SRT API Functions

  • [Library Initialization](#Library-Initialization) * [srt_startup](#srt_startup) * [srt_cleanup](#srt_cleanup)
  • [Creating and configuring sockets](#Creating-and-configuring-sockets) * [srt_socket](#srt_socket) * [srt_bind](#srt_bind) * [srt_create_socket](#srt_create_socket) * [srt_bind_peerof](#srt_bind-peerof) * [srt_getsockstate](#srt_getsockstate) * [rt_getsndbuffer](#rt_getsndbuffer) * [srt_close](#srt_close)
  • [Connecting](#Connecting) * [srt_listen](#srt_listen) * [srt_accept](#srt_accept) * [srt_connect](#srt_connect) * [srt_connect_debug](#srt_connect_debug) * [srt_rendezvous](#srt_rendezvous)
  • [Options and properties](#Options-and-properties) * [srt_getpeername](#srt_getpeername) * [srt_getsockname](#srt_getsockname) * [srt_getsockopt, srt_getsockflag](#srt_getsockopt-srt_getsockflag) * [srt_setsockopt, srt_setsockflag](#srt_setsockopt-srt_setsockflag)
  • [Helper data types for transmission](#Helper-data-types-for-transmission) * [SRT_MSGCTRL](#SRT_MSGCTRL)
  • [Transmission](#Transmission) * [srt_send, srt_sendmsg, srt_sendmsg2](#srt_send-srt_sendmsg-srt_sendmsg2) * [srt_recv, srt_recvmsg, srt_recvmsg2](#srt_recv-srt_recvmsg-srt_recvmsg2) * [srt_sendfile, srt_recvfile](#srt_sendfile-srt_recvfile)
  • [Diagnostics](#Diagnostics) * [srt_getlasterror_str](#srt_getlasterror_str) * [srt_getlasterror](#srt_getlasterror) * [srt_strerror](#srt_strerror) * [srt_clearlasterror](#srt_clearlasterror)
  • [Performance tracking](#Performance-tracking) * [srt_bstats, srt_bistats](#srt_bstats-srt_bistats)
  • [Asynchronous operations (epoll)](#Asynchronous-operations-epoll) * [srt_epoll_create](#srt_epoll_create) * [srt_epoll_add_usock, srt_epoll_add_ssock, srt_epoll_update_usock, srt_epoll_update_ssock](#srt_epoll_add_usock-srt_epoll_add_ssock-srt_epoll_update_usock-srt_epoll_update_ssock) * [srt_epoll_remove_usock, srt_epoll_remove_ssock](#srt_epoll_remove_usock-srt_epoll_remove_ssock) * [srt_epoll_wait](#srt_epoll_wait) * [srt_epoll_release](#srt_epoll_release)
  • [Logging control](#Logging-control) * [srt_setloglevel](#srt_setloglevel) * [srt_addlogfa, srt_dellogfa, srt_resetlogfa](#srt_addlogfa-srt_dellogfa-srt_resetlogfa) * [srt_setloghandler](#srt_setloghandler) * [srt_setlogflags](#srt_setlogflags)

Library initialization

### srt_startup ` int srt_startup(void); `

This function shall be called at the start of an application that uses the SRT library. It provides all necessary platform-specific initializations, sets up global data, and starts the SRT GC thread. If this function isn’t explicitly called, it will be called automatically when creating the first socket. However, relying on this behavior is strongly discouraged.

  • Returns:
    • 0 = successfully run, or already started
    • 1 = this is the first startup, but the GC thread is already running
    • -1 = failed
  • Errors:
    • SRT_ECONNSETUP (with error code set): Reported when required system

resource(s) failed to initialize. This is currently used only on Windows to report a failure from WSAStartup.

### srt_cleanup ` int srt_cleanup(void); `

This function cleans up all global SRT resources and shall be called just before exiting the application that uses the SRT library. This cleanup function will still be called from the C++ global destructor, if not called by the application, although relying on this behavior is stronly discouraged.

  • Returns:
    • 0 (A possibility to return other values is reserved for future use)

IMPORTANT: Note that the startup/cleanup calls have an instance counter. This means that if you call srt_startup multiple times, you need to call the srt_cleanup function exactly the same number of times.

Creating and configuring sockets

### srt_socket ` SRTSOCKET srt_socket(int af, int type, int protocol); `

Creates an SRT socket:

  • af: family (either AF_INET or AF_INET6)
  • type, protocol: ignored

NOTE: The UDT library uses the type parameter to specify file or message mode by specifying that SOCK_STREAM corresponds to a TCP-like file transmission mode, and SOCK_DGRAM corresponds to an SCTP-like message transmission mode. SRT still supports these modes. However, this is controlled by the SRTO_MESSAGEAPI socket option when the transmission type is file (SRTO_TRANSTYPE set to SRTT_FILE) and the only reasonable value for the type parameter here is SOCK_DGRAM.

  • Returns:
    • a valid socket ID on success
    • INVALID_SOCKET (-1) on error
  • Errors:
    • SRT_ENOTBUF: not enough memory to allocate required resources

NOTE: This is probably a design flaw (BUG?). Usually underlying system errors are reported by SRT_ECONNSETUP.

### srt_create_socket ` SRTSOCKET srt_create_socket(); `

Creates a socket in AF_INET family only.

NOTE: In future the address family may be removed from initial socket configuration, which will free the user from specifying it in srt_socket. The srt_create_socket function will be used for all families. The family will be specified only with the first srt_bind or srt_connect, and in this case SRT_EINVPARAM will not be used (see [srt_bind](#srt_bind) below).

