Provided by: libgetