.\" gd_getdata.3.  The gd_getdata man page.
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.\" Copyright (C) 2008-2016 D. V. Wiebe
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.TH gd_getdata 3 "25 December 2016" "Version 0.10.0" "GETDATA"

.SH NAME
gd_getdata \(em retrieve data from a Dirfile database

.SH SYNOPSIS
.SC
.B #include <getdata.h>
.HP
.BI "size_t gd_getdata(DIRFILE *" dirfile ", const char *" field_code ", off_t"
.IB first_frame ", off_t " first_sample ", size_t " num_frames ", size_t"
.IB num_samples ", gd_type_t " return_type ", void *" data_out );
.EC

.SH DESCRIPTION
The
.FN gd_getdata
function queries a dirfile(5) database specified by
.ARG dirfile
for the field
.ARG field_code .
It fetches
.ARG num_frames
frames plus
.ARG num_samples
samples from this field, starting 
.ARG first_sample
samples past frame
.ARG first_frame . 
The data is converted to the data type specified by
.ARG return_type ,
and stored in the user-supplied buffer
.ARG data_out .

The 
.ARG field_code
may contain one of the representation suffixes listed in dirfile-format(5).
If it does,
.FN gd_getdata
will compute the appropriate complex norm before returning the data.

The 
.ARG dirfile
argument must point to a valid DIRFILE object previously created by a call to
.F3 gd_open .
The argument
.ARG data_out
must point to a valid memory location of sufficient size to hold all data
requested.

Unless using
.B GD_HERE 
(see below), the first sample returned will be
.IP
.IR first_frame " * " samples_per_frame " + " first_sample
.PP
as measured from the start of the dirfile, where
.ARG samples_per_frame
is the number of samples per frame as returned by
.F3 gd_spf .
The number of samples fetched is, similarly,
.IP
.IR num_frames " * " samples_per_frame " + " num_samples .
.PP
Although calling
.FN gd_getdata
using both samples and frames is possible, the function is typically called
with either
.ARG num_samples
and
.ARG first_sample ,
or
.ARG num_frames
and
.ARG first_frames ,
equal to zero.

Instead of explicitly specifying the origin of the read, the caller may pass the
special symbol
.B GD_HERE
as
.ARG first_frame .
This will result in the read occurring at the current position of the I/O
pointer for the field (see
.B GetData I/O Pointers
below for a discussion of field I/O pointers).  In this case, the value of
.ARG first_sample
is ignored.

When reading a
.B SINDIR
field,
.ARG return_type
must be
.BR GD_STRING .
For all other field types, the
.ARG return_type
argument should be one of the following symbols, which indicates the desired
return type of the data:
.RS
.DD GD_UINT8
unsigned 8-bit integer
.DD GD_INT8
signed (two's complement) 8-bit integer
.DD GD_UINT16
unsigned 16-bit integer
.DD GD_INT16
signed (two's complement) 16-bit integer
.DD GD_UINT32
unsigned 32-bit integer
.DD GD_INT32
signed (two's complement) 32-bit integer
.DD GD_UINT64
unsigned 64-bit integer
.DD GD_INT64
signed (two's complement) 64-bit integer
.DD GD_FLOAT32
IEEE-754 standard 32-bit single precision floating point number
.DD GD_FLOAT64
IEEE-754 standard 64-bit double precision floating point number
.DD GD_COMPLEX64
C99-conformant 64-bit single precision complex number
.DD GD_COMPLEX128
C99-conformant 128-bit double precision complex number
.DD GD_NULL
the null type: the database is queried as usual, but no data is returned.
In this case,
.ARG data_out
is ignored and may be NULL.
.RE

The return type of the data need not be the same as the type of the data stored
in the database.  Type conversion will be performed as necessary to return the
requested type.  If the
.ARG field_code
does not indicate a representation, but conversion from a complex value to a
purely real one is required, only the real portion of the requested vector will
be returned.

Upon successful completion, the I/O pointer of the field will be on the sample
immediately following the last sample returned, if possible.  On error, the
position of the I/O pointer is not specified, and may not even be well defined.

