Manuals: libslink


The purpose of this library is to provide all needed functionality for communicating with a SeedLink server in a generalized way. This library facilitates rapid SeedLink client development as the client does not need to know any details of the SeedLink protocol. The library is intended to work with all existing versions of SeedLink and will not use features that a given SeedLink server does not support. The library is written in C.


The library is thread-safe on Unix-like platforms as long as each thread manages it’s own SeedLink Connection Description (see below). The library is definitely not thread-safe under Win32.

The usage philosophy is that a so-called SeedLink Connection Description (SLCD) stipulates all aspects of a connection to a server. In libslink the SLCD is implemented as a C struct which is used by most of the utility functions. The struct is given below with descriptions of each parameter:

typedef struct slcd_s
    SLstream   *streams;
    char       *sladdr;
    char       *begin_time;
    char       *end_time;
    short int	resume;
    short int   multistation;
    short int   dialup;
    short int   batchmode;
    short int   lastpkttime;
    short int   terminate;
    int         keepalive;
    int         netto;
    int         netdly;
    float       protocol_ver;
    char       *info;
    int         link;
    SLstat     *stat;
    SLlog      *log;
  } SLCD;

Parameter descriptions

Parameter Description
streams A pointer to struct that specifies streams to be requested from the server; this is the beginning of the “stream chain”. The user should not need to interface directly with the stream chain as there are helper functions to populate the chain; namely sl_addstream() and sl_setuniparams()
sladdr The address of the SeedLink server to connect to in 'host:port' format. Example: "".
begin_time, end_time If these parameters are set the connection is configured for time window extraction. The end_time is optional. When no end_time is set the behavior is server dependent but can be expected, with current server versions, to indicate that the server should keep the window open-ended and not close the connection. The format is 'year,month,day,hour,min,sec', i.e. “2002,05,03,13,00,00”, for both parameters. If these parameters are set they will override the use of any sequence numbers (specified in the stream chain) to resume a data stream.
resume A flag to control the inclusion of sequence numbers with the DATA or FETCH commands. If this flag is true sequence numbers will be appended to the DATA/FETCH command instructing the remote server to start sending data from the supplied sequence number for a given stream. If false no sequence numbers will be sent and the server will start the stream from the next arriving data packet. The default is 1 (true).
multistation A flag to indicate that the connection should be configured in multi-station mode. If you use sl_addstream() or sl_setuniparams() to populate the stream chain this parameter is set automatically. The default is 0 (false).
dialup A flag to indicate that the connection should be configured in so-called dial-up mode. This is a mode of operation where the server will close the connection when the end of the selected buffers have been reached, i.e. the most current data has been sent. The default is 0 (false).
batchmode A flag to indicate that the connection should be configured in batch mode, 1=requested and 2=activated. This is a mode of operation intended to speed up negotiation with the server by minimizing acknowledgement checks. The default is 0 (false).
lastpkttime A flag to indicate if the last packet time should be appended to a DATA or FETCH command during connection negotiation. The last packet time is only (optionally) sent to SeedLink servers version 2.93 or greater. In combination with the last packet sequence number, this information is used by the remote server to decide if the last packet sequence number is valid for the current buffer. Basically this should prevent large amounts of overlap data from being sent if the client has not been connected in a while and is a basic sanity check. The default is 0 (false).
terminate A flag to indicate that the connection to the remote server should be shutdown and the internal buffer should be flushed of complete packets. The routine sl_terminate() will set this flag; it is recommended to use sl_terminate() in a signal handling routine. The default is, of course, 0 (false).

The following parameters have specific meaning for the connection management function described below, sl_collect():

Parameter Description
keepalive Interval, in seconds, to send keepalive requests to the server. Default is 0 (disabled).
netto Network timeout, in seconds. If no packets are received within this period the connection will be closed and re-opened. Default is 600 seconds. A value of 0 disables this feature.
netdly Network connection delay, in seconds. This parameter controls how much time the manager will wait until attempting to re-connect. Default is 30 seconds.

The following parameters are maintained by the library routines and should not be changed by the calling program:

Parameter Description
protocol_ver This is the version of the SeedLink protocol in use. This value is parsed from the server’s response to the HELLO command and will be set to 0.0 when the version could not be detected. Internal library routines use this value to check if specific features should be used with the currently connected server. If the version could not be detected the default action is to allow all features. The first line of response to the HELLO command is expected to be in the format: "seedlink v#.# <optional text>" where 'seedlink' is case insensitive and '#.#' is the protocol version.
info This parameter indicates that an INFO packet should be requested from the server during the next call to sl_collect(). The client should request INFO packets via the sl_request_info() function and not use this value directly.
link This is the file descriptor associated with the connected network socket. This value will be -1 when not connected to a server.

