--- source_url: https://www.pubnub.com/docs/sdks/posix-c/api-reference/configuration title: Configuration API for POSIX C SDK updated_at: 2026-05-21T15:47:13.457Z sdk_name: PubNub POSIX C SDK --- > Documentation Index > For a curated overview of PubNub documentation, see: https://www.pubnub.com/docs/llms.txt > For the full list of all documentation pages, see: https://www.pubnub.com/docs/llms-full.txt # Configuration API for POSIX C SDK PubNub POSIX C SDK POSIX C complete API reference for building real-time applications on PubNub, including basic usage and sample code. ## Configuration This page describes the preprocessor definitions (macros) that configure C‑Core. They live in the `pubnub_config.h` header file. The header file differs by platform. Some definitions that do not apply to a platform may be omitted. Any definitions that you find in `pubnub_config.h` and that are not listed here should not be changed. ### PUBNUB_CTX_MAX: maximum number of contexts Maximum number of PubNub contexts that can be used at the same time. Applies only to statically allocated contexts. On hosted platforms (POSIX, Windows...), contexts are dynamic by default, so this is ignored. Choose the allocation module (static or dynamic) at link time. A context is used to publish messages or subscribe to (get) them. Each context consumes memory. Set a realistic limit. A typical configuration may consist of a single PubNub context for channel subscription and another PubNub context that will periodically publish messages about device status (with timeout lower than message generation frequency). This only needs two contexts. Another typical setup may have a single subscription context and maintain a pool of contexts for each publish call triggered by an external event (for example, a button push). This would need N+1 contexts, N being the number of external events. You can use a single context, but you cannot publish and subscribe on it at the same time. This may cause lost messages. ### PUBNUB_BUF_MAXLEN: size of HTTP buffer Size of the HTTP buffer, in bytes. It heavily impacts the context memory size and sets the upper bound on the URL‑encoded message size. For messages up to 2 KB, set about `2500` for `PUBNUB_BUF_MAXLEN`. For larger messages, increase the buffer. You can set this via compiler options. If not, change the default value in the header. ### PUBNUB_DYNAMIC_REPLY_BUFFER: use dynamic reply buffer Set to `false` (0) to use a static buffer and then set its size via `PUBNUB_REPLY_MAXLEN`. Set to `true` (anything !=0) to use a dynamic buffer, that is, dynamically try to allocate as much memory as needed for the buffer. Default on hosted platforms (Windows, POSIX...) is to use a dynamic reply buffer. Be aware that this can involve large amounts of data (megabytes). If memory is constrained, prefer a static reply buffer. If the reply exceeds the size of your statically allocated buffer, the transaction will fail. Choose the option that best suits your application. ### PUBNUB_REPLY_MAXLEN: reply static length This is only significant if `PUBNUB_DYNAMIC_REPLY_BUFFER` is `true`. In that case it defines the size, in octets, of the reply buffer. It will hold the whole (HTTP) body of the reply, not the (HTTP) headers. Replies longer than this are discarded and an error is reported. Large subscribe replies can cause lost messages. ### PUBNUB_ORIGIN: the DNS hostname of PubNub This is the DNS hostname for the PubNub network. In most cases, do not change it. If enabled (see `PUBNUB_ORIGIN_SETTABLE`), you can set `origin` at runtime. But, in the case that you do need to change it at compile time, but not at runtime and want to save some RAM in the context, it's OK to change this macro. ### PUBNUB_ORIGIN_SETTABLE: is origin settable at runtime If `true (!=0)`, the origin can be changed at runtime. This adds a small memory cost. If `false (==0)`, the origin cannot be changed at runtime. Use this to reduce RAM usage. ### PUBNUB_DEFAULT_TRANSACTION_TIMER: duration of transaction timeout Duration of the transaction timeout set during context initialization, in milliseconds. Timeout duration in the context can be changed by the user after initialization (at runtime). This is used only if timer support is enabled (default on hosted platforms). ### PUBNUB_CALLBACK_THREAD_STACK_SIZE_KB: size of the PubNub thread Stack size (in kilobytes) for the polling thread when using the callback interface. Smaller values save memory, but avoid less than 64 KB. Set `0` to use the default. Ignored by the sync interface. ### PUBNUB_PROXY_API: enable proxy support Set `true (!=0)` to enable proxy support (default on hosted platforms). Configure host, port, and protocol with the C‑Core APIs. Set `false (==0)` to disable proxy support and reduce code size. Depending on your build, you may also need to exclude proxy modules from compilation and linking. See the sample makefiles. ### PUBNUB_MAX_PROXY_HOSTNAME_LENGTH: max length of proxy hostname Maximum length (characters) of the proxy host name stored in the context. Set this to fit your proxy names. One host name is stored per context. ### PUBNUB_ONLY_PUBSUB: use only publish&subscribe Set `true (!=0)` to support only Publish and Subscribe. Default is `false` on hosted platforms. This reduces code size if you do not need other transactions. If you compile modules not needed in this mode, the build may warn. Many linkers drop unused modules automatically, but behavior depends on your toolchain. You can set this via compiler options or edit the header default. ## Initialization There is no installation of this SDK. Just clone the Git(hub) repo, or download a ZIP or tarball of a release from [https://github.com/pubnub/c-core/releases](https://github.com/pubnub/c-core/releases) It has the code and example `Makefiles` to get you started. :::tip Makefiles Use the `Makefiles` as a starting point in your own projects (whether they are based on Make or some other build tool / system or IDE). ::: #### Makefile without SSL/TLS support The `Makefile` for POSIX without SSL/TLS support is available at [/posix/posix.mk](https://github.com/pubnub/c-core/blob/master/posix/posix.mk) See [/posix/README.md](https://github.com/pubnub/c-core/blob/master/posix/README.md) for info on how to build on POSIX (and `POSIX-like`) systems. #### Makefile with SSL/TLS support The `Makefile` for POSIX with SSL/TLS, (via OpenSSL) is available at [/openssl/posix.mk](https://github.com/pubnub/c-core/blob/master/openssl/posix.mk) in the repo. See [/openssl/README.md](https://github.com/pubnub/c-core/blob/master/openssl/README.md) for info on how to build w/OpenSSL on POSIX and other OpenSSL related data. #### Including the header Your calling pattern (synchronous vs callback) determines which header to import. Learn more [later in this guide](#calling-patterns). ##### Sync If using the synchronous (sync) pattern, import only `pubnub_sync.h`: ```c #include "pubnub_sync.h" ``` ##### Callback If using the callback pattern, import `pubnub_callback.h` and `pthread.h`: ```c #include "pubnub_callback.h" #include ``` #### Memory allocation This client uses dynamic memory for contexts. Create a context with `pubnub_alloc()` (check the return value) and free it with `pubnub_free()`. :::note Required UUID Always set the `UUID` to uniquely identify the user or device that connects to PubNub. This `UUID` should be persisted, and should remain unchanged for the lifetime of the user or the device. If you don't set the `UUID`, you won't be able to connect to PubNub. ::: ```c #include "pubnub_alloc.h" #include int main() { pubnub_t *ctx = pubnub_alloc(); if (NULL == ctx) { puts("Couldn't allocate a Pubnub context"); return -1; } /* Do something with ctx... and then: */ pubnub_free(ctx); return 0 } ``` #### Timers There is one timer: the total transaction timer. It starts when a transaction starts and stops when it finishes. If it expires, the transaction is cancelled locally. If a publish already reached the server, cancellation does not roll it back. If the transaction timer expires, the outcome is `timeout`. The actual duration can exceed what you set due to platform factors, but it is usually close. Set the timer after initializing the context and before starting transactions. The setting applies to all transaction types. #### Thread safety C‑Core supports thread‑safe operation, though, for performance, you may think about not using it. To use thread‑safety support, define the preprocessor symbol `PUBNUB_THREADSAFE` (just define it, the value does not matter). ##### Thread-safe usage Thread safety is internal to the SDK. It protects the PubNub context, not your application data. If you signal from callbacks to worker threads, synchronize your own data. If you compiled thread-safety support in, you are free to access the same context from different threads, pretty much in any way you wish. However, there are some advised guidelines you should follow: * With the sync blocking interface, waiting threads may block for a long time. Avoid this if possible. * With the sync non‑blocking interface and `pubnub_await`, behavior is similar to the blocking interface. * With