--- name: "nbd_aio_pread" section: "3" platform: "linux" package: "libnbd" version: "1.24.4-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/nbd_aio_pread" --- # nbd\_aio\_pread(3) read from the NBD server ## NAME nbd\_aio\_pread - read from the NBD server ## SYNOPSIS ``` #include typedef struct { int (*callback) (void *user_data, int *error); void *user_data; void (*free) (void *user_data); } nbd_completion_callback; int64_t nbd_aio_pread ( struct nbd_handle *h, void *buf, size_t count, uint64_t offset, nbd_completion_callback completion_callback, uint32_t flags ); ``` ## DESCRIPTION Issue a read command to the NBD server. To check if the command completed, call [**nbd\_aio\_command\_completed**(3)](/nbd_aio_command_completed/3.md). Or supply the optional `"completion_callback"` which will be invoked as described in "Completion callbacks" in [**libnbd**(3)](/libnbd/3.md). Note that you must ensure `"buf"` is valid until the command has completed. Furthermore, if the `"error"` parameter to `"completion_callback"` is set or if [**nbd\_aio\_command\_completed**(3)](/nbd_aio_command_completed/3.md) reports failure, and if [**nbd\_get\_pread\_initialize**(3)](/nbd_get_pread_initialize/3.md) returns true, then libnbd sanitized `"buf"`, but it is unspecified whether the contents of `"buf"` will read as zero or as partial results from the server. If [**nbd\_get\_pread\_initialize**(3)](/nbd_get_pread_initialize/3.md) returns false, then libnbd did not sanitize `"buf"`, and the contents are undefined on failure. Other parameters behave as documented in [**nbd\_pread**(3)](/nbd_pread/3.md). 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)](/nbd_set_strict_mode/3.md) function can be used to alter which scenarios should await a server reply rather than failing fast. ## RETURN VALUE This call returns the 64 bit cookie of the command. The cookie is ≥ `1`. Cookies are unique (per libnbd handle, not globally). ## ERRORS On error `-1` is returned. Refer to "ERROR HANDLING" in [**libnbd**(3)](/libnbd/3.md) 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)](/libnbd/3.md). ## HANDLE STATE nbd\_aio\_pread 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_AIO_PREAD 1 ``` ## EXAMPLE This example is also available as *examples/aio-connect-read.c* in the libnbd source code. ``` /* This example shows how to use the AIO (asynchronous) low * level API to connect to a server and read the disk. * * Here are a few ways to try this example: * * nbdkit -U - linuxdisk . \ * --run './aio-connect-read $unixsocket' * * nbdkit -U - floppy . \ * --run './aio-connect-read $unixsocket' * * nbdkit -U - pattern size=1M \ * --run './aio-connect-read $unixsocket' */ #include #include #include #include #include #include #include #define NR_SECTORS 32 #define SECTOR_SIZE 512 struct data { uint64_t offset; char sector[SECTOR_SIZE]; }; static int hexdump (void *user_data, int *error) { struct data *data = user_data; FILE *pp; if (*error) { errno = *error; perror ("failed to read"); exit (EXIT_FAILURE); } printf ("sector at offset 0x%" PRIx64 ":\n", data->offset); pp = popen ("hexdump -C", "w"); if (pp == NULL) { perror ("popen: hexdump"); exit (EXIT_FAILURE); } fwrite (data->sector, SECTOR_SIZE, 1, pp); pclose (pp); printf ("\n"); /* Returning 1 from the callback automatically retires * the command. */ return 1; } static struct data data[NR_SECTORS]; int main (int argc, char *argv[]) { struct nbd_handle *nbd; size_t i; 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. * This only starts the connection. */ if (nbd_aio_connect_unix (nbd, argv[1]) == -1) { fprintf (stderr, "%s\n", nbd_get_error ()); exit (EXIT_FAILURE); } /* Wait for the connection to complete. The use of * nbd_poll here is only as an example. You could also * integrate this with poll(2), glib or another main * loop. Read libnbd(3) and the source file lib/poll.c. */ while (!nbd_aio_is_ready (nbd)) { if (nbd_poll (nbd, -1) == -1) { fprintf (stderr, "%s\n", nbd_get_error ()); exit (EXIT_FAILURE); } } assert (nbd_get_size (nbd) >= NR_SECTORS * SECTOR_SIZE); /* Issue read commands for the first NR sectors. */ for (i = 0; i < NR_SECTORS; ++i) { data[i].offset = i * SECTOR_SIZE; /* The callback (hexdump) is called when the command * completes. The buffer must continue to exist while * the command is running. */ if (nbd_aio_pread (nbd, data[i].sector, SECTOR_SIZE, data[i].offset, (nbd_completion_callback) { .callback = hexdump, .user_data = &data[i], }, 0) == -1) { fprintf (stderr, "%s\n", nbd_get_error ()); exit (EXIT_FAILURE); } } /* Run the main loop until all the commands have * completed and retired. Again the use of nbd_poll * here is only as an example. */ while (nbd_aio_in_flight (nbd) > 0) { if (nbd_poll (nbd, -1) == -1) { fprintf (stderr, "%s\n", nbd_get_error ()); exit (EXIT_FAILURE); } } /* Close the libnbd handle. */ nbd_close (nbd); exit (EXIT_SUCCESS); } ``` ## SEE ALSO [**nbd\_aio\_command\_completed**(3)](/nbd_aio_command_completed/3.md), [**nbd\_aio\_pread\_structured**(3)](/nbd_aio_pread_structured/3.md), [**nbd\_create**(3)](/nbd_create/3.md), [**nbd\_get\_pread\_initialize**(3)](/nbd_get_pread_initialize/3.md), [**nbd\_pread**(3)](/nbd_pread/3.md), [**nbd\_set\_pread\_initialize**(3)](/nbd_set_pread_initialize/3.md), [**nbd\_set\_strict\_mode**(3)](/nbd_set_strict_mode/3.md), "Issuing asynchronous commands" in [**libnbd**(3)](/libnbd/3.md), [**libnbd**(3)](/libnbd/3.md). ## AUTHORS Eric Blake Richard W.M. Jones ## COPYRIGHT Copyright Red Hat ## LICENSE This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA