[clang-tools-extra] [llvm] [flang] [clang] [flang ]GETLOG runtime and extension implementation: get login username (PR #70917)

[via cfe-commits]

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@@ -37,5 +75,19 @@ void FORTRAN_PROCEDURE_NAME(getarg)(
   (void)RTNAME(GetCommandArgument)(
       n, &value, nullptr, nullptr, __FILE__, __LINE__);
 }
+
+void FORTRAN_PROCEDURE_NAME(getlog)(std::int8_t *arg, std::int64_t length) {
+  std::array<char, LOGIN_NAME_MAX + 1> str;
+  int error = getlogin_r(str.data(), str.size());
+  assert(error == 0 && "getlogin_r returned an error");
+
+  // Trim space from right/end
+  int i = str.size();
+  while (' ' == str[--i]) {
+    str[i] = 0;
+  }
+  strncpy(reinterpret_cast<char *>(arg), str.data(), length);
+}
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jeanPerier wrote:

I think you also need to pad the result argument with blanks for all the characters of the result that were not filled with the login  (including the copied `0` terminator)  (if it was copied).
Padding with blanks is the usual Fortran convention for INTENT(OUT) string passed to intrinsics, GETLOG does not mention it, but I tested with gfortran and it does pad with blanks.

Example on some system I am called `jp`

```
  character(10) :: string
  string = "ZZZZZZZZZZ"
  call getlog(string)
  write(*, "(AA)") string, "X"
end
```
outputs  `jp        X`, so the 8 characters after the login name were padded with blanks.

Maybe you could share and re-use the copyBufferAndPad logic at:
https://github.com/llvm/llvm-project/blob/1b781ee9284ded54aabd88690a726b615c29cc2d/flang/runtime/time-intrinsic.cpp#L282

https://github.com/llvm/llvm-project/pull/70917