the sync non‑blocking interface and without `pubnub_await`, waiting threads block less. The main useful action is cancelling a transaction from another thread. * Avoid calling `pubnub_await` or `pubnub_last_result` from different threads; it makes debugging harder. * With the callback interface, avoid calling SDK functions from your callback except small helpers. It simplifies debugging. ##### Thread-unsafe usage If you compile without thread‑safety support, the SDK is not thread safe. Do not use one context from more than one thread at the same time. In multithreaded code: 1. Prefer using a context from only the thread that created it 2. If that is not possible, add your own synchronization (for example, pthread condition variables, mutexes, or message queues) 3. With the callback interface, you can start a transaction in one thread and then use the context only in the callback. This is safe. :::note Threads and contexts Keep in mind that it is perfectly safe to use different contexts from different threads at the same time. To each (thread) its own (context). ::: #### Transaction and operation The Posix C SDK operates as a set of transactions. A transaction is initiated by the client SDK and is defined as a single message exchange between the SDK and PubNub service. Every interaction that the client SDK initiates with PubNub is sequenced as a series of transactions which ultimately results in a PubNub service-specific operation. #### Status and Events The SDK provides a set of status and event identifiers which can help developers interact with the library. The status identifier codes are returned as part of the SDK's API invocation. These are used by the developer to check for status of transactions or for detecting normal / abnormal conditions in an API call. Some of the commonly used status codes are as follows 1. `PNR_OK`: Success, the transaction finished successfully 2. `PNR_STARTED`: The previously initiated transaction has started. 3. `PNR_IN_PROGRESS`: Indicates that the previous transaction with PubNub service is still in progress. Refer to the API docs for a complete list of status identifiers supported by the library. `Events` refer to the PubNub REST operations which are initiated by the client SDK. The most common example of events are subscribe and publish. A client subscribing for a channel is a subscribe event and a client publishing a message on a channel is a publish event. Some of the common event identifiers are as follows: 1. `PBTT_SUBSCRIBE`: Subscriber operation 2. `PBTT_PUBLISH`: Publish operation Refer to the API docs for a complete list of operations supported by the SDK. #### Calling patterns This SDK provides `sync` and `callback` (notification) interfaces for retrieving the outcome of a PubNub request/transaction/operation. ##### Sync The `sync` interface works like this: 1. Start a transaction (say, publish - using `pubnub_publish()`) 2. Either `pubnub_await()` the outcome, or use your own loop in which you check `if (PNR_STARTED != pubnub_last_result())` 3. Handle the outcome as you wish This is illustrated in the `Hello World` example below (which is the same for any platform that supports `sync` interface). ##### Callback The `callback` interface is somewhat more flexible, uses less CPU resources, but is, in general, a little harder to use. One way to use it is to emulate the `sync` interface: 1. Create a callback function (`my_callback`) per the prototype required by `pubnub_register_callback()` 2. In `my_callback()`, use a condition variable to signal that outcome was received 3. Set the callback via `pubnub_register_callback()` 4. Start a transaction (say, publish - using `pubnub_publish()`) 5. Wait on the condition variable (the same one used in `my_callback`) 6. Handle the outcome as you wish This is illustrated in the `Hello World` example below, using pthreads condition variable. Obviously, on platforms that don't support pthreads you will use some similar API (for example, SetEvent/WaitForSingleObject on Windows). There are other ways to use the callback interface, like the `state machine` or similar, where the callback will handle the outcome of a transaction but will also start the next Pubnub transaction, or do other stuff that it needs to do. This is very application specific, so we don't provide an example here. ##### Proxy configuration :::note Set the proxy You need to link in the required modules and set the proxy. Use int `pubnub_set_proxy_manual()` from `pubnub_proxy.h`. If the proxy server is an authenticating one, you may need to set the username/password, with `pubnub_set_proxy_authentication_username_password()`. See a [sample makefile](https://github.com/pubnub/c-core/blob/master/posix/posix.mk). ::: Use this function to initialize the PubNub client context and set account credentials such as `publish_key` and `subscribe_key`. ### Method(s) To `Initialize` PubNub you can use the following method(s) in the Posix C SDK: ```c void pubnub_init (pubnub_t *p, const char *publish_key, const char *subscribe_key) ``` | Parameter | Type | Required | Default | Description | | --- | --- | --- | --- | --- | | p | pubnub_t* | Yes | | Pointer to the Context to initialize (use `pubnub_alloc()` to obtain it) | | publish_key | char | Yes | | Pointer to the `string` of the key to use when publishing messages. | | subscribe_key | char | Yes | | Pointer to the `string` of the key to use when subscribing to messages | ### Sample code #### Initialize the PubNub client API :::note Required UUID Always set the `UUID` to uniquely identify the user or device that connects to PubNub. This `UUID` should be persisted, and should remain unchanged for the lifetime of the user or the device. If you don't set the `UUID`, you won't be able to connect to PubNub. ::: ```c pubnub_t *ctx = pubnub_alloc(); if (NULL == ctx) { puts("Couldn't allocate a Pubnub context"); return -1; } pubnub_init(ctx, "demo", "demo"); pubnub_set_uuid(ctx, "myUniqueUUID"); pubnub_set_ssl_options(ctx, true, true, true); ``` ### Returns Returns a PubNub instance for APIs like `pubnub_publish()`, `pubnub_subscribe()`, `pubnub_history()`, and `pubnub_here_now()`. ### Other examples #### Initialize the client :::note Required UUID Always set the `UUID` to uniquely identify the user or device that connects to PubNub. This `UUID` should be persisted, and should remain unchanged for the lifetime of the user or the device. If you don't set the `UUID`, you won't be able to connect to PubNub. ::: ```c pubnub_init(ctx, /*publish key*/"demo", /*subscribe key*/"demo"); pubnub_set_uuid(ctx, "myUniqueUUID"); ``` #### Initialization for a Read-Only client In the case where a client will only read messages and never publish to a channel, you can simply omit the `publish_key` when initializing the client: :::note Required UUID Always set the `UUID` to uniquely identify the user or device that connects to PubNub. This `UUID` should be persisted, and should remain unchanged for the lifetime of the user or the device. If you don't set the `UUID`, you won't be able to connect to PubNub. ::: ```c pubnub_init(ctx, "", "demo"); ``` #### Use a custom UUID Set a custom `UUID` to identify your users. :::note Required UUID Always set the `UUID` to uniquely identify the user or device that connects to PubNub. This `UUID` should be persisted, and should remain unchanged for the lifetime of the user or the device. If you don't set the `UUID`, you won't be able to connect to PubNub. ::: ```c pubnub_t *pn = pubnub_alloc(); pubnub_init(pn, "myPublishKey", "mySubscribeKey"); pubnub_set_uuid(pn, "myUniqueUUID"); ``` #### Initializing with SSL enabled This examples demonstrates how to enable PubNub Transport Layer Encryption with `SSL`. Just initialize the client with second set to `true`. The hard work is done, now the PubNub API takes care of the rest. Just subscribe and publish as usual and you are good to go. :::note Required UUID Always set the `UUID` to uniquely identify the user or device that connects to PubNub. This `UUID` should be persisted, and should remain unchanged for the lifetime of the user or the device. If you don't set the `UUID`, you won't be able to connect to PubNub. ::: ```c pubnub_t *ctx = pubnub_alloc(); if (NULL == ctx) { puts("Couldn't allocate a Pubnub context"); return -1; } pubnub_init(ctx, "demo", "demo"); pubnub_set_uuid(ctx, "myUniqueUUID"); pubnub_set_ssl_options(ctx, true, true, true); ``` ## UUID These functions are used to set/get a user ID on the fly. ### Method(s) To set/get `UUID` you can use the following method(s) in Posix C SDK: #### Set UUID ```c void pubnub_set_uuid (pubnub_t *p, const char *uuid) ``` | Parameter | Description | | --- | --- | | `p` *Type: pubnub_t* | Pointer to PubNub context | | `uuid` *Type: const char* | Pointer to `uuid` `string` | #### Get UUID ```c char const *pubnub_uuid_get(pubnub_t *p) ``` | Parameter | Description | | --- | --- | | `p` *Type: pubnub_t* | Pointer to pubnub client context. | After `pubnub_init()`, it will return NULL until you change it to non-NULL via `pubnub_set_uuid()`. ### Sample code #### Set UUID :::note Required UUID Always set the `UUID` to uniquely identify