[flang] Implement FNUM() (#159433)

The GNU Fortran library function FNUM(u) returns the UNIX file
descriptor that corresponds to an open Fortran unit number, if any;
otherwise -1.

This implementation is a library extension only, not an intrinsic.

3 files changed, 11 insertions, 0 deletions

diff --git a/flang-rt/include/flang-rt/runtime/file.h b/flang-rt/include/flang-rt/runtime/file.h
index 468a759214b8..25942c053bbe 100644
--- a/flang-rt/include/flang-rt/runtime/file.h
+++ b/flang-rt/include/flang-rt/runtime/file.h
@@ -27,6 +27,7 @@ class OpenFile {
 public:
   using FileOffset = std::int64_t;
 
+  int fd() const { return fd_; }
   const char *path() const { return path_.get(); }
   std::size_t pathLength() const { return pathLength_; }
   void set_path(OwningPtr<char> &&, std::size_t bytes);
diff --git a/flang-rt/lib/runtime/extensions.cpp b/flang-rt/lib/runtime/extensions.cpp
index 2c42597a5654..19e75143705a 100644
--- a/flang-rt/lib/runtime/extensions.cpp
+++ b/flang-rt/lib/runtime/extensions.cpp
@@ -424,6 +424,15 @@ std::int64_t RTNAME(Ftell)(int unitNumber) {
     return -1;
   }
 }
+
+std::int32_t FORTRAN_PROCEDURE_NAME(fnum)(const int &unitNumber) {
+  if (ExternalFileUnit * unit{ExternalFileUnit::LookUp(unitNumber)}) {
+    return unit->fd();
+  } else {
+    return -1;
+  }
+}
+
 } // namespace io
 
 } // extern "C"
diff --git a/flang-rt/lib/runtime/unit.h b/flang-rt/lib/runtime/unit.h
index 5ea52d1907f6..7aeea0931e01 100644
--- a/flang-rt/lib/runtime/unit.h
+++ b/flang-rt/lib/runtime/unit.h
@@ -59,6 +59,7 @@ class PseudoOpenFile {
 public:
   using FileOffset = std::int64_t;
 
+  RT_API_ATTRS int fd() const { return 1 /*stdout*/; }
   RT_API_ATTRS const char *path() const { return nullptr; }
   RT_API_ATTRS std::size_t pathLength() const { return 0; }
   RT_API_ATTRS void set_path(OwningPtr<char> &&, std::size_t bytes) {}