# zmq_ctx_set_ext(3) Manual Page

## NAME

zmq_ctx_set_ext - set extended context options

## SYNOPSIS

int zmq_ctx_set_ext (void *context, int option_name, const void *option_value, size_t option_len);

## DESCRIPTION

The zmq_ctx_set_ext() function shall set the option specified by the option_name argument to the value pointed to by the option_value argument for the ØMQ context pointed to by the context argument. The option_len argument is the size of the option value in bytes. For options taking a value of type "character string", the provided byte data should either contain no zero bytes, or end in a single zero byte (terminating ASCII NUL character).

The zmq_ctx_set_ext() function accepts all the option names accepted by zmq_ctx_set(). Options that make most sense to set using zmq_ctx_set_ext() instead of zmq_ctx_set() are the following options:

### ZMQ_THREAD_NAME_PREFIX: Set name prefix for I/O threads

The ZMQ_THREAD_NAME_PREFIX argument sets a string prefix to each thread created for the internal context’s thread pool. This option is only supported on Linux. This option is useful to help debugging done via "top -H" or "gdb"; in case multiple processes on the system are using ZeroMQ it is useful to provide through this context option an application-specific prefix to distinguish ZeroMQ background threads that belong to different processes. This option only applies before creating any sockets on the context.

Option value type | character string
Option value unit | N/A
Default value | empty string

## RETURN VALUE

The zmq_ctx_set_ext() function returns zero if successful. Otherwise it returns `-1` and sets errno to one of the values defined below.

## ERRORS

EINVAL

  The requested option option_name is unknown.

EFAULT

  The provided context is invalid.

## EXAMPLE

Setting a prefix on internal ZMQ thread names:

    void *context = zmq_ctx_new ();
    const char prefix[] = "MyApp";
    size_t prefixLen = sizeof(prefix);
    zmq_ctx_set (context, ZMQ_THREAD_NAME_PREFIX, &prefix, &prefixLen);

    char buff[256];
    size_t buffLen = sizeof(buff);
    int rc = zmq_ctx_get (context, ZMQ_THREAD_NAME_PREFIX, &buff, &buffLen);
    assert (rc == 0);
    assert (buffLen == prefixLen);

## SEE ALSO

zmq_ctx_set(3) zmq(7)

## AUTHORS

This page was written by the ØMQ community. To make a change please read the ØMQ Contribution Policy at http://www.zeromq.org/docs:contributing.