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# bson_reserve_buffer()

## Synopsis

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

## Parameters

- `bson`: An initialized bson_t.

- `size`: The length in bytes of the new buffer.

## Description

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().

## Returns

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

## Example

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>

    bool
    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;
    }

    int
    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;
    }

See also

bson_concat()
bson_init()
bson_init_from_json()
bson_init_static()
bson_new()
bson_new_from_buffer()
bson_new_from_data()
bson_new_from_json()
bson_reinit()
bson_reserve_buffer()
bson_sized_new()