nbd_pwrite3

Usage

 #include <libnbd.h>
 int nbd_pwrite (
       struct nbd_handle *h, const void *buf, size_t count,
       uint64_t offset, uint32_t flags
     );

Overview

Issue a write command to the NBD server, writing the data in "buf" to the range starting at "offset" and ending at "offset" + "count" - 1. NBD can only write all or nothing using this call. The call returns when the command has been acknowledged by the server, or there is an error. Note this will generally return an error if nbd_is_read_only(3) is true.

Note that libnbd defaults to enforcing a maximum write buffer of the lesser of 64MiB or any maximum payload size advertised by the server; attempts to exceed this will generally result in a client-side "ERANGE" error, rather than a server-side disconnection. The actual limit can be learned with nbd_get_block_size(3).

The "flags" parameter may be 0 for no flags, or may contain "LIBNBD_CMD_FLAG_FUA" meaning that the server should not return until the data has been committed to permanent storage (if that is supported - some servers cannot do this, see nbd_can_fua(3)). For convenience, unless nbd_set_strict_flags(3) was used to disable "LIBNBD_STRICT_AUTO_FLAG", libnbd ignores the presence or absence of the flag "LIBNBD_CMD_FLAG_PAYLOAD_LEN" in "flags", while correctly using the flag over the wire according to whether extended headers were negotiated.

By default, libnbd will reject attempts to use this function with parameters that are likely to result in server failure, such as requesting an unknown command flag. The nbd_set_strict_mode(3) function can be used to alter which scenarios should await a server reply rather than failing fast.

Return value

If the call is successful the function returns 0.

Errors

On error -1 is returned.

Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

The following parameters must not be NULL: "h", "buf". For more information see "Non-NULL parameters" in libnbd(3).

Handle state

nbd_pwrite can be called when the handle is in the following state:

 ┌─────────────────────────────────────┬─────────────────────────┐
 │ Handle created, before connecting   │ ❌ error                │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ❌ error                │
 │ Connected to the server             │ ✅ allowed              │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │
 └─────────────────────────────────────┴─────────────────────────┘

Version

This function first appeared in libnbd 1.0.

If you need to test if this function is available at compile time check if the following macro is defined:

 #define LIBNBD_HAVE_NBD_PWRITE 1

Examples

This example is also available as examples/reads-and-writes.c in the libnbd source code.

 /* This example shows how to do synchronous reads
  * and writes randomly over the first megabyte of an
  * NBD server.  Note this will destroy any existing
  * content on the NBD server.
  *
  * To test it with nbdkit and a RAM disk:
  *
  * nbdkit -U - memory 1M \
  *     --run './simple-reads-and-writes $unixsocket'
  */
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
 #include <inttypes.h>
 #include <assert.h>
 #include <time.h>
 #include <libnbd.h>
 int
 main (int argc, char *argv[])
 {
   struct nbd_handle *nbd;
   char buf[512];
   size_t i;
   int64_t exportsize;
   uint64_t offset;
   srand (time (NULL));
   if (argc != 2) {
     fprintf (stderr, "%s socket\n", argv[0]);
     exit (EXIT_FAILURE);
   }
   /* Create the libnbd handle. */
   nbd = nbd_create ();
   if (nbd == NULL) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
   /* Connect to the NBD server over a
    * Unix domain socket.
    */
   if (nbd_connect_unix (nbd, argv[1]) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
   /* Get the size of the disk and check
    * it's large enough.
    */
   exportsize = nbd_get_size (nbd);
   if (exportsize == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
   assert (exportsize >= sizeof buf);
   /* Check that the server is writable. */
   if (nbd_is_read_only (nbd) == 1) {
     fprintf (stderr, "%s: "
              "error: this NBD export is read-only\n",
              argv[0]);
     exit (EXIT_FAILURE);
   }
   for (i = 0; i < sizeof buf; ++i)
     buf[i] = rand ();
   /* 1000 writes. */
   for (i = 0; i < 1000; ++i) {
     offset = rand () % (exportsize - sizeof buf);
     if (nbd_pwrite (nbd, buf, sizeof buf,
                     offset, 0) == -1) {
       fprintf (stderr, "%s\n", nbd_get_error ());
       exit (EXIT_FAILURE);
     }
   }
   /* 1000 reads and writes. */
   for (i = 0; i < 1000; ++i) {
     offset = rand () % (exportsize - sizeof buf);
     if (nbd_pread (nbd, buf, sizeof buf,
                    offset, 0) == -1) {
       fprintf (stderr, "%s\n", nbd_get_error ());
       exit (EXIT_FAILURE);
     }
     offset = rand () % (exportsize - sizeof buf);
     if (nbd_pwrite (nbd, buf, sizeof buf,
                     offset, 0) == -1) {
       fprintf (stderr, "%s\n", nbd_get_error ());
       exit (EXIT_FAILURE);
     }
   }
   /* Sends a graceful shutdown to the server. */
   if (nbd_shutdown (nbd, 0) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
   nbd_close (nbd);
   exit (EXIT_SUCCESS);
 }