libc: add support for open_memstream

Adds support for POSIX interface open_memstream() that allows writing
to dynamic memory buffer stream. The stream is dynamically reallocated
as the buffer grows with initial size set to zero.

The caller has to free the buffer after the stream is closed.

The implementation uses fopencookie() for custom stream operations and
callbacks.

Signed-off-by: Michal Lenc <michallenc@seznam.cz>

include/stdio.h

@@ -254,6 +254,7 @@ FAR FILE *fopencookie(FAR void *cookie, FAR const char *mode,
 /* Memory buffer stream */
 
 FAR FILE *fmemopen(FAR void *buf, size_t size, FAR const char *mode);
+FAR FILE *open_memstream(FAR char **bufp, FAR size_t *sizep);
 
 /* Operations on paths */
 

libs/libc/stdio/Make.defs

@@ -48,7 +48,7 @@ CSRCS += lib_feof.c lib_ferror.c lib_rewind.c lib_clearerr.c
 CSRCS += lib_scanf.c lib_vscanf.c lib_fscanf.c lib_vfscanf.c lib_tmpfile.c
 CSRCS += lib_setbuf.c lib_setvbuf.c lib_libfilelock.c lib_libgetstreams.c
 CSRCS += lib_setbuffer.c lib_fputwc.c lib_putwc.c lib_fputws.c
-CSRCS += lib_fopencookie.c lib_fmemopen.c
+CSRCS += lib_fopencookie.c lib_fmemopen.c lib_open_memstream.c
 endif
 
 # Add the stdio directory to the build

libs/libc/stdio/lib_open_memstream.c

@@ -0,0 +1,227 @@
+/****************************************************************************
+ * libs/libc/stdio/lib_open_memstream.c
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <string.h>
+#include <assert.h>
+#include <errno.h>
+
+#include "libc.h"
+
+/****************************************************************************
+ * Private Types
+ ****************************************************************************/
+
+struct memstream_cookie_s
+{
+  FAR char **buf;   /* Memory buffer */
+  char saved;       /* char at sizep before '\0' */
+  size_t *sizep;
+  off_t size;       /* Allocated buffer size */
+  off_t end;        /* Maximum position we have written to */
+  off_t pos;        /* Current position in the buffer */
+};
+
+/****************************************************************************
+ * Private Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: memstream_write
+ ****************************************************************************/
+
+static ssize_t memstream_write(FAR void *c, FAR const char *buf,
+                               size_t size)
+{
+  FAR struct memstream_cookie_s *memstream_cookie =
+    (FAR struct memstream_cookie_s *)c;
+  FAR char *buf_grow;
+  size_t new_size;
+
+  if (memstream_cookie->pos + size + 1 > memstream_cookie->size)
+    {
+      /* We have to reallocate the buffer. */
+
+      new_size = memstream_cookie->pos + size + 1;
+      buf_grow = lib_realloc(*memstream_cookie->buf, new_size);
+      if (buf_grow == NULL)
+        {
+          return -ENOMEM;
+        }
+
+      memset(buf_grow + memstream_cookie->end, 0,
+            new_size - memstream_cookie->size);
+
+      *memstream_cookie->buf = buf_grow;
+      memstream_cookie->size = new_size;
+    }
+
+  memcpy(*memstream_cookie->buf + memstream_cookie->pos, buf, size);
+
+  /* POSIX: If a write moves the position to a value larger than
+   * the current length, the current length shall be set to this position.
+   * In this case a null character shall be appended to the current buffer.
+   */
+
+  memstream_cookie->pos += size;
+  if (memstream_cookie->pos > memstream_cookie->end)
+    {
+      memstream_cookie->end = memstream_cookie->pos;
+    }
+  else
+    {
+      memstream_cookie->saved = *(*memstream_cookie->buf +
+        memstream_cookie->pos);
+    }
+
+  *memstream_cookie->sizep = memstream_cookie->pos;
+  *(*memstream_cookie->buf + memstream_cookie->pos) = '\0';
+
+  return size;
+}
+
+/****************************************************************************
+ * Name: memstream_seek
+ ****************************************************************************/
+
+static off_t memstream_seek(FAR void *c, FAR off_t *offset, int whence)
+{
+  FAR struct memstream_cookie_s *memstream_cookie =
+    (FAR struct memstream_cookie_s *)c;
+  off_t new_offset;
+
+  switch (whence)
+    {
+      case SEEK_SET:
+        new_offset = *offset;
+        break;
+      case SEEK_END:
+        new_offset = memstream_cookie->end + *offset;
+        break;
+      case SEEK_CUR:
+        new_offset = memstream_cookie->pos + *offset;
+        break;
+      default:
+        set_errno(ENOTSUP);
+        return -1;
+    }
+
+  /* Seek to negative value or value larger than maximum size shall fail. */
+
+  if (new_offset < 0 || new_offset > memstream_cookie->end)
+    {
+      set_errno(EINVAL);
+      return -1;
+    }
+
+  if (memstream_cookie->pos < memstream_cookie->end)
+    {
+      /* Retrieve saved character if we painted in already written area. */
+
+      *(*memstream_cookie->buf + memstream_cookie->pos) =
+        memstream_cookie->saved;
+    }
+
+  memstream_cookie->pos = new_offset;
+  if (memstream_cookie->pos < memstream_cookie->end)
+    {
+      /* We go backwards, therefore we have to write null character
+       * at memstream_cookie->pos. But we might want to keep this
+       * character for future seeks, so keep it in memstream_cookie->saved.
+       */
+
+      memstream_cookie->saved = *(*memstream_cookie->buf +
+        memstream_cookie->pos);
+      *(*memstream_cookie->buf + memstream_cookie->pos) = '\0';
+      *memstream_cookie->sizep = memstream_cookie->pos;
+    }
+  else
+    {
+      *memstream_cookie->sizep = memstream_cookie->end;
+    }
+
+  *offset = new_offset;
+  return new_offset;
+}
+
+/****************************************************************************
+ * Name: memstream_close
+ ****************************************************************************/
+
+static int memstream_close(FAR void *c)
+{
+  FAR struct memstream_cookie_s *memstream_cookie =
+    (FAR struct memstream_cookie_s *)c;
+
+  lib_free(memstream_cookie);
+  return 0;
+}
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+FAR FILE *open_memstream(FAR char **bufp, FAR size_t *sizep)
+{
+  cookie_io_functions_t memstream_io;
+  FAR struct memstream_cookie_s *memstream_cookie;
+  FAR FILE *filep;
+
+  if (bufp == NULL || sizep == NULL)
+    {
+      set_errno(EINVAL);
+      return NULL;
+    }
+
+  memstream_cookie = lib_zalloc(sizeof(struct memstream_cookie_s));
+  if (memstream_cookie == NULL)
+    {
+      set_errno(ENOMEM);
+      return NULL;
+    }
+
+  *bufp = NULL;
+  *sizep = 0;
+  memstream_cookie->buf   = bufp;
+  memstream_cookie->sizep = sizep;
+
+  memstream_io.read   = NULL;
+  memstream_io.write  = memstream_write;
+  memstream_io.seek   = memstream_seek;
+  memstream_io.close  = memstream_close;
+
+  filep = fopencookie(memstream_cookie, "w", memstream_io);
+  if (filep == NULL)
+    {
+      lib_free(memstream_cookie);
+      return NULL;
+    }
+
+  return filep;
+}