,6 +1167,24 @@ pub fn abort() -> ! {
     unsafe { ::sys::abort_internal() };
 }
 
+/// Returns the OS-assigned process identifier associated with this process.
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```no_run
+/// use std::process:current_pid;
+///
+/// println!("My pid is {}", current_pid());
+/// ```
+///
+///
+#[unstable(feature = "getpid", issue = "44971", reason = "recently added")]
+pub fn current_pid() -> u32 {
+    ::sys::os::getpid()
+}
+
 #[cfg(all(test, not(target_os = "emscripten")))]
 mod tests {
     use io::prelude::*;

src/libstd/sys/redox/os.rs

@@ -209,3 +209,7 @@ pub fn exit(code: i32) -> ! {
     let _ = syscall::exit(code as usize);
     unreachable!();
 }
+
+pub fn getpid() -> u32 {
+    syscall::getpid().unwrap() as u32
+}

src/libstd/sys/unix/os.rs

@@ -513,3 +513,7 @@ pub fn home_dir() -> Option<PathBuf> {
 pub fn exit(code: i32) -> ! {
     unsafe { libc::exit(code as c_int) }
 }
+
+pub fn getpid() -> u32 {
+    unsafe { libc::getpid() as u32 }
+}

src/libstd/sys/windows/os.rs

@@ -318,6 +318,10 @@ pub fn exit(code: i32) -> ! {
     unsafe { c::ExitProcess(code as c::UINT) }
 }
 
+pub fn getpid() -> u32 {
+    unsafe { c::GetCurrentProcessId() as u32 }
+}
+
 #[cfg(test)]
 mod tests {
     use io::Error;