### srt_bind ` int srt_bind(SRTSOCKET u, const struct sockaddr* name, int namelen); `

Binds a socket to a local address and port. Binding specifies the local network interface and the UDP port number to be used for the socket. When the local address is a form of INADDR_ANY, then it’s bound to all interfaces. When the port number is 0, then the port number will be system-allocated if necessary.

This call is obligatory for a listening socket before calling srt_listen and for rendezvous mode before calling srt_connect, otherwise it’s optional. For a listening socket it defines the network interface and the port where the listener should expect a call request. In case of rendezvous mode (when the socket has set SRTO_RENDEZVOUS to true, in this mode both parties connect to one another) it defines the network interface and port from which packets will be sent to the peer and to which the peer is expected to send packets.

For a connecting socket this call can set up the outgoing port to be used in the communication. It is allowed that multiple SRT sockets share one local outgoing port, as long as SRTO_REUSEADDR is set to true (default). Without this call the port will be automatically selected by the system.

See **NOTE* below under* [srt_create_socket](#srt_create_socket).

  • Returns:
    • SRT_ERROR (-1) on error, otherwise 0
  • Errors:
    • SRT_EINVSOCK: Socket passed as u designates no valid socket
    • SRT_EINVOP: Socket already bound
    • SRT_EINVPARAM: Address family in name is not one set for srt_socket
    • SRT_ECONNSETUP: Internal creation of a UDP socket failed
    • SRT_ESOCKFAIL: Internal configuration of a UDP socket (bind, setsockopt) failed

NOTE: SRT_EINVPARAM will not be the case in future, when the family is removed from the initial socket configuration, see [srt_create_socket](#srt_create_socket) above.

### srt_bind_peerof

` int srt_bind_peerof(SRTSOCKET u, UDPSOCKET udpsock); `

A version of srt_bind that acquires a given UDP socket instead of creating one.

### srt_getsockstate

` SRT_SOCKSTATUS srt_getsockstate(SRTSOCKET u); `

Gets the current status of the socket. Possible states are:

  • SRTS_INIT: Created, but not bound
  • SRTS_OPENED: Created and bound, but not in use yet.
  • SRTS_LISTENING: Socket is in listening state
  • SRTS_CONNECTING: The connect operation was initiated, but not yet

finished. This may also mean that it has timed out; you can only know that after getting a socket error report from srt_epoll_wait. In blocking mode it’s not possible because srt_connect does not return until the socket is connected or failed due to timeout or interrupted call. * SRTS_CONNECTED: The socket is connected and ready for transmission. * SRTS_BROKEN: The socket was connected, but the connection was broken * SRTS_CLOSING: The socket may still be open and active, but closing is requested, so no further operations will be accepted (active operations will be completed before closing) * SRTS_CLOSED: The socket has been closed, but not yet removed by the GC thread * SRTS_NONEXIST: The specified number does not correspond to a valid socket.

### srt_getsndbuffer

` int srt_getsndbuffer(SRTSOCKET sock, size_t* blocks, size_t* bytes); `

Retrieves information about the sender buffer.

  • sock: Socket to test
  • blocks: Written information about buffer blocks in use
  • bytes: Written information about bytes in use

This function can be used for diagnostics. It is especially useful when the socket needs to be closed asynchronously.

### srt_close

` int srt_close(SRTSOCKET u); `

Closes the socket and frees all used resources. Note that underlying UDP sockets may be shared between sockets, so these are freed only with the last user closed.

  • Returns:
    • SRT_ERROR (-1) in case of error, otherwise 0
  • Errors:
    • SRT_EINVSOCK: Socket u indicates no valid socket ID

Connecting

### srt_listen ` int srt_listen(SRTSOCKET u, int backlog); `

This sets up the listening state on a socket with a backlog setting that defines how many sockets may be allowed to wait until they are accepted (excessive connection requests are rejected in advance).

  • Returns:
    • SRT_ERROR (-1) in case of error, otherwise 0.
  • Errors:
    • SRT_EINVPARAM: Value of backlog is 0 or negative.
    • SRT_EINVSOCK: Socket u indicates no valid SRT socket.
    • SRT_EUNBOUNDSOCK: srt_bind has not yet been called on that socket.
    • SRT_ERDVNOSERV: SRTO_RENDEZVOUS flag is set to true on specified socket.
    • SRT_EINVOP: Internal error (should not happen when SRT_EUNBOUNDSOCK is reported).
    • SRT_ECONNSOCK: The socket is already connected.
    • SRT_EDUPLISTEN: The address used in srt_bind by this socket is already

occupied by another listening socket. Binding multiple sockets to one IP address and port is allowed, as long as SRTO_REUSEADDR is set to true, but only one of these sockets can be set up as a listener.

### srt_accept

` SRTSOCKET srt_accept(SRTSOCKET lsn, struct sockaddr* addr, int* addrlen); `

Accepts a pending connection and creates a new socket to handle it. The socket that is connected to a remote party is returned.

  • lsn: the listener socket previously configured by srt_listen
  • addr: the IP address and port specification for the remote party
  • addrlen: INPUT: size of addr pointed object. OUTPUT: real size of the

returned object

NOTE: addr is allowed to be NULL, in which case it’s understood that the application is not interested in the address from which the connection originated. Otherwise addr should specify an object into which the address will be written, and addrlen must also specify a variable to contain the object size.

  • Returns:
    • A valid socket ID for the connection, on success
    • SRT_ERROR (-1) on failure
  • Errors:
    • SRT_EINVPARAM: NULL specified as addrlen, when addr is not NULL
    • SRT_EINVSOCK: lsn designates no valid socket ID. Can also mean Internal
Error when an error occurred while creating an accepted socket (BUG?)
  • SRT_ENOLISTEN: lsn is not set up as a listener (srt_listen not called,
or the listener socket has already been closed)
  • SRT_ERDVNOSERV: Internal error (if no SRT_ENOLISTEN reported, it means

that the socket could not be set up as rendezvous because srt_listen does not allow it)

  • SRT_EASYNCRCV: No connection reported so far. This error is reported only

when the lsn listener socket was configured as non-blocking for reading (SRTO_RCVSYN set to false); otherwise the call blocks until a connection is reported or an error occurs

### srt_connect

` int srt_connect(SRTSOCKET u, const struct sockaddr* name, int namelen); `

Connects a socket to a remote party with a specified address and port.