the user or device that connects to PubNub. This `UUID` should be persisted, and should remain unchanged for the lifetime of the user or the device. If you don't set the `UUID`, you won't be able to connect to PubNub. ::: ```c pubnub_t *ctx = pubnub_alloc(); if (NULL == ctx) { puts("Couldn't allocate a Pubnub context"); return -1; } pubnub_init(ctx, "myPublishKey", "mySubscribeKey"); pubnub_set_uuid(ctx, "myUniqueUUID"); ``` #### Get UUID ```c printf("UUID is %s", pubnub_uuid_get(ctx)); ``` ### Returns Get UUID returns the following output: | Type | Description | | --- | --- | | `char const*` | `UUID` for context. Null if not set. | ### Other examples #### Initializing with a custom uuid :::note Required UUID Always set the `UUID` to uniquely identify the user or device that connects to PubNub. This `UUID` should be persisted, and should remain unchanged for the lifetime of the user or the device. If you don't set the `UUID`, you won't be able to connect to PubNub. ::: ```c pubnub_t *ctx = pubnub_alloc(); if (NULL == ctx) { puts("Couldn't allocate a Pubnub context"); return -1; } struct Pubnub_UUID uuid; char random_uuid; if (0 == pubnub_generate_uuid_v4_random(&uuid)) { random_uuid = pubnub_uuid_to_string(&uuid).uuid; pubnub_init(ctx, "myPublishKey", "mySubscribeKey"); pubnub_set_uuid(ctx, random_uuid); } pubnub_free(ctx); ``` ## Authentication key Setter and getter for users auth key. ### Method(s) #### Set auth key ```c pubnub_set_auth(pubnub_t* p, const char* auth) ``` | Parameter | Description | | --- | --- | | `p` *Type: pubnub_t* | Pointer to pubnub client context | | `auth`Type: const char* | Pointer to `auth` `string`. NULL to unset | #### Get auth key ```c char const *pubnub_auth_get(pubnub_t *p) ``` | Parameter | Description | | --- | --- | | `p` *Type: pubnub_t* | Pointer to pubnub client context | ### Sample code #### Set auth key ```c pubnub_t *ctx = pubnub_alloc(); if (NULL == ctx) { puts("Couldn't allocate a Pubnub context"); return -1; } pubnub_init(ctx, "demo", "demo"); pubnub_set_auth(ctx, "my_new_authkey"); ``` #### Get auth key ```c pubnub_t *ctx = pubnub_alloc(); if (NULL == ctx) { puts("Couldn't allocate a Pubnub context"); return -1; } pubnub_init(ctx, "demo", "demo"); pubnub_set_auth(ctx, "my_auth_key"); printf("Auth Key is %s", pubnub_auth_get(ctx)); ``` ### Returns `Get Auth key` returns the `current authentication key`. ## Origin Sets the origin (DNS host) for context `p`. If runtime origin is disabled, the call fails. NULL resets the origin to the default. ### Method To set the origin for a PubNub context use: ```c int pubnub_origin_set(pubnub_t *p, char const *origin) ``` | Parameter | Description | | --- | --- | | `p` *Type: pubnub_t * | PubNub context to set `origin` for | | `Origin`Type: char const* | The `origin` to use for context `p`. If NULL, the default `origin` will be set. To request a custom domain, contact support and follow the [request process](https://www.pubnub.com/docs/general/setup/data-security#request-process). | ### Sample code To set the origin to the European data center explicitly: ```c pubnub_origin_set(pn, "ps.pndsn.com"); ``` ### Returns | Type | Description | | --- | --- | | `int` | 0: success, -1: fail | ## Set SSL options Sets the SSL options for a context. ### Method(s) ```c void pubnub_set_ssl_options(pubnub_t *p, bool useSSL, bool ignoreSecureConnectionRequirement) ``` | Parameter | Description | | --- | --- | | `p` *Type: pubnub_t* | Pointer to pubnub client context | | `useSSL` *Type: bool | Should the PubNub client establish the connection to PubNub using `SSL`?Default: `true` | | `ignoreSecureConnectionRequirement` *Type: bool | ignoreSecureConnectionRequirement When `SSL` is enabled, should the client fallback to a non-SSL connection if it experiences issues handshaking across local proxies, firewalls, etc?Default: `true` | ### Sample code ```c pubnub_set_ssl_options(ctx, true, true); ``` ### Returns `None` ## Terms in this document * **Origin** - The subdomain used to establish a connection to the PubNub network that allows your application's traffic to appear like it's coming from your own domain. * **PubNub** - PubNub is a real-time messaging platform that provides APIs and SDKs for building scalable applications. It handles the complex infrastructure of real-time communication, including: Message delivery and persistence, Presence detection, Access control, Push notifications, File sharing, Serverless processing with Functions and Events & Actions, Analytics and monitoring with BizOps Workspace, AI-powered insights with Illuminate.