Supply win32 pread and pwrite

CHANGES.md

@@ -1,8 +1,8 @@
 ## Unreleased
 
 - Support all version of cmdliner (#386)
-- Support MSVC compiler. Still missing implementations of pread and
-  pwrite. (#383)
+- Support MSVC compiler. Supply Windows implementations of pread and pwrite.
+  (#383)
 
 # 1.6.0 (2022-02-12)
 

src/unix/pread.c

@@ -5,8 +5,53 @@
 #include <caml/unixsupport.h>
 
 #ifdef _MSC_VER
+#include <Windows.h>
 #include <BaseTsd.h>
+#include <Intsafe.h>
 typedef SSIZE_T ssize_t;
+typedef HANDLE file_descr_t;
+#define File_descr_val Handle_val
+
+ssize_t pread(file_descr_t fd, void *buf, size_t count, size_t offset)
+{
+  OVERLAPPED oOverlap;
+  BOOL bResult;
+  DWORD dwError;
+  ssize_t ret;
+  HRESULT hr;
+
+  /* https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-readfile#synchronization-and-file-position */
+  oOverlap.Internal     = 0;
+  oOverlap.InternalHigh = 0;
+  oOverlap.Offset       = LODWORD(offset);
+  oOverlap.OffsetHigh   = HIDWORD(offset);
+  oOverlap.hEvent       = 0;
+  bResult = ReadFile(fd, buf, count, NULL, &oOverlap);
+
+  if (!bResult) {
+    /* ReadFile failed, or is completing asynchrously if opened with
+      FILE_FLAG_OVERLAPPED. OCaml does not use FILE_FLAG_OVERLAPPED. */
+    dwError = GetLastError();
+    if (dwError == ERROR_HANDLE_EOF) {
+      /* pread returns 0 on EOF */
+      ret = 0;
+    } else {
+      /* pread returns -1 on any error */
+      ret = -1;
+    }
+  } else {
+    /* pread returns number of bytes read */
+    hr = ULongPtrToSSIZET(oOverlap.InternalHigh, &ret);
+    if(hr != S_OK) {
+      /* overflow. too many bytes read */
+      ret = -1;
+    }
+  }
+  return ret;
+}
+#else
+typedef size_t file_descr_t;
+#define File_descr_val Int_val
 #endif
 
 CAMLprim value caml_index_pread_int
@@ -15,7 +60,7 @@ CAMLprim value caml_index_pread_int
   CAMLparam5(v_fd, v_fd_off, v_buf, v_buf_off, v_len);
 
   ssize_t ret;
-  size_t fd = Int_val(v_fd);
+  file_descr_t fd = File_descr_val(v_fd);
   size_t fd_off = Long_val(v_fd_off);
   size_t buf_off = Long_val(v_buf_off);
   size_t len = Long_val(v_len);
@@ -34,7 +79,7 @@ CAMLprim value caml_index_pread_int64
   CAMLparam5(v_fd, v_fd_off, v_buf, v_buf_off, v_len);
 
   ssize_t ret;
-  size_t fd = Int_val(v_fd);
+  file_descr_t fd = File_descr_val(v_fd);
   size_t fd_off = Int64_val(v_fd_off);
   size_t buf_off = Long_val(v_buf_off);
   size_t len = Long_val(v_len);

src/unix/pwrite.c

@@ -5,8 +5,50 @@
 #include <caml/unixsupport.h>
 
 #ifdef _MSC_VER
+#include <Windows.h>
 #include <BaseTsd.h>
+#include <Intsafe.h>
 typedef SSIZE_T ssize_t;
+typedef HANDLE file_descr_t;
+#define File_descr_val Handle_val
+
+ssize_t pwrite(file_descr_t fd, const void *buf, size_t count, size_t offset)
+{
+  OVERLAPPED oOverlap;
+  DWORD dwBytesWritten;
+  BOOL bResult;
+  DWORD dwError;
+  ssize_t ret;
+  HRESULT hr;
+
+  /* https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-writefile#synchronization-and-file-position */
+  /*    &dwBytesWritten - ignored but non-NULL for Windows 7 compatibility */
+  oOverlap.Internal     = 0;
+  oOverlap.InternalHigh = 0;
+  oOverlap.Offset       = LODWORD(offset);
+  oOverlap.OffsetHigh   = HIDWORD(offset);
+  oOverlap.hEvent       = 0;
+  bResult = WriteFile(fd, buf, count, &dwBytesWritten, &oOverlap);
+
+  if (!bResult) {
+    /* WriteFile failed, or is completing asynchrously if opened with
+      FILE_FLAG_OVERLAPPED. OCaml does not use FILE_FLAG_OVERLAPPED. */
+    dwError = GetLastError();
+    /* pwrite returns -1 on any error */
+    ret = -1;
+  } else {
+    /* pwrite returns number of bytes written */
+    hr = ULongPtrToSSIZET(oOverlap.InternalHigh, &ret);
+    if(hr != S_OK) {
+      /* overflow. too many bytes written */
+      ret = -1;
+    }
+  }
+  return ret;
+}
+#else
+typedef size_t file_descr_t;
+#define File_descr_val Int_val
 #endif
 
 CAMLprim value caml_index_pwrite_int
@@ -15,7 +57,7 @@ CAMLprim value caml_index_pwrite_int
   CAMLparam5(v_fd, v_fd_off, v_buf, v_buf_off, v_len);
 
   ssize_t ret;
-  size_t fd = Int_val(v_fd);
+  file_descr_t fd = File_descr_val(v_fd);
   size_t fd_off = Long_val(v_fd_off);
   size_t buf_off = Long_val(v_buf_off);
   size_t len = Long_val(v_len);
@@ -34,7 +76,7 @@ CAMLprim value caml_index_pwrite_int64
   CAMLparam5(v_fd, v_fd_off, v_buf, v_buf_off, v_len);
 
   ssize_t ret;
-  size_t fd = Int_val(v_fd);
+  file_descr_t fd = File_descr_val(v_fd);
   size_t fd_off = Int64_val(v_fd_off);
   size_t buf_off = Long_val(v_buf_off);
   size_t len = Long_val(v_len);