  • u: SRT socket. This must be a freshly created socket that has not yet been

used for anything except possibly srt_bind. * name: specification of the remote address and port * namelen: size of the object passed by name

NOTES: 1. See NOTE regarding family under [srt_create_socket](#srt_create_socket), and SRT_EINVPARAM error under [srt_bind](#srt_bind) above. 2. The socket used here may be bound from upside so that it uses a predefined network interface or local outgoing port. If not, it behaves as if it was bound to INADDR_ANY (which binds on all interfaces) and port 0 (which makes the system assign the port automatically).

  • Returns:
    • SRT_ERROR (-1) in case of error, otherwise 0
  • Errors:
    • SRT_EINVSOCK: Socket u indicates no valid socket ID
    • SRT_EINVPARAM: Address family in name is not one set for srt_socket
    • SRT_ERDVUNBOUND: Socket u has set SRTO_RENDEZVOUS to true, but srt_bind

hasn’t yet been called on it. The srt_connect function is also used to connect a rendezvous socket, but rendezvous sockets must be explicitly bound to a local interface prior to connecting. Non-rendezvous sockets (caller sockets) can be left without binding - the call to srt_connect will bind them automatically.

  • SRT_ECONNSOCK: Socket u is already connected

### srt_connect_debug

` int srt_connect_debug(SRTSOCKET u, const struct sockaddr* name, int namelen, int forced_isn); `

This function is for developers only and can be used for testing. It does the same thing as [srt_connect](#srt_connect), with the exception that it allows specifying the Initial Sequence Number for data transmission. Normally this value is generated randomly.

### srt_rendezvous ``` int srt_rendezvous(SRTSOCKET u, const struct sockaddr* local_name, int local_namelen,

const struct sockaddr* remote_name, int remote_namelen);

``` Performs a rendezvous connection. This is a shortcut for doing bind locally, setting the SRTO_RENDEZVOUS option to true, and doing srt_connect.

  • u: socket to connect
  • local_name: specifies the local network interface and port to bind
  • remote_name: specifies the remote party’s IP address and port

NOTE: The port value shall be the same in local_name and remote_name.

Options and properties

### srt_getpeername ` int srt_getpeername(SRTSOCKET u, struct sockaddr* name, int* namelen); `

Retrieves the remote address to which the socket is connected.

  • Returns:
    • SRT_ERROR (-1) in case of error, otherwise 0
  • Errors:
    • SRT_EINVSOCK: Socket u indicates no valid socket ID
    • SRT_ENOCONN: Socket u isn’t connected, so there’s no remote address to return

### srt_getsockname ` int srt_getsockname(SRTSOCKET u, struct sockaddr* name, int* namelen); `

Extracts the address to which the socket was bound. Although you should know the address(es) that you have used for binding yourself, this function can be useful for extracting the local outgoing port number when it was specified as 0 with binding for system autoselection. With this function you can extract the port number after it has been autoselected.

  • Returns:
    • SRT_ERROR (-1) in case of error, otherwise 0
  • Errors:
    • SRT_EINVSOCK: Socket u indicates no valid socket ID
    • SRT_ENOCONN: Socket u isn’t bound, so there’s no local

address to return (BUG? It should rather be SRT_EUNBOUNDSOCK)

### srt_getsockopt, srt_getsockflag ` int srt_getsockopt(SRTSOCKET u, int level /*ignored*/, SRT_SOCKOPT opt, void* optval, int* optlen); int srt_getsockflag(SRTSOCKET u, SRT_SOCKOPT opt, void* optval, int* optlen); `

Gets the value of the given socket option. The first version (srt_getsockopt) respects the BSD socket API convention, although the “level” parameter is ignored. The second version (srt_getsockflag) omits the “level” parameter completely.

Options correspond to various data types, so you need to know what data type is assigned to a particular option, and to pass a variable of the appropriate data type. Specifications are provided in the apps/socketoptions.hpp file at the srt_options object declaration.

  • Returns:
    • SRT_ERROR (-1) in case of error, otherwise 0
  • Errors:
    • SRT_EINVSOCK: Socket u indicates no valid socket ID
    • SRT_EINVOP: Option opt indicates no valid option

### srt_setsockopt, srt_setsockflag

` int srt_setsockopt(SRTSOCKET u, int level /*ignored*/, SRT_SOCKOPT opt, const void* optval, int optlen); int srt_setsockflag(SRTSOCKET u, SRT_SOCKOPT opt, const void* optval, int optlen); `

Sets a value for a socket option. The first version (srt_setsockopt) respects the BSD socket API convention, although the “level” parameter is ignored. The second version (srt_setsockflag) omits the “level” parameter completely.

Options correspond to various data types, so you need to know what data type is assigned to a particular option, and to pass a variable of the appropriate data type with the option value to be set.

  • Returns:
    • SRT_ERROR (-1) in case of error, otherwise 0

-Errors:

  • SRT_EINVSOCK: Socket u indicates no valid socket ID
  • SRT_EINVOP: Option opt indicates no valid option
  • Various other errors that may result from problems when setting a specific

option (see option description for details).

Helper data types for transmission

### SRT_MSGCTRL

The SRT_MSGCTRL structure:

``` typedef struct SRT_MsgCtrl_ {

int flags; // Left for future int msgttl; // TTL for a message, default -1 (delivered always) int inorder; // Whether a message is allowed to supersede partially lost one. Unused in stream and live mode. int boundary; //0:mid pkt, 1(01b):end of frame, 2(11b):complete frame, 3(10b): start of frame uint64_t srctime; // source timestamp (usec), 0: use internal time int32_t pktseq; // sequence number of the first packet in received message (unused for sending) int32_t msgno; // message number (output value for both sending and receiving)

} SRT_MSGCTRL; ```

The SRT_MSGCTRL structure is used in srt_sendmsg2 and srt_recvmsg2 calls and specifies some special extra parameters:

  • flags: [IN, OUT]. RESERVED FOR FUTURE USE (should be 0). This is

intended to specify some special options controlling the details of how the called function should work.

