[flang][runtime] Add ACCESS library procedure

flang/docs/Intrinsics.md

@@ -657,6 +657,14 @@ CALL CO_REDUCE
 CALL CO_SUM
 ```
 
+### Inquiry Functions
+ACCESS (GNU extension) is not supported on Windows. Otherwise:
+```
+CHARACTER(LEN=*) :: path = 'path/to/file'
+IF (ACCESS(path, 'rwx')) &
+  ...
+```
+
 ## Non-standard intrinsics
 ### PGI
 ```

flang/include/flang/Runtime/extensions.h

@@ -44,5 +44,9 @@ std::int64_t RTNAME(Signal)(std::int64_t number, void (*handler)(int));
 // GNU extension subroutine SLEEP(SECONDS)
 void RTNAME(Sleep)(std::int64_t seconds);
 
+// GNU extension function ACCESS(NAME, MODE)
+std::int64_t FORTRAN_PROCEDURE_NAME(access)(const char *name,
+    std::int64_t nameLength, const char *mode, std::int64_t modeLength);
+
 } // extern "C"
 #endif // FORTRAN_RUNTIME_EXTENSIONS_H_

flang/runtime/extensions.cpp

@@ -17,6 +17,7 @@
 #include "flang/Runtime/entry-names.h"
 #include "flang/Runtime/io-api.h"
 #include <chrono>
+#include <cstring>
 #include <ctime>
 #include <signal.h>
 #include <thread>
@@ -138,5 +139,77 @@ void RTNAME(Sleep)(std::int64_t seconds) {
   std::this_thread::sleep_for(std::chrono::seconds(seconds));
 }
 
+// TODO: not supported on Windows
+#ifndef _WIN32
+std::int64_t FORTRAN_PROCEDURE_NAME(access)(const char *name,
+    std::int64_t nameLength, const char *mode, std::int64_t modeLength) {
+  std::int64_t ret = -1;
+  if (nameLength <= 0 || modeLength <= 0 || !name || !mode) {
+    return ret;
+  }
+
+  // ensure name is null terminated
+  char *newName = nullptr;
+  if (name[nameLength - 1] != '\0') {
+    newName = static_cast<char *>(std::malloc(nameLength + 1));
+    std::memcpy(newName, name, nameLength);
+    newName[nameLength] = '\0';
+    name = newName;
+  }
+
+  // calculate mode
+  bool read = false;
+  bool write = false;
+  bool execute = false;
+  bool exists = false;
+  int imode = 0;
+
+  for (std::int64_t i = 0; i < modeLength; ++i) {
+    switch (mode[i]) {
+    case 'r':
+      read = true;
+      break;
+    case 'w':
+      write = true;
+      break;
+    case 'x':
+      execute = true;
+      break;
+    case ' ':
+      exists = true;
+      break;
+    default:
+      // invalid mode
+      goto cleanup;
+    }
+  }
+  if (!read && !write && !execute && !exists) {
+    // invalid mode
+    goto cleanup;
+  }
+
+  if (!read && !write && !execute) {
+    imode = F_OK;
+  } else {
+    if (read) {
+      imode |= R_OK;
+    }
+    if (write) {
+      imode |= W_OK;
+    }
+    if (execute) {
+      imode |= X_OK;
+    }
+  }
+  ret = access(name, imode);
+
+cleanup:
+  if (newName) {
+    free(newName);
+  }
+  return ret;
+}
+#endif
+
 } // namespace Fortran::runtime
 } // extern "C"