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Added migration guide for libnanomsg.
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This is anticipation of removing the compatibility layer.
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gdamore committed Sep 7, 2024
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7 changes: 3 additions & 4 deletions README.md
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Expand Up @@ -170,11 +170,10 @@ Purchases of the book help fund continued development of NNG.
Some demonstration programs have been created to help serve as examples.
These are located in the `demo` directory.

## Legacy Compatibility
## Migrating from `libnanomsg`

A legacy `libnanomsg` compatible API is available, and while it offers
less capability than the modern NNG API, it may serve as a transition aid.
Please see [_nng_compat_(3compat)](docs/man/nng_compat.3compat.adoc) for details.
Please see the [migration guide](docs/migrating.md) for converting applications
from legacy `libnanomsg` to `NNG`.

## Commercial Support

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115 changes: 115 additions & 0 deletions docs/migrating.md
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# Migrating from libnanomsg

Previous version of NNG offered direct API compatibility with _libnanomsg_,
but that support is no longer offered in this version.

If your application is still using legacy _libnanomsg_ APIs, you will need to
update it for this version of NNG.

## Header Files

Most applications can replace all `#include <nn/*.h>` statements with `#include <nng/nng.h>`.

## Link Libraries

Replace `-lnanomsg` with `-lnng`.
It may be necessary to include additional system libraries, such as `-lpthread`, depending on your system.

## Types

Sockets, dialers, and listeners in _libnanomsg_ are simple integers.
In NNG, these are `struct` types.

Messages are quite different in NNG, with the absence of the POSIX message control
headers.

The `struct nn_msghdr` structure has no equivalent. See `nng_msg` for the
NNG approach to messages. Likewise there is no `struct nn_cmsghdr` equivalent.

## API Conversions

| Nanomsg API | NNG Equivalent | Notes |
| ------------------- | ------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------- |
| `nn_strerror` | `nng_strerror` |
| `nn_errno` | No equivalent. Errors are redirectly rather than through `errno`. |
| `nn_socket` | Use the appropriate protocol constructor, such as `nng_req0_open`. |
| `nn_close` | `nng_close` |
| `nn_bind` | `nng_listen`, `nng_listener_create` | Allocating a listener with `nng_lister_create` and configuring it offers more capabilities. |
| `nn_connect` | `nng_dial`, `nng_dialer_create` | Allocating a dialer with `nng_dialer_create` and configuring it offers more capabilities. |
| `nn_shutdown` | `nng_lister_close`, `nng_dialer_close` |
| `nn_allocmsg` | `nng_msg_alloc` | There are significant semantic differences. |
| `nn_freemsg` | `nng_msg_free` |
| `nn_reallocmsg` | `nng_msg_realloc` |
| `nn_send` | `nng_send` |
| `nn_recv` | `nng_recv` |
| `nn_sendmsg` | `nng_sendmsg` |
| `nn_getsockopt` | `nng_socket_get` | NNG has typed accessors for options, and also separate functions for dialers and listeners. |
| `nn_setsockopt` | `nng_socket_set` |
| `nn_device` | `nng_device` |
| `nn_poll` | None | Can be constructed using `nng_aio`. Few if any applications ever used this API. |
| `nn_term` | None | The `nng_fini` API can do this, but is not recommended except when debugging memory leaks. |
| `nn_get_statistic` | `nng_stats_get` | The statistics in NNG are completely different, with different semantics and no stability guarantees. |
| `NN_POLLIN` | None | Used only with `nn_poll`. |
| `NN_POLLOUT` | None | Used only with `nn_poll`. |
| `NN_MSG` | `NNG_FLAG_ALLOC` | See `nng_send` and `nng_recv` for details. |
| `NN_CMSG_ALIGN` | None |
| `NN_CMSG_FIRSTHDR` | None |
| `NN_CMSG_NXTHDR` | None |
| `NN_CMSG_DATA` | None |
| `NN_CMSG_LEN` | None |
| `NN_CMSG_SPACE` | None |
| `struct nn_iovec` | `nng_iov` |
| `struct nn_msghdr` | `nng_msg` |
| `struct nn_cmsghdr` | `nng_msg` and `nng_msg_header` |