  • msgttl: [IN]. In message and live mode only, specifies the TTL for

sending messages (in [ms]). Not used for receiving messages. A packet is scheduled for sending by this call and then waits in the sender buffer to be picked up at the moment when all previously scheduled data are already sent, which may be blocked when the data are scheduled faster than the network can afford to send. Default -1 means to wait indefinitely. If specified, then the packet waits for an opportunity to be sent over the network only up to this TTL, and then, if still not sent, the packet is discarded.

  • inorder: [IN]. In message mode only, specifies that sent messages should

be extracted by the receiver in the order of sending. This can be meaningful if a packet loss has happened, and a particular message must wait for retransmission so that it can be reassembled and then delivered. When this flag is false, the message can be delivered even if there are any previous messages still waiting for completion.

  • boundary: RESERVED FOR FUTURE USE. Intended to be used in a special mode

when you are allowed to send or retrieve a part of the message.

  • srctime:
    • [IN] Sender only. Specifies the application-provided timestamp. If not used

(specified as 0), the current system time (absolute microseconds since epoch) is used.

  • [OUT] Receiver only. Specifies the time when the packet was intended to be

delivered to the receiver.

  • pktseq: Receiver only. Reports the sequence number for the packet carrying

out the payload being returned. If the payload is carried out by more than one UDP packet, only the sequence of the first one is reported. Note that in live mode there’s always one UDP packet per message.

  • msgno: Message number that can be sent by both sender and receiver,

although it is required that this value remain monotonic in subsequent send calls. Normally message numbers start with 1 and increase with every message sent.

Helpers for `SRT_MSGCTRL`:

` void srt_msgctrl_init(SRT_MSGCTRL* mctrl); const SRT_MSGCTRL srt_msgctrl_default; `

Helpers for getting an object of SRT_MSGCTRL type ready to use. The first is a function that fills the object with default values. The second is a constant object and can be used as a source for assignment. Note that you cannot pass this constant object into any of the API functions because they require it to be mutable, as they use some fields to output values.

Transmission

### srt_send, srt_sendmsg, srt_sendmsg2 ` int srt_send(SRTSOCKET u, const char* buf, int len); int srt_sendmsg(SRTSOCKET u, const char* buf, int len, int ttl/* = -1*/, int inorder/* = false*/); int srt_sendmsg2(SRTSOCKET u, const char* buf, int len, SRT_MSGCTRL *mctrl); `

Sends a payload to a remote party over a given socket.

  • u: Socket used to send. The socket must be connected for this operation.
  • buf: Points to the buffer containing the payload to send.
  • len: Size of the payload specified in buf.
  • ttl: Time (in [ms]) to wait for a possibility to send. See description of

the [SRT_MSGCTRL::msgttl](#SRT_MSGCTRL) field. * inorder: Required to be received in the order of sending. See [SRT_MSGCTRL::inorder](#SRT_MSGCTRL). * mctrl: An object of [SRT_MSGCTRL](#SRT_MSGCTRL) type that contains extra parameters, including ttl and inorder.

The way this function works is determined by the mode set in options, and it has specific requirements:

1. In file/stream mode, the payload is byte-based. You are not required to know the size of the data, although they are only guaranteed to be received in the same byte order.

2. In file/message mode, the payload that you send using this function is a single message that you intend to be received as a whole. In other words, a single call to this function determines a message’s boundaries.

3. In live mode, you are only allowed to send up to the length of SRTO_PAYLOADSIZE, which can’t be larger than 1456 bytes (1316 default).

  • Returns:
    • Size of the data sent, if successful. Note that in file/stream mode the

returned size may be less than len, which means that it didn’t send the whole contents of the buffer. You would need to call this function again with the rest of the buffer next time to send it completely. In both file/message and live mode the successful return is always equal to len

  • In case of error, SRT_ERROR (-1)

  • Errors:

    • SRT_ENOCONN: Socket u used when the operation is not connected.

    • SRT_ECONNLOST: Socket u used for the operation has lost its connection.

    • SRT_EINVALMSGAPI: Incorrect API usage in message mode: * live mode: trying to send more bytes at once than SRTO_PAYLOADSIZE

    • SRT_EINVALBUFFERAPI: Incorrect API usage in stream mode: * Reserved for future use. The congestion controller object

      used for this mode doesn’t use any restrictions on this call for now, but this may change in future.

    • SRT_ELARGEMSG: Message to be sent can’t fit in the sending buffer (that is,

it exceeds the current total space in the sending buffer in bytes). This means that the sender buffer is too small, or the application is trying to send a larger message than initially predicted.

  • SRT_EASYNCSND: There’s no free space currently in the buffer to schedule

the payload. This is only reported in non-blocking mode (SRTO_SNDSYN set to false); in blocking mode the call is blocked until enough free space in the sending buffer becomes available.

  • SRT_ETIMEOUT: The condition described above still persists and the timeout

has passed. This is only reported in blocking mode when SRTO_SNDTIMEO is set to a value other than -1.