.SS Behaviour While Reading Specific Field Types

.TP
.BR MPLEX :
Reading an
.B MPLEX
field typically requires GetData to read data before the range returned in order
to determine the value of the first sample returned.  This can become expensive
if the encoding of the underlying RAW data does not support seeking backwards
(which is true of most compression encodings).  How much preceding data GetData
searches for the initial value of the returned data can be adjusted, or the
lookback disabled completely, using
.F3 gd_mplex_lookback .
If the initial value of the field is not found in the data searched, GetData
will fill the returned vector, up to the next available sample of the
mulitplexed field, with zero for integer return types, or IEEE-754-conforming
NaN (not-a-number) for floating point return types, as it does when providing
data before the beginning-of-field.

GetData caches the value of the last sample from every
.B MPLEX
it reads so that a subsequent read of the field starting from the following
sample (either through an explicit starting sample given by the caller or else
implicitly using
.BR GD_HERE )
will not need to scan the field backwards.  This cache is invalidated if a
different return type is used, or if an intervening operation moves the field's
I/O pointer.

.TP
.BR SINDIR :
The only allowed
.ARG return_type
when reading
.B SINDIR
data is
.BR GD_STRING .
The
.ARG data
argument should be of type
.BR "const char **" ,
and be large enough to hold one pointer for each sample requested.  It will be
filled with pointers to read-only string data.  The caller should not free the
returned string pointers.  For convenience when allocating buffers, the
.B GD_STRING
constant has the property:
.nh
.BI "GD_SIZE(GD_STRING) == sizeof(" "const char *" )\fR.
.hy
On samples where the index vector is out of range of the
.BR SARRAY ,
and also on samples before the index vector's frame offset, the value stored in
.ARG data
will be the NULL pointer.

.TP
.BR PHASE :
A forward-shifted
.B PHASE
field will always encounter the end-of-field marker before its input field does.
This has ramifications when reading
streaming data
with
.FN gd_getdata
and using
.F3 gd_nframes
to gauge field lengths (that is: a
forward-shifted
.B PHASE
field always has less data in it than
.F3 gd_nframes
implies that it does).  As with any other field,
.FN gd_getdata
will return a short count whenever a read from a
.B PHASE
field encounters the end-of-field marker.

Backward-shifted
.B PHASE
fields do not suffer from this problem, since
.FN gd_getdata
pads reads past the beginning-of-field marker with NaN or zero as appropriate.
Database creators who wish to use the
.B PHASE
field type with streaming data are encouraged to work around this limitation
by only using backward-shifted 
.B PHASE
fields, by writing
.B RAW
data at the maximal frame lag, and then back-shifting all data which should have
been written earlier.  Another possible work-around is to write
systematically less data to the reference
.B RAW
field in proportion to the maximal forward phase shift.  This method will work
with applications which respect the database size reported by
.F3 gd_nframes
resulting in these applications effectively ignoring all frames past the frame
containing the maximally forward-shifted
.B PHASE
field's end-of-field marker.

.TP
.BR WINDOW :
The samples of a
.B WINDOW
for which the field conditional is false will be filled with either zero for
integer return types, or IEEE-754-conforming NaN (not-a-number) for floating
point return types.

.SH RETURN VALUE
In all cases,
.FN gd_getdata
returns the number of samples (not bytes) successfully read from the database.
If the end-of-field is encountered before the requested number of samples have
been read, a short count will result.  this is not an error.

Requests for data before the beginning-of-field marker, which may have
been shifted from frame zero by a
.B PHASE
field or
.B /FRAMEOFFSET
directive, will result in the the data being padded at the front by NaN or zero,
depending on whether the return type is of floating point or integral type.