There are a number of functions to assist with the creation of a SLCD (see the man pages for programming details):

Function Description
sl_newslcd() allocate and initialize a SLCD struct with default values.
sl_freeslcd() free all memory associated with a SLCD.
sl_addstream() add a stream entry to the stream chain; multi-station mode is implied.
sl_setuniparams() set the stream parameters for uni-station mode.
sl_read_streamlist() read a list of streams from a file and add them to the stream chain; multi-station mode is implied.
sl_parse_streamlist() parse a list of stream from a string and add them to the stream chain; multi-station mode is implied.

Using the connection manager

A function is provided to manage a connection to a SeedLink server based on the parameters in the connection description.

Function Description
sl_collect() using the parameters in the connection description this function will maintain a persistent connect to a server. Designed to run in a tight loop at the heart of a client program this routine blocks until a packet is received, then setting the slpack pointer to the packet and returning a positive integer. Zero (0) is returned when either 1) the requested time window has been fulfilled, 2) the termination sequence is completed or 3) an error occurred.

If the info parameter is set in the connection description the manager will request the specified INFO during the next iteration of sl_collect(). In-stream INFO requests can be made by using the sl_request_info() function.

The manager uses the keepalive, netto and netdly parameters in the way specified above.
sl_collect_nb() This is a non-blocking version of sl_collect() it will always return whether a packet is received or not. If not called at regular short intervals this function might not be able to properly manage the connection, i.e. update internal timers for timeouts, send keepalives, etc. For this reason this is NOT A RECOMMENDED INTERFACE. If you really need to use this function try to call it at regular intervals, in other words, don’t spend too much time processing or sleeping in-between calls.
sl_terminate() set the terminate flag in the connection description. This will cause sl_collect() to close the network connection, finish passing received packets to the caller and finally return 0.

It is highly recommended to implement signal handling to call sl_terminate() for smooth program termination.

Using state files

The state of each stream, the sequence number and time stamp in the stream chain, can be saved to and recovered from state files:

  • sl_savestate() : save current sequence numbers and time stamps to a file.
  • sl_recoverstate() : recover sequence numbers and time stamps from a file.

Using these functions a SeedLink client can be shut down and re-started without a gap in the data stream as long as all necessary data still exists in the SeedLink buffer.

Controlling output from the library functions

All of the log and diagnostic messages emitted by the library functions use the same interface. The output from this interface can be controlled. This is useful when the library will be embedded in a larger system with a custom logging facility. See the man page for more details.

  • sl_loginit() : initialize the verbosity level of the library functions, also set the functions and prefixes used for log, diagnostic and error messages.
  • sl_log() : the central logging facility. Behavior is controlled by the settings specified with sl_loginit().

The default destination for log messages is standard output (stdout), while all diagnostic (including error) messages go to standard error (stderr). Most of the internal messages emitted by the library are considered diagnostic and will, by default, go to standard error.

The default prefix for log and diagnostic messages is nothing. The default prefix for diagnostic error messages is “error: “.

There are re-entrant versions of these functions that operation either on the logging parameters in a SLCD or directly on a logging parameter SLlog struct. They are intended for use in threaded programs or where a complex logging scheme is desired. See the man pages for more details.

Handling of Mini-SEED records

Some functions are included for parsing mini-SEED record headers and unpacking/decompressing data samples. As of version 1.0 the data formats supported are INT-16, INT-32, Steim-1 and Steim-2.

  • sl_msr_parse_() : parse the fixed section data header and blockettes 100, 1000 and 1001. Optionally unpacking/decompressing data samples.
  • sl_msr_print() : print header values out using sl_log(). The level of information printed can be controlled.

There are also functions to return the (double precision) sample rate and the record start time as a (double precision) POSIX epoch, these are sl_msr_dsamprate() and sl_msr_depochstime() respectively. Consult the manual pages for further details.

Illustrative program example

For a real example of libslink usage see the program in the ‘example’ directory. Below is a simple, step-by-step, schematic view of common library usage:

sl_newslcd() // initialize new connection description.
// set a server address
// set other desired parameters; begin/end time or batchmode
sl_parse_streamlist() // parse a stream list string
sl_recover_state() // recover state information from a file
while ( sl_collect() != 0 )
    [ do something with the data record ]
sl_savestate()        - save state information to a file

See Also

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