  • SRT_EPEERERR: This is reported only in the case where, as a stream is being

received by a peer, the srt_recvfile function encounters an error during a write operation on a file. This is reported by a UMSG_PEERERROR message from the peer, and the agent sets the appropriate flag internally. This flag persists up to the moment when the connection is broken or closed.

### srt_recv, srt_recvmsg, srt_recvmsg2

` int srt_recv(SRTSOCKET u, char* buf, int len); int srt_recvmsg(SRTSOCKET u, char* buf, int len); int srt_recvmsg2(SRTSOCKET u, char *buf, int len, SRT_MSGCTRL *mctrl); `

Extracts the payload waiting to be received. Note that srt_recv and srt_recvmsg are identical functions, two different names being kept for historical reasons. In the UDT predecessor the application was required to use either the UDT::recv version for stream mode and UDT::recvmsg for message mode. In SRT this distinction is resolved internally by the SRTO_MESSAGEAPI flag.

  • u: Socket used to send. The socket must be connected for this operation.
  • buf: Points to the buffer to which the payload is copied
  • len: Size of the payload specified in buf
  • mctrl: An object of [SRT_MSGCTRL](#SRT_MSGCTRL) type that contains extra

parameters

The way this function works is determined by the mode set in options, and it has specific requirements:

1. In file/stream mode, as many bytes as possible are retrieved, that is, only so many bytes that fit in the buffer and are currently available. Any data that is available but not extracted this time will be available next time.

2. In file/message mode, exactly one message is retrieved, with the boundaries defined at the moment of sending. If some parts of the messages are already retrieved, but not the whole message, nothing will be received (the function blocks or returns SRT_EASYNCRCV). If the message to be returned does not fit in the buffer, nothing will be received and the error is reported.

3. In live mode, the function behaves as in file/message mode, although the number of bytes retrieved will be at most the size of SRTO_PAYLOADSIZE. In this mode, however, with default settings of SRTO_TSBPDMODE and SRTO_TLPKTDROP, the message will be received only when its time to play has come, and until then it will be kept in the receiver buffer; also, when the time to play has come for a message that is next to the currently lost one, it will be delivered and the lost one dropped.

  • Returns:
    • Size (>0) of the data received, if successful.
    • 0, if the connection has been closed
    • SRT_ERROR (-1) when an error occurs
  • Errors:
    • SRT_ENOCONN: Socket u used for the operation is not connected.
    • SRT_ECONNLOST: Socket u used for the operation has lost connection

(this is reported only if the connection was unexpectedly broken, not when it was closed by the foreign host).

  • SRT_EINVALMSGAPI: Incorrect API usage in message mode: * live mode: size of the buffer is less than SRTO_PAYLOADSIZE

  • SRT_EINVALBUFFERAPI: Incorrect API usage in stream mode: * Currently not in use. File congestion control used for stream mode

    does not restrict the parameters. ???

  • SRT_ELARGEMSG: Message to be sent can’t fit in the sending buffer (that is,

it exceeds the current total space in the sending buffer in bytes). This means that the sender buffer is too small, or the application is trying to send a larger message than initially intended.

  • SRT_EASYNCRCV: There are no data currently waiting for delivery. This

happens only in non-blocking mode (when SRTO_RCVSYN is set to false). In blocking mode the call is blocked until the data are ready. How this is defined, depends on the mode:

  • In live mode (with SRTO_TSBPDMODE on), at least one packet must

be present in the receiver buffer and its time to play be in the past * In file/message mode, one full message must be available,

  • the next one waiting if there are no messages with inorder = false, or

possibly the first message ready with inorder = false

  • In file/stream mode, it is expected to have at least one byte of data

still not extracted * SRT_ETIMEOUT: The readiness condition described above is still not achieved

and the timeout has passed. This is only reported in blocking mode when SRTO_RCVTIMEO is set to a value other than -1.

### srt_sendfile, srt_recvfile

` int64_t srt_sendfile(SRTSOCKET u, const char* path, int64_t* offset, int64_t size, int block); int64_t srt_recvfile(SRTSOCKET u, const char* path, int64_t* offset, int64_t size, int block); `

These are functions dedicated to sending and receiving a file. You need to call this function just once for the whole file, although you need to know the size of the file prior to sending and also define the size of a single block that should be internally retrieved and written into a file in a single step. This influences only the performance of the internal operations; from the application perspective you just have one call that exits only when the transmission is complete.

  • u: Socket used for transmission. The socket must be connected.
  • path: Path to the file that should be read or written.
  • offset: Needed to pass or retrieve the offset used to read or write to a file
  • size: Size of transfer (file size, if offset is at 0)
  • block: Size of the single block to read at once before writing it to a file

The following values are recommended for the block parameter:

` #define SRT_DEFAULT_SENDFILE_BLOCK 364000 #define SRT_DEFAULT_RECVFILE_BLOCK 7280000 `

You need to pass them to the srt_sendfile or srt_recvfile function if you don’t know what value to chose.

  • Returns:

    • Size (>0) of the transmitted data of a file. It may be less than size, if

    the size was greater than the free space in the buffer, in which case you have to send rest of the file next time. * -1 in case of error.

  • Errors:

    • SRT_ENOCONN: Socket u used for the operation is not connected.
    • SRT_ECONNLOST: Socket u used for the operation has lost its connection.
    • SRT_EINVALBUFFERAPI: When socket has SRTO_MESSAGEAPI = true or

    SRTO_TSBPDMODE = true.

