From 7c9ae677067f24e16f964dbf090e6a579a3e7c26 Mon Sep 17 00:00:00 2001
From: "Edgar San Martin, Jr." <[EMAIL REDACTED]>
Date: Mon, 12 Jan 2026 14:39:06 -0500
Subject: [PATCH] Add write buffering to Windows SDL_IOStream (#12424) (#14794)
---
src/io/SDL_iostream.c | 151 ++++++++++++++++++++++++++++++++++++------
1 file changed, 130 insertions(+), 21 deletions(-)
diff --git a/src/io/SDL_iostream.c b/src/io/SDL_iostream.c
index 9639a66148c88..0f91022671ba3 100644
--- a/src/io/SDL_iostream.c
+++ b/src/io/SDL_iostream.c
@@ -70,6 +70,9 @@ typedef struct IOStreamWindowsData
void *data;
size_t size;
size_t left;
+ void *write_data;
+ size_t write_pos;
+ bool writable;
bool append;
bool autoclose;
} IOStreamWindowsData;
@@ -80,7 +83,8 @@ typedef struct IOStreamWindowsData
#define INVALID_SET_FILE_POINTER 0xFFFFFFFF
#endif
-#define READAHEAD_BUFFER_SIZE 1024
+#define READAHEAD_BUFFER_SIZE 1024
+#define WRITEBEHIND_BUFFER_SIZE 1024
static HANDLE SDLCALL windows_file_open(const char *filename, const char *mode)
{
@@ -150,6 +154,41 @@ static HANDLE SDLCALL windows_file_open(const char *filename, const char *mode)
return h;
}
+static bool windows_flush_write_buffer(IOStreamWindowsData *iodata,
+ SDL_IOStatus *status)
+{
+ if (iodata->write_pos == 0) {
+ return true; // Nothing to flush
+ }
+
+ // In append mode, seek to EOF before writing
+ if (iodata->append) {
+ LARGE_INTEGER windowsoffset;
+ windowsoffset.QuadPart = 0;
+ if (!SetFilePointerEx(iodata->h, windowsoffset,
+ &windowsoffset, FILE_END)) {
+ if (status) {
+ *status = SDL_IO_STATUS_ERROR;
+ }
+ WIN_SetError("Error seeking in datastream");
+ return false;
+ }
+ }
+
+ DWORD bytes;
+ if (!WriteFile(iodata->h, iodata->write_data,
+ (DWORD)iodata->write_pos, &bytes, NULL)) {
+ if (status) {
+ *status = SDL_IO_STATUS_ERROR;
+ }
+ WIN_SetError("Error writing to datastream");
+ return false;
+ }
+
+ iodata->write_pos = 0;
+ return true;
+}
+
static Sint64 SDLCALL windows_file_size(void *userdata)
{
IOStreamWindowsData *iodata = (IOStreamWindowsData *) userdata;
@@ -168,6 +207,10 @@ static Sint64 SDLCALL windows_file_seek(void *userdata, Sint64 offset, SDL_IOWhe
DWORD windowswhence;
LARGE_INTEGER windowsoffset;
+ if (!windows_flush_write_buffer(iodata, NULL)) {
+ return -1;
+ }
+
// FIXME: We may be able to satisfy the seek within buffered data
if ((whence == SDL_IO_SEEK_CUR) && (iodata->left)) {
offset -= iodata->left;
@@ -204,6 +247,10 @@ static size_t SDLCALL windows_file_read(void *userdata, void *ptr, size_t size,
size_t read_ahead;
DWORD bytes;
+ if (!windows_flush_write_buffer(iodata, status)) {
+ return 0;
+ }
+
if (iodata->left > 0) {
void *data = (char *)iodata->data +
iodata->size -
@@ -270,11 +317,20 @@ static size_t SDLCALL windows_file_read(void *userdata, void *ptr, size_t size,
static size_t SDLCALL windows_file_write(void *userdata, const void *ptr, size_t size, SDL_IOStatus *status)
{
- IOStreamWindowsData *iodata = (IOStreamWindowsData *) userdata;
- DWORD bytes;
+ IOStreamWindowsData *iodata = (IOStreamWindowsData *)userdata;
+ const Uint8 *src = (const Uint8 *)ptr;
+ size_t remaining = size;
+ size_t total_written = 0;
+ if (!iodata->writable) {
+ *status = SDL_IO_STATUS_READONLY;
+ return 0;
+ }
+
+ // Invalidate read-ahead buffer if it has data
if (iodata->left) {
- if (!SetFilePointer(iodata->h, -(LONG)iodata->left, NULL, FILE_CURRENT)) {
+ if (!SetFilePointer(iodata->h, -(LONG)iodata->left,
+ NULL, FILE_CURRENT)) {
*status = SDL_IO_STATUS_ERROR;
WIN_SetError("Error seeking in datastream");
return 0;
@@ -282,30 +338,66 @@ static size_t SDLCALL windows_file_write(void *userdata, const void *ptr, size_t
iodata->left = 0;
}
