# nng_listener_get(3) Manual Page

## NAME

nng_listener_get - get listener option

## SYNOPSIS

    #include <nng/nng.h>

    int nng_listener_get(nng_listener l, const char *opt, void *val, size_t *valszp);

    int nng_listener_get_bool(nng_listener l, const char *opt, bool *bvalp);

    int nng_listener_get_int(nng_listener l, const char *opt, int *ivalp);

    int nng_listener_get_ms(nng_listener l, const char *opt, nng_duration *durp);

    int nng_listener_get_ptr(nng_listener l, const char *opt, void **ptr);

    int nng_listener_get_size(nng_listener l, const char *opt, size_t *zp);

    int nng_listener_get_addr(nng_listener l, const char *opt, nng_sockaddr *sap);

    int nng_listener_get_string(nng_listener l, const char *opt, char **strp);

    int nng_listener_get_uint64(nng_listener l, const char *opt, uint64_t *u64p);

## DESCRIPTION

The `nng_listener_get()` functions are used to retrieve option values for the listener l. The actual options that may be retrieved in this way vary, and many are documented in nng_options(5).

Additionally some transport-specific options are documented with the transports themselves.

### Forms

In all of these forms, the option opt is retrieved from the listener l. The forms vary based on the type of the option they take.

The details of the type, size, and semantics of the option will depend on the actual option, and will be documented with the option itself.

`nng_listener_get()`

  This function is untyped and can be used to retrieve the value of any option. The caller must store a pointer to a buffer to receive the value in val, and the size of the buffer shall be stored at the location referenced by valszp.
  When the function returns, the actual size of the data copied (or that would have been copied if sufficient space were present) is stored at the location referenced by valszp. If the caller’s buffer is not large enough to hold the entire object, then the copy is truncated. Therefore the caller should validate that the returned size in valszp does not exceed the original buffer size to check for truncation.
  It is acceptable to pass `NULL` for val if the value in valszp is zero. This can be used to determine the size of the buffer needed to receive the object.

| It may be easier to use one of the typed forms of this function.

`nng_listener_get_bool()`

  This function is for options which take a Boolean (`bool`). The value will be stored at bvalp.

`nng_listener_get_int()`

  This function is for options which take an integer (`int`). The value will be stored at ivalp.

`nng_listener_get_ms()`

  This function is used to retrieve time durations (such as timeouts), stored in durp as a number of milliseconds.

`nng_listener_get_ptr()`

  This function is used to retrieve a pointer, ptr, to structured data. The data referenced by ptr is generally managed using other functions. Note that this form is somewhat special in that the object is generally not copied, but instead the pointer to the object is copied.

`nng_listener_get_size()`

  This function is used to retrieve a size into the pointer zp, typically for buffer sizes, message maximum sizes, and similar options.

`nng_listener_get_addr()`

  This function is used to retrieve an `nng_sockaddr` into the value referenced by sap.

`nng_listener_get_string()`

  This function is used to retrieve a string into strp. This string is created from the source using `nng_strdup()` and consequently must be freed by the caller using `nng_strfree()` when it is no longer needed.

`nng_listener_get_uint64()`

  This function is used to retrieve a 64-bit unsigned value into the value referenced by u64p. This is typically used for options related to identifiers, network numbers, and similar.

## RETURN VALUES

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

## ERRORS

`NNG_EBADTYPE` | Incorrect type for option.
`NNG_ECLOSED` | Parameter l does not refer to an open listener.
`NNG_EINVAL` | Size of destination val too small for object.
`NNG_ENOMEM` | Insufficient memory exists.
`NNG_ENOTSUP` | The option opt is not supported.
`NNG_EWRITEONLY` | The option opt is write-only.

## SEE ALSO

nng_listen(3), nng_listener_create(3) nng_listener_set(3) nng_socket_get(3), nng_strdup(3), nng_strerror(3), nng_strfree(3), nng_duration(5), nng_listener(5), nng_options(5), nng_sockaddr(5), nng(7)