(BUG?: Looxlike MESSAGEAPI isn’t checked)
  • SRT_EINVRDOFF: There is a mistake in offset or size parameters, which

should match the index availability and size of the bytes available since offset index. This is actually reported for srt_sendfile when the seekg or tellg operations resulted in error. * SRT_EINVWROFF: Like above, reported for srt_recvfile and seekp/tellp. * SRT_ERDPERM: The read from file operation has failed (srt_sendfile). * SRT_EWRPERM: The write to file operation has failed (srt_recvfile).

Diagnostics

General notes concerning the “getlasterror” diagnostic functions: when an API function ends up with error, this error information is stored in a thread-local storage. This means that you’ll get the error of the operation that was last performed as long as you call this diagnostic function just after the failed function has returned. In any other situation the information provided by the diagnostic function is undefined.

### srt_getlasterror

` int srt_getlasterror(int* errno_loc); `

Get the numeric code of the last error. Additionally, in the variable passed as errno_loc the system error value is returned, or 0 if there was no system error associated with the last error. The system error is:

  • On POSIX systems, the value from errno
  • On Windows, the result from GetLastError() call

### srt_strerror ` const char* srt_strerror(int code, int errnoval); `

Returns a string message that represents a given SRT error code and possibly the errno value, if not 0.

NOTE: This function isn’t thread safe. It uses a static variable to hold the error description. There’s no problem with using it in a multithreaded environment, as long as only one thread in the whole application calls this function at the moment

### srt_getlasterror_str ` const char* srt_getlasterror_str(void); `

Get the text message for the last error. It’s a shortcut to calling first srt_getlasterror and then passing the returned value into srt_strerror. Note that, in contradiction to srt_strerror, this function is thread safe.

### srt_clearlasterror

` void srt_clearlasterror(void); `

This function clears the last error. After this call, the srt_getlasterror will report a “successful” code.

Performance tracking

General note concerning sequence numbers used in SRT: they are 32-bit “circular numbers” with the most significant bit not included, so for example 0x7FFFFFFF shifted by 3 forward becomes 2. As far as any comparison is concerned, it can be only spoken about a “distance” rather than difference, which is an integer value expressing an offset to be added to one sequence in order to get the second one. This distance is only valid as long as the threshold value isn’t exceeded, so it’s stated that all sequence numbers that are anywhere taken into account were systematically updated and they are kept in the range between 0 and half of the maximum 0x7FFFFFFF. Hence the distance counting procedure always assumes that the sequence number are in the required range already, so for a numbers like 0x7FFFFFF0 and 0x10, for which the “numeric difference” would be 0x7FFFFFE0, the “distance” is 0x20.

### srt_bstats, srt_bistats ``` // perfmon with Byte counters for better bitrate estimation. int srt_bstats(SRTSOCKET u, SRT_TRACEBSTATS * perf, int clear);

// permon with Byte counters and instantaneous stats instead of moving averages for Snd/Rcvbuffer sizes. int srt_bistats(SRTSOCKET u, SRT_TRACEBSTATS * perf, int clear, int instantaneous); ```

Reports the current statistics

  • u: Socket from which to get statistics
  • perf: Pointer to an object to be written with the statistics
  • clear: 1 if the statistics should be cleared after retrieval
  • instantaneous: 1 if the statistics should use instant data, not moving averages

SRT_TRACEBSTATS is an alias to struct CBytePerfMon. Most of the fields are reasonably well described in the header file comments. Here are descriptions of some less obvious fields in this structure (instant measurements):

  • usPktSndPeriod: This is the minimum time (sending period) that must be kept

between two packets sent consecutively over the link used by this socket. Note that sockets sharing one outgoing port use the same underlying UDP socket and therefore the same link and the same sender queue. usPktSndPeriod is the inversion of the maximum sending speed. It isn’t the EXACT time interval between two consecutive sendings because in the case where the time spent by the application between two consecutive sendings exceeds usPktSndPeriod, the next packet will be sent immediately. The extra “wasted” time will be accounted for at the next sending.

  • pktFlowWindow: The “flow window” in packets. It is the amount of free space

on the peer receiver, stating that this socket represents the sender. When this value drops to zero, the next packet sent will be dropped by the receiver without processing. In file mode this may cause a slowdown of sending in order to wait until the receiver makes more space available, after it eventually extracts the packets waiting in its receiver buffer; in live mode the receiver buffer contents should normally occupy not more than half of the buffer size (default 8192). If pktFlowWindow value is less than that and becomes even less in the next reports, it means that the receiver application on the peer side cannot process the incoming stream fast enough and this may lead do a dropped connection.

  • pktCongestionWindow: The “congestion window” in packets. In file mode

this value starts at 16 and is increased with every number of reported acknowledged packets, and then is also updated based on the receiver-reported delivery rate. It represents the maximum number of packets that can be safely sent now without causing congestion. The higher this value, the faster the packets can be sent. In live mode this field is not used.

  • pktFlightSize: The number of packets in flight. This is the distance

between the packet sequence number that was last reported by an ACK message and the sequence number of the packet just sent (at the moment when the statistics are being read).

NOTE: ACKs are received periodically, so this value is most accurate just after receiving an ACK and becomes a little exaggerated over time until the next ACK arrives. This is because with a new packet sent and the sent sequence increased the ACK number stays the same for a moment, which increases this value, but the exact number of packets arrived since the last ACK report is unknown. Possibly a new statistical data can be added which holds only the distance between the ACK sequence and the sent sequence at the moment when ACK arrives and isn’t updated until the next ACK arrives. The difference between this value and pktFlightSize would show then the number of packets whose fate is unknown at the moment.

