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.\"Copyright 2010 (c) EPFL
.TH EGD_GET_DATA 3 2010 "EPFL" "EEGDEV library manual"
.SH NAME
egd_get_data - peek buffered data
.SH SYNOPSIS
.LP
.B #include <eegdev.h>
.sp
.BI "ssize_t egd_get_available(struct eegdev* " dev ");"
.br
.BI "ssize_t egd_get_data(struct eegdev* " dev ", size_t " ns ", ...);"
.br
.SH DESCRIPTION
.LP
\fBegd_get_available\fP() returns the number of samples that have been
buffered by the device referenced by \fIdev\fP and that have not been read
yet.
.LP
\fBegd_get_data\fP() peeks the \fIns\fP next samples from the buffered data
acquired by the device referenced by \fIdev\fP and fills the arrays
provided in the variable list of arguments with the obtained data. If all
requested samples have been already acquired, the function returns
immediately. Otherwise, the call blocks until the requested data is
available, the acquisition stops or a problem occurs. In the last two cases,
the data read may be less than requested.
.LP
The arrays provided in the variable list of argument are filled following
the formats specified by previous call to \fBegd_acq_setup\fP(3). In
particular, the number of arrays supplied in the variable list of argument
and their size should be consistent with the number of arrays and strides
specified by the call to \fBegd_acq_setup\fP(3).
.SH "RETURN VALUE"
.LP
\fBegd_get_available\fP() returns the number of unread samples in case of
succes. Otherwise, \-1 is returned and \fIerrno\fP is set accordingly.
.LP
In case of success, \fBegd_get_data\fP() returns the number of read samples
(which can be less than the requested number). Otherwise, \-1 is returned
and \fIerrno\fP is set accordingly.
.SH ERRORS
.LP
\fBegd_get_available\fP() and \fBegd_get_data\fP() will fail if:
.TP
.B EINVAL
\fIdev\fP is NULL.
.TP
.B ENOMEM
The internal ringbuffer of the device referenced by \fIdev\fP is full.
.TP
.B EAGAIN
The underlying hardware referenced by \fIdev\fP has encountered a loss of
connection, maybe due some cable disconnected or a power switch set to off.
.TP
.B EIO
The underlying hardware referenced by \fIdev\fP has encountered a loss of
synchronization for an unknown reason.
.SH NOTES
Please be aware that the user has no obligation to make all the calls to
\fBegd_get_data\fP() and \fBegd_get_available\fP() during the acquisition.
He can also peform some of them after the acquisition which will correspond
to get the remaining buffered data.
.LP
For example, it might happened that a user want to wait for an certain
external event to occur before stopping the acquisition. In this situation,
the usual workflow would be to start the acquisition, get regurlarly some
data while scanning the event to occur. When this happens, the acquisition
is immediately stopped. However at the moment of stopping the acquisition,
there might still be some buffered data which could be important.
Calling \fBegd_get_available\fP() after \fBegd_stop\fP(3) would then return
the size of the remaining data that could be obtained with
\fBegd_get_data\fP().
.SH EXAMPLE
.RS
.nf
#define NEEG 8
#define NTRI 1
#define NS 4
int ns_tot;
ssize_t ns_read;
float eegarr[NEEG*NS];
int32_t triarr[NTRI*NS];
struct grpconf grp[2];
unsigned int strides[2];
\fB/* Assume that a device has been successfully opened, i.e. there
is a valid 'dev' variable of type struct eegdev* */\fP
strides[0] = NEEG*sizeof(float);
strides[1] = NTRI*sizeof(int32_t);
grp[0].sensortype = egd_sensor_type("eeg");
grp[0].index = 0;
grp[0].iarray = 0;
grp[0].arr_offset = 0;
grp[0].nch = NEEG;
grp[0].datatype = EGD_FLOAT;
grp[1].sensortype = egd_sensor_type("trigger");
grp[1].index = 0;
grp[1].iarray = 1;
grp[1].arr_offset = 0;
grp[1].nch = NTRI;
grp[1].datatype = EGD_INT32;
\fB/* Setup how to get the data */\fP
egd_acq_setup(dev, 2, strides, 2, grp);
\fB/* Start the acquisition.
From now, all incoming samples will be buffered */\fP
egd_start(dev);
ns_tot = 0;
while (ns_tot < 1000) {
\fB/* Get the data */\fP
ns_read = egd_get_data(dev, NS, eegarr, triarr);
if (ns_read < 0) {
\fB/* Handle failure */\fP
}
ns_tot += ns_read;
\fB/* do something with the new data */\fP
}
\fB/* Stop the acquisition, i.e. no new data is buffered */\fP
egd_stop(dev);
.fi
.RE
.SH "SEE ALSO"
.BR egd_acq_setup (3),
.BR egd_start (3),
.BR egd_stop (3)
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