# nng_http_handler_alloc(3http) Manual Page

## NAME

nng_http_handler_alloc - allocate HTTP server handler

## SYNOPSIS

    #include <nng/nng.h>
    #include <nng/supplemental/http/http.h>

    typedef struct nng_http_handler nng_http_handler;

    int nng_http_handler_alloc(nng_http_handler **hp, const char *path,
        void (*func)(nng_aio *);

    int nng_http_handler_alloc_directory(nng_http_handler **hp, const char *path,
        const char *dirname);

    int nng_http_handler_alloc_file(nng_http_handler **hp, const char *path,
        const char *filename);

    int nng_http_handler_alloc_redirect(nng_http_handler **hp, const char *path,
        uint16_t status, const char *location);

    int nng_http_handler_alloc_static(nng_http_handler **hp, const char *path,
        const void *data, size_t size, const char *content_type);

## DESCRIPTION

The `nng_http_handler_alloc()` family of functions allocate a handler which will be used to process requests coming into an HTTP server. On success, a pointer to the handler is stored at the located pointed to by hp.

Every handler has a Request-URI to which it refers, which is determined by the path argument. Only the path component of the Request URI is considered when determining whether the handler should be called.

Additionally each handler has a method it is registered to handle (the default is `GET`, see `nng_http_handler_set_method()`), and optionally a 'Host' header it can be matched against (see `nng_http_handler_set_host()`).

In some cases, a handler may reference a logical tree rather (directory) rather than just a single element. (See `nng_http_handler_set_tree()`).

### Custom Handler

The generic (first) form of this creates a handler that uses a user-supplied function to process HTTP requests. This function uses the asynchronous I/O framework. The function takes a pointer to an `nng_aio` structure.

The aio will be passed with the following input values (retrieved with `nng_aio_get_input()`):

0: `nng_http_req *` request

  The client’s HTTP request.

1: `nng_http_handler *` handler

  Pointer to the handler object.

2: `nng_http_conn *` conn

  The underlying HTTP connection.

The handler should create an `nng_http_res *` response (such as via `nng_http_res_alloc()` or `nng_http_res_alloc_error()`) and store that in as the first output (index 0) with `nng_aio_set_output()`.

Alternatively, the handler may send the HTTP response (and any associated body data) itself using the connection. In that case the output at index 0 of the aio should be NULL.

Finally, using the `nng_aio_finish()` function, the aio should be completed successfully. If any non-zero status is returned back to the caller instead, then a generic 500 response will be created and sent, if possible, and the connection will be closed.

The aio may be scheduled for deferred completion using the `nng_aio_defer()` function.

| The callback function should NOT call `nng_aio_begin()`, as that will already have been done by the server framework.

### Directory Handler

The second member of this family, `nng_http_handler_alloc_directory()`, creates a handler configured to serve a directory tree. The uri is taken as the root, and files are served from the directory tree rooted at path.

When the client Request-URI resolves to a directory in the file system, the handler looks first for a file named `index.html` or `index.htm`. If one is found, then that file is returned back to the client. If no such index file exists, then an `NNG_HTTP_STATUS_NOT_FOUND` (404) error is sent back to the client.

The `Content-Type` will be set automatically based upon the extension of the requested file name. If a content type cannot be determined from the extension, then `application/octet-stream` is used.

The directory handler is created as a tree handler initially in exclusive mode (see nng_http_handler_set_tree_exclusive). This can be changed by calling nng_http_handler_set_tree(3http) after creating the directory handler.

### File Handler

The third member of this family, `nng_http_handler_alloc_file()`, creates a handler to serve up a single file; it does not traverse directories or search for `index.html` or `index.htm` files.

The `Content-Type` will be set automatically based upon the extension of the requested file name. If a content type cannot be determined from the extension, then `application/octet-stream` is used.

### Redirect Handler

The fourth member is used to arrange for a server redirect from one URL to another. The reply will be with status code status, which should be a 3XX code such as 301, and a `Location:` header will contain the URL referenced by location, with any residual suffix from the request URI appended.

| Use `nng_http_handler_set_tree()` to redirect an entire tree. For example, it is possible to redirect an entire HTTP site to another HTTPS site by specifying `/` as the path and then using the base of the new site, such as `https://newsite.example.com` as the new location.

| Be sure to use the appropriate value for status. Permanent redirection should use 301 and temporary redirections should use 307. In REST APIs, using a redirection to supply the new location of an object created with `POST` should use 303.

### Static Handler

The fifth member of this family, `nng_http_handler_alloc_static()`, creates a handler to serve up fixed content located in program data. The client is sent the data, with `Content-Length` of size bytes, and `Content-Type` of content_type.

## RETURN VALUES

These functions return 0 on success, and non-zero otherwise.

## ERRORS

`NNG_EINVAL` | An invalid path was specified.
`NNG_ENOMEM` | Insufficient free memory exists to allocate a message.
`NNG_ENOTSUP` | No support for HTTP in the library.

## SEE ALSO

nng_aio_defer(3), nng_aio_finish(3), nng_aio_get_input(3), nng_aio_set_output(3), nng_http_handler_collect_body(3http), nng_http_handler_free(3http), nng_http_handler_set_host(3http), nng_http_handler_set_method(3http), nng_http_handler_set_tree(3http), nng_http_handler_set_tree_exclusive(3http), nng_http_res_alloc(3http), nng_http_res_alloc_error(3http), nng_http_server_add_handler(3http), nng_strerror(3), nng_aio(5), nng(7)