- // if in append mode, we must go to the EOF before write
- if (iodata->append) {
- LARGE_INTEGER windowsoffset;
- windowsoffset.QuadPart = 0;
- if (!SetFilePointerEx(iodata->h, windowsoffset, &windowsoffset, FILE_END)) {
+ // For large writes, flush buffer and write directly
+ if (size >= WRITEBEHIND_BUFFER_SIZE) {
+ if (!windows_flush_write_buffer(iodata, status)) {
+ return 0;
+ }
+
+ // In append mode, seek to EOF before direct write
+ if (iodata->append) {
+ LARGE_INTEGER windowsoffset;
+ windowsoffset.QuadPart = 0;
+ if (!SetFilePointerEx(iodata->h, windowsoffset,
+ &windowsoffset, FILE_END)) {
+ *status = SDL_IO_STATUS_ERROR;
+ WIN_SetError("Error seeking in datastream");
+ return 0;
+ }
+ }
+
+ DWORD bytes;
+ if (!WriteFile(iodata->h, ptr, (DWORD)size, &bytes, NULL)) {
*status = SDL_IO_STATUS_ERROR;
- WIN_SetError("Error seeking in datastream");
+ WIN_SetError("Error writing to datastream");
return 0;
+ } else if (bytes == 0 && size > 0) {
+ *status = SDL_IO_STATUS_NOT_READY;
}
+ return bytes;
}
- if (!WriteFile(iodata->h, ptr, (DWORD)size, &bytes, NULL)) {
- *status = SDL_IO_STATUS_ERROR;
- WIN_SetError("Error writing to datastream");
- return 0;
- } else if (bytes == 0 && size > 0) {
- *status = SDL_IO_STATUS_NOT_READY;
+ // Buffer small writes
+ while (remaining > 0) {
+ size_t space = WRITEBEHIND_BUFFER_SIZE - iodata->write_pos;
+ size_t to_buffer = SDL_min(remaining, space);
+
+ SDL_memcpy((char *)iodata->write_data + iodata->write_pos,
+ src, to_buffer);
+ iodata->write_pos += to_buffer;
+ src += to_buffer;
+ remaining -= to_buffer;
+ total_written += to_buffer;
+
+ if (iodata->write_pos == WRITEBEHIND_BUFFER_SIZE) {
+ if (!windows_flush_write_buffer(iodata, status)) {
+ return total_written;
+ }
+ }
}
- return bytes;
+
+ return total_written;
}
static bool SDLCALL windows_file_flush(void *userdata, SDL_IOStatus *status)
{
- IOStreamWindowsData *iodata = (IOStreamWindowsData *) userdata;
+ IOStreamWindowsData *iodata = (IOStreamWindowsData *)userdata;
+
+ if (!windows_flush_write_buffer(iodata, status)) {
+ return false;
+ }
+
+ // Sync to disk
if (!FlushFileBuffers(iodata->h)) {
return WIN_SetError("Error flushing datastream");
}
@@ -314,16 +406,23 @@ static bool SDLCALL windows_file_flush(void *userdata, SDL_IOStatus *status)
static bool SDLCALL windows_file_close(void *userdata)
{
- IOStreamWindowsData *iodata = (IOStreamWindowsData *) userdata;
+ IOStreamWindowsData *iodata = (IOStreamWindowsData *)userdata;
+ bool result = true;
+
+ if (!windows_flush_write_buffer(iodata, NULL)) {
+ result = false;
+ }
+
if (iodata->h != INVALID_HANDLE_VALUE) {
if (iodata->autoclose) {
CloseHandle(iodata->h);
}
- iodata->h = INVALID_HANDLE_VALUE; // to be sure
+ iodata->h = INVALID_HANDLE_VALUE;
}
SDL_free(iodata->data);
+ SDL_free(iodata->write_data);
SDL_free(iodata);
- return true;
+ return result;
}
SDL_IOStream *SDL_IOFromHandle(HANDLE handle, const char *mode, bool autoclose)
@@ -348,6 +447,9 @@ SDL_IOStream *SDL_IOFromHandle(HANDLE handle, const char *mode, bool autoclose)
iface.close = windows_file_close;
iodata->h = handle;
+ iodata->writable = (SDL_strchr(mode, 'w') != NULL) ||
+ (SDL_strchr(mode, 'a') != NULL) ||
+ (SDL_strchr(mode, '+') != NULL);
iodata->append = (SDL_strchr(mode, 'a') != NULL);
iodata->autoclose = autoclose;
@@ -357,6 +459,13 @@ SDL_IOStream *SDL_IOFromHandle(HANDLE handle, const char *mode, bool autoclose)
return NULL;
}
+ iodata->write_data = (char *)SDL_malloc(WRITEBEHIND_BUFFER_SIZE);
+ if (!iodata->write_data) {
+ iface.close(iodata);
+ return NULL;
+ }
+ iodata->write_pos = 0;
+
SDL_IOStream *iostr = SDL_OpenIO(&iface, iodata);
if (!iostr) {
iface.close(iodata);