  • msRTT: The RTT (Round-Trip time) is the sum of two STT (Single-Trip time)

values, one from agent to peer, and one from peer to agent. Note that the measurement method is different than on TCP; SRT measures only the “reverse RTT”, that is, the time measured at the receiver between sending a UMSG_ACK message until receiving the sender-responded UMSG_ACKACK message (with the same journal). This happens to be a little different to the “forward RTT” as measured in TCP, which is the time between sending a data packet of a particular sequence number and receiving UMSG_ACK with a sequence number that is later by 1. Forward RTT isn’t being measured or reported in SRT, although some research works have shown that these values, even though shuold be the same, happen to differ, that is, “reverse RTT” seems to be more optimistic.

  • mbpsBandwidth: The bandwidth in Mb/s. The bandwidth is measured at the

receiver, which sends back a running average calculation to the sender with the ACK message.

  • byteAvailSndBuf: The number of bytes available in the sender buffer. This

value decreases with data scheduled for sending by the application, and increases with every ACK received from the receiver, after the packets are sent over the UDP link.

  • ` byteAvailRcvBuf`: The number of bytes available in the receiver buffer.

This value increases after the application extracts the data from the socket (uses one of srt_recv* functions) and decreases with every packet received from the sender over the UDP link.

  • mbpsMaxBW: The maximum bandwidth in Mb/s. Usually this is the setting from

the SRTO_MAXBW option, which may include the value 0 (unlimited). Under certain conditions a nonzero value might be be provided by the appropriate congestion control module, although none of the built-in congestion control modules currently uses it.

  • byteMSS: Same as a value from SRTO_MSS option, “Message Segment Size”.

It’s the size of the MTU unit (size of the UDP packet used for transport, including all possible headers, that is Ethernet, IP and UDP), default 1500.

  • pktSndBuf: The number of packets in the send buffer that are already

scheduled for sending or even possibly sent, but not yet acknowledged.

  • byteSndBuf: Same as pktSndBuf, in bytes.
  • msSndBuf: Same as pktSndBuf, but expressed as a time interval between the

oldest and the latest packet scheduled for sending.

  • msSndTsbPdDelay: If SRTO_TSBPDMODE is on (default for live mode), it

returns the value of SRTO_PEERLATENCY, otherwise 0.

  • pktRcvBuf: Number of packets in the receiver buffer. Note that in live mode

(with SRTO_TSBPDMODE turned on, default) some packets must stay in the buffer and will not be signed off to the application until the “time to play” comes. In file mode (both stream and message) it means that all that is above 0 can (and shall) be read right now.

  • byteRcvBuf: Like pktRcvBuf, in bytes.
  • msRcvBuf: Time interval between the first and last available packets in the

receiver buffer. Note that this range includes all packets regardless of whether they are ready to play or not (regarding the live mode)..

  • msRcvTsbPdDelay: If SRTO_TSBPDMODE is on (default for live mode), it

returns the value of SRTO_RCVLATENCY; otherwise 0.

Asynchronous operations (epoll)

The epoll system is currently the only method for using multiple sockets in one thread with having the blocking operation moved to epoll waiting so that it can block on multiple sockets at once. That is, instead of blocking a single reading or writing operation, as it’s in blocking mode, it blocks until at least one of the sockets subscribed for a single waiting call in given operation mode is ready to do this operation without blocking. It’s usually combined with setting the nonblocking mode on a socket, which in SRT is set separately for reading and writing (SRTO_RCVSYN and SRTO_SNDSYN respectively) in order to ensure that in case of some internal error in the application (or even possibly a bug in SRT that has reported a spurious readiness report) the operation will end up with error rather than cause blocking, which would be more dangerous for the application in this case (SRT_EASYNCRCV and SRT_EASYNCRCV respectively).

The epoll system, similar to the one on Linux, relies on eid objects managed internally in SRT, which can be subscribed to particular sockets and the readiness status of particular operations. The srt_epoll_wait function can then be used to block until any readiness status in the whole eid is set.

### srt_epoll_create ` int srt_epoll_create(void); `

Creates a new epoll container.

  • Returns:
    • valid EID on success
    • -1 on failure
  • Errors:
    • SRT_ECONNSETUP: System operation failed. This is on systems that use a

special method for the system part of epoll and therefore associated resources, like epoll on Linux.

### srt_epoll_add_usock, srt_epoll_add_ssock, srt_epoll_update_usock, srt_epoll_update_ssock

` int srt_epoll_add_usock(int eid, SRTSOCKET u, const int* events); int srt_epoll_add_ssock(int eid, SYSSOCKET s, const int* events); int srt_epoll_update_usock(int eid, SRTSOCKET u, const int* events); int srt_epoll_update_ssock(int eid, SYSSOCKET s, const int* events); `

Adds a socket to a container, or updates an existing socket subscription.

The _usock suffix refers to a user socket (SRT socket). The _ssock suffix refers to a system socket.

The _add_ functions add new sockets. The _update_ functions act on a socket that is in the container already and just allow changes in the subscription details. For example, if you have already subscribed a socket with SRT_EPOLL_OUT to wait until it’s connected, to change it into poll for read-readiness, you use this function on that same socket with a variable set to SRT_EPOLL_IN. This will not only change the event type which is polled on the socket, but also remove any readiness status for flags that are no longer set. It is discouraged to perform socket removal and adding back (instead of using _update_) because this way you may miss an event that could happen in a short moment between these two calls.

  • eid: epoll container id
  • u: SRT socket
  • s: system socket
  • events: points to a variable set to epoll flags, or NULL if

you want to subscribe a socket for all possible events

  • Returns:
    • 0 if successful, otherwise -1
  • Errors:
    • SRT_EINVPOLLID: eid designates no valid EID object

BUG?: for add_ssock the system error results in an empty CUDTException() call which actually results in SRT_SUCCESS. For cases like that the SRT_ECONNSETUP code is predicted.

