uint8_t *
bson_reserve_buffer (bson_t *bson, uint32_t size);


  • bson: An initialized bson_t.
  • size: The length in bytes of the new buffer.


Grow the internal buffer of bson to size and set the document length to size. Useful for eliminating copies when reading BSON bytes from a stream.

First, initialize bson with bson_init() or bson_new(), then call this function. After it returns, the length of bson is set to size but its contents are uninitialized memory: you must fill the contents with a BSON document of the correct length before any other operations.

The document must be freed with bson_destroy().


A pointer to the internal buffer, which is at least size bytes, or NULL if the space could not be allocated.


Use bson_reserve_buffer to write a function that takes a bson_t pointer and reads a file into it directly:

#include <stdio.h>
#include <bson/bson.h>

read_into (bson_t *bson, FILE *fp)
   uint8_t *buffer;
   long size;

   if (fseek (fp, 0L, SEEK_END) < 0) {
      perror ("Couldn't get file size");
      return 1;

   size = ftell (fp);
   if (size == EOF) {
      perror ("Couldn't get file size");
      return 1;

   if (size > INT32_MAX) {
      fprintf (stderr, "File too large\n");
      return 1;

   /* reserve buffer space - bson is temporarily invalid */
   buffer = bson_reserve_buffer (bson, (uint32_t) size);
   if (!buffer) {
      fprintf (stderr, "Couldn't reserve %ld bytes", size);
      return false;

   /* read file directly into the buffer */
   rewind (fp);
   if (fread ((void *) buffer, 1, (size_t) size, fp) < (size_t) size) {
      perror ("Couldn't read file");
      return false;

   return true;

main ()
   FILE *fp;
   char *json;

   /* stack-allocated, initialized bson_t */
   bson_t bson = BSON_INITIALIZER;

   if (!(fp = fopen ("document.bson", "rb"))) {
      perror ("Couldn't read file");
      return 1;

   read_into (&bson, fp);
   fclose (fp);

   json = bson_as_canonical_extended_json (&bson, NULL);
   printf ("%s\n", json);

   bson_free (json);
   bson_destroy (&bson);

   return 0;