On error, this function returns zero and stores a negative-valued error code in
the
.B DIRFILE
object which may be retrieved by a subsequent call to
.F3 gd_error .
Possible error codes are:
.DD GD_E_ALLOC
The library was unable to allocate memory.
.DD GD_E_BAD_CODE
The field specified by
.ARG field_code ,
or one of the fields it uses for input, was not found in the database.
.DD GD_E_BAD_DIRFILE
An invalid
.ARG dirfile
was supplied.
.DD GD_E_BAD_SCALAR
A scalar field used in the definition of the field was not found, or was not of
scalar type.
.DD GD_E_BAD_TYPE
An invalid
.ARG return_type
was specified.
.DD GD_E_DIMENSION
The supplied
.ARG field_code
referred to a 
.BR CONST ,\~ CARRAY ,
or 
.B STRING
field.  The caller should use
.F3 gd_get_constant ,\~ gd_get_carray (3) ,
or
.F3 gd_get_string
instead.  Or, a scalar field was found where a vector field was expected in
the definition of
.ARG field_code
or one of its inputs.
.DD GD_E_DOMAIN
An immediate read was attempted using
.BR GD_HERE ,
but the I/O pointer of the field was not well defined because two or more of
the field's inputs did not agree as to the location of the I/O pointer.
.DD GD_E_INTERNAL_ERROR
An internal error occurred in the library while trying to perform the task.
This indicates a bug in the library.  Please report the incident to the
maintainer.
.DD GD_E_IO
An error occurred while trying to open or read from a file on disk containing
a raw field or LINTERP table.
.DD GD_E_LUT
A LINTERP table was malformed.
.DD GD_E_RANGE
An attempt was made to read data outside the addressable Dirfile range (more
than 2**63 samples past the start of the dirfile).
.DD GD_E_RECURSE_LEVEL
Too many levels of recursion were encountered while trying to resolve
.ARG field_code .
This usually indicates a circular dependency in field specification in the
dirfile.
.DD GD_E_UNKNOWN_ENCODING
The encoding scheme of a RAW field could not be determined.  This may also
indicate that the binary file associated with the RAW field could not be found.
.DD GD_E_UNSUPPORTED
Reading from dirfiles with the encoding scheme of the specified dirfile is not
supported by the library.  See
dirfile-encoding(5)
for details on dirfile encoding schemes.
.PP
A descriptive error string for the error may be obtained by calling
.F3 gd_error_string .

.SH NOTES
To save memory,
.FN gd_getdata
uses the memory pointed to by
.ARG data_out
as scratch space while computing derived fields.  As a result, if an error is
encountered during the computation, the contents of this memory buffer are
unspecified, and may have been modified by this call, even though
.FN gd_getdata
will report zero samples returned on error.

Reading slim-compressed data (see defile-encoding(5)), may cause unexpected
memory usage.  This is because slimlib internally caches open decompressed files
as they are read, and GetData doesn't close data files between
.FN gd_getdata
calls for efficiency's sake.  Memory used by this internal slimlib buffer can be
reclaimed by calling
.F3 gd_raw_close
on fields when finished reading them.

When operating on a platform whose
.B size_t
is
.IR N -bytes
wide, a single call of
.FN gd_getdata
will never return more than (2**(\fIN\fR-1) - 1) samples.  The request will
be truncated at (2**(\fIN\fR-\fIM\fR) - 1) samples, where \fIM\fR is the size,
in bytes, of the largest data type used to calculate the returned field.  If a
larger request is specified, less data than requested will be returned, without
raising an error.  This limit is imposed even when 
.ARG return_type
is
.B GD_NULL
or when reading from the
.I INDEX
field (i.e., even when no actual I/O or calculation occurs).  In all cases, the
actual amount of data is returned.

.SH GETDATA I/O POINTERS
This is a general discussion of field I/O pointers in the GetData library, and
contains information not directly applicable to
.FN gd_getdata .

Every
.B RAW
field in an open Dirfile has an
.I I/O pointer
which indicates the library's current read and write poisition in the field.
These I/O pointers are useful when performing sequential reads or writes on
Dirfile fields (see
.B GD_HERE
in the description above).  The value of the I/O pointer of a field is reported
by
.F3 gd_tell .

Derived fields have virtual I/O pointers arising from the I/O pointers of their
input fields.  These virtual I/O pointers may be valid (when all input fields
agree on their position in the dirfile) or invalid (when the input fields are
not in agreement).  The I/O pointer of some derived fields is
.I always
invalid.  The usual reason for this is the derived field simultaneously reading
from two different places in the same
.B RAW
field.  For example, given the following Dirfile metadata specification:

.RS
a \fBRAW UINT8\fR 1
.br
b \fBPHASE\fR a 1
.br
c \fBLINCOM\fR 2 a 1 0 b 1 0
.RE

the derived field
.I c
never has a valid I/O pointer, since any particular sample of
.I c
ultimately involves reading from more than one place in the
.B RAW
field
.IR a .
Attempting to perform sequential reads or writes (with
.BR GD_HERE )
on a derived field when its I/O pointer is invalid will result in an error
(specifically,
.BR GD_E_DOMAIN ).

The implicit
.I INDEX
field has an effective I/O pointer than mostly behaves like a true
.B RAW
field I/O pointer, except that it permits simultaneous reads from multiple
locations.  So, given the following metadata specification:

.RS
d \fBPHASE\fR INDEX 1
.br
e \fBLINCOM\fR 2 INDEX 1 0 d 1 0
.RE

the I/O pointer of the derived field
.I e
will always be valid, unlike the similarly defined
.I c
above.  The virtual I/O pointer of a derived field will change in response to
movement of the
.B RAW
I/O pointers underlying the derived fields inputs, and vice versa: moving the
I/O pointer of a derived field will move the I/O pointer of the
.B RAW
fields from which it ultimately derives.  As a result, the I/O pointer of
any particular field may move in unexpected ways if multiple fields are
manipulated at the same time.

When a Dirfile is first opened, the I/O pointer of every
.B RAW
field is set to the beginning-of-frame
(the value returned by
.F3 gd_bof ),
as is the I/O pointer of any newly-created
.B RAW
field.

The following library calls cause I/O pointers to move:
.TP
.BR gd_getdata "() and " gd_putdata (3)
These functions move the I/O pointer of affected fields to the sample
immediately following the last sample read or written, both when performed at
an absolutely specified position and when called for a sequential read or write
using
.BR GD_HERE .
When reading a derived field which simultaneously reads from more than one place
in a
.B RAW
field (such as
.I c
above), the position of that
.B RAW
field's I/O pointer is unspecified (that is: it is not specified which input
field is read first).
.TP
.F3 gd_seek
This function is used to manipulate I/O pointers directly.
.TP
.BR gd_flush "(3) and " gd_raw_close (3)
These functions set the I/O pointer of any
.B RAW
field which is closed back to the beginning-of-field.
.TP
.I calls which result in modifications to raw data files:
this may happen when calling any of:
.F3 gd_alter_encoding ,
.F3 gd_alter_endianness ,
.F3 gd_alter_frameoffset ,
.F3 gd_alter_entry ,
.F3 gd_alter_raw ,
.F3 gd_alter_spec ,
.F3 gd_malter_spec ,
.F3 gd_move ,
or
.F3 gd_rename ;
these functions close affected
.B RAW
fields before making changes to the raw data files, and so reset the
corresponding I/O pointers to the beginning-of-field.

.PP
In general, when these calls fail, the I/O pointers of affected fields may be
anything, even out-of-bounds or invalid.  After an error, the caller should
issue an explicit
.F3 gd_seek
to repoisition I/O pointers before attempting further sequential operations.

.SH HISTORY
The function
.FN getdata
appeared in GetData-0.3.0.

The
.B GD_COMPLEX64
and
.B GD_COMPLEX128
data types appeared in GetData-0.6.0.

In GetData-0.7.0, this function was renamed to
.FN gd_getdata .

The
.B GD_HERE
symbol used for sequential reads appeared in GetData-0.8.0.

The
.B GD_STRING
data type appeared in GetData-0.10.0.

.SH SEE ALSO
.F3 GD_SIZE ,
.F3 gd_error ,
.F3 gd_error_string ,
.F3 gd_get_constant ,
.F3 gd_get_string ,
.F3 gd_mplex_lookback ,
.F3 gd_nframes ,
.F3 gd_open ,
.F3 gd_raw_close ,
.F3 gd_seek ,
.F3 gd_spf ,
.F3 gd_putdata ,
dirfile(5), dirfile-encoding(5)