### srt_epoll_remove_usock, srt_epoll_remove_ssock

` int srt_epoll_remove_usock(int eid, SRTSOCKET u); int srt_epoll_remove_ssock(int eid, SYSSOCKET s); `

Removes a specified socket from an epoll container and clears all readiness states recorded for that socket.

The _usock suffix refers to a user socket (SRT socket). The _ssock suffix refers to a system socket.

  • Returns:
    • 0 if successful, otherwise -1
  • Errors:
    • SRT_EINVPOLLID: eid designates no valid EID object

### srt_epoll_wait ``` int srt_epoll_wait(int eid, SRTSOCKET* readfds, int* rnum, SRTSOCKET* writefds, int* wnum, int64_t msTimeOut,

SYSSOCKET* lrfds, int* lrnum, SYSSOCKET* lwfds, int* lwnum);

```

Blocks the call until any readiness state occurs in the epoll container. Mind that the readiness states reported in epoll are permanent, not edge-triggered.

Readiness can be on a socket in the container for the event type as per subscription. The first readiness state causes this function to exit, but all ready sockets are reported. This function blocks until the timeout. If timeout is 0, it exits immediately after checking. If timeout is -1, it blocks indefinitely until a readiness state occurs.

  • eid: epoll container
  • readfds and rnum: A pointer and length of an array to write SRT sockets that are read-ready
  • writefds and wnum: A pointer and length of an array to write SRT sockets that are write-ready
  • msTimeOut: Timeout specified in milliseconds, or special values (0 or -1)
  • lwfds and lwnum:A pointer and length of an array to write system sockets that are read-ready
  • lwfds and lwnum:A pointer and length of an array to write system sockets that are write-ready

Note that there is no space here to report sockets for which it’s already known that the operation will end up with error (athough such a state is known internally). If an error occurred on a socket then that socket is reported in both read-ready and write-ready arrays, regardless of what event types it was subscribed for. Usually then you subscribe given socket for only read readiness, for example (SRT_EPOLL_IN), but pass both arrays for read and write readiness. This socket will not be reported in the write readiness array even if it’s write ready (because this isn’t what it was subscribed for), but it will be reported there, if the next operation on this socket is about to be erroneous. On such sockets you can still perform an operation, just you should expect that it will always report and error. On the other hand that’s the only way to know what kind of error has occurred on the socket.

  • Returns:
    • The number (>0) of ready sockets, of whatever kind (if any)
    • -1 in case of error
  • Errors:
    • SRT_EINVPOLLID: eid designates no valid EID object
    • SRT_ETIMEOUT: Up to msTimeOut no sockets subscribed in eid were ready.

This is reported only if msTimeOut was >=0, otherwise the function waits indefinitely.

### srt_epoll_release ` int srt_epoll_release(int eid); `

Deletes the epoll container.

  • Returns:
    • The number (>0) of ready sockets, of whatever kind (if any)
    • -1 in case of error
  • Errors:
    • SRT_EINVPOLLID: eid designates no valid EID object

Logging control

SRT has a widely used system of logs, as this is usually the only way to determine how the internals are working, without changing the rules by the act of tracing. Logs are split into levels (5 levels out of those defined by syslog are in use) and additional filtering is possible on FA (functional area). By default only up to the Note log level are displayed and from all FAs.

Logging can only be manipulated globally, with no regard to a specific socket. This is because lots of operations in SRT are not dedicated to any particular socket, and some are shared between sockets.

### srt_setloglevel

` void srt_setloglevel(int ll); `

Sets the minimum severity for logging. A particular log entry is displayed only if it has a severity greater than or equal to the minimum. Setting this value to LOG_DEBUG turns on all levels.

The constants for this value are those from <sys/syslog.h> (for Windows, refer to common/win/syslog_defs.h). The only meaningful are:

  • LOG_DEBUG: Highly detailed and very frequent messages
  • LOG_NOTICE: Occasionally displayed information
  • LOG_WARNING: Unusual behavior
  • LOG_ERR: Abnormal behavior
  • LOG_CRIT: Error that makes the current socket unusable

### srt_addlogfa, srt_dellogfa, srt_resetlogfa

` void srt_addlogfa(int fa); void srt_dellogfa(int fa); void srt_resetlogfa(const int* fara, size_t fara_size); `

A functional area (FA) is an additional filtering mechanism for logging. You can set up logging to display logs only from selected FAs. The list of FAs is collected in srt.h file, as identified by the SRT_LOGFA_ prefix. They are not enumerated here because they may be changed very often.

All FAs are turned on by default, except potentially dangerous ones (such as SRT_LOGFA_HAICRYPT). The reaons is that they may display either some security information that shall remain in the memory only (so, only if strictly required for the development), or some duplicated information (so you may want to turn this FA on, while turning off the others).

### srt_setloghandler ` void srt_setloghandler(void* opaque, SRT_LOG_HANDLER_FN* handler); typedef void SRT_LOG_HANDLER_FN(void* opaque, int level, const char* file, int line, const char* area, const char* message); `

By default logs are printed to standard error stream. This function replaces the sending to a stream with a handler function that will receive them.

### srt_setlogflags ` void srt_setlogflags(int flags); `

When you set a log handler with srt_setloghandler, you may also want to configure which parts of the log information you do not wish to be passed in the log line (the message parameter). A user’s logging facility may, for example, not wish to get the current time or log level marker, as it will provide this information on its own.

The following flags are available, as collected in logging_api.h public header:

  • SRT_LOGF_DISABLE_TIME: Do not provide the time in the header
  • SRT_LOGF_DISABLE_THREADNAME: Do not provide the thread name in the header
  • SRT_LOGF_DISABLE_SEVERITY: Do not provide severity information in the header
  • SRT_LOGF_DISABLE_EOL: Do not add the end-of-line character to the log line

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