## Options

The following options are changed.

| Nanomsg Option | NNG Eqvaivalent | Notes |
| ---------------------- | -------------------- | ------------------------------------------------------- |
| `NN_LINGER` | None | NNG does not support tuning this. |
| `NN_SNDBUF` | `NNG_OPT_SENDBUF` | NNG value is given in messages, not bytes. |
| `NN_RCVBUF` | `NNG_OPT_RECVBUF` | NNG value is given in messages, not bytes. |
| `NN_SNDTIMEO` | `NNG_OPT_SENDTIMEO` |
| `NN_RCVTIMEO` | `NNG_OPT_RECVTIMEO` |
| `NN_RECONNECT_IVL` | `NNG_OPT_RECONNMINT` |
| `NN_RECONNECT_IVL_MAX` | `NNG_OPT_RECONNMAXT` |
| `NN_SNDPRIO` | None | Not supported in NNG yet. |
| `NN_RCVPRIO` | None | Not supported in NNG yet. |
| `NN_RCVFD` | `NNG_OPT_RECVFD` |
| `NN_SNDFD` | `NNG_OPT_SENDFD` |
| `NN_DOMAIN` | None | NNG options are not divided by domain or protocol. |
| `NN_PROTOCOL` | `NNG_OPT_PROTO` | See also `NNG_OPT_PROTONAME`. |
| `NN_IPV4ONLY` | None | Use URL such as `tcp4://` to obtain this functionality. |
| `NN_SOCKET_NAME` | `NNG_OPT_SOCKNAME` |
| `NN_MAXTTL` | `NNG_OPT_MAXTTL` |

## Error Codes

Most of the error codes have similar names in NNG, just prefixed with `NNG_`.
There are some exceptions. Be aware that the numeric values are _not_ the same.

| Nanomsg Error | NNG Error | Notes |
| -------------- | ------------------------------------------------------------------------------ | ---------------------------------------------------------------------------------- |
| `EINTR` | `NNG_EINTR` | |
| `ENOMEM` | `NNG_ENOMEM` | |
| `EINVAL` | `NNG_EINVAL`, `NNG_EADDRINVAL`, `NNG_EBADTYPE`, `NNG_EAMBIGUOUS` | NNG discrimates between different types of errors. |
| `EBUSY` | `NNG_EBUSY` | |
| `ETIMEDOUT` | `NNG_ETIMEDOUT` | |
| `ECONNREFUSED` | `NNG_ECONNREFUSED` | |
| `EBADF` | `NNG_ECLOSED`, `NNG_ECANCELED` | Canceling an operation returns differently than using an invalid or closed object. |
| `EAGAIN` | `NNG_EAGAIN` |
| `ENOTSUP` | `NNG_ENOTSUP` |
| `EADDRINUSE` | `NNG_EADDRINUSE` |
| `EFSM` | `NNG_ESTATE` | Not a legal POSIX _errno_ value. |
| `ENOENT` | `NNG_ENOENT` |
| `EPROTO` | `NNG_EPROTO` |
| `EHOSTUNREACH` | `NNG_EUNREACHABLE` |
| `EACCCES` | `NNG_EPERM`, `NNG_EWRITEONLY`, `NNG_EREADONLY`, `NNG_ECRYPTO`, `NNG_EPEERAUTH` | NNG has more fine grained reasons for access failures. |
| `EMSGSIZE` | `NNG_EMSGSIZE` |
| `ECONNABORTED` | `NNG_ECONNABORTED` |
| `ECONNRESET` | `NNG_ECONNRESET` |
| `EEXIST` | `NNG_EEXIST` |
| `EMFILE` | `NNG_ENOFILES` |
| `ENOSPC` | `NNG_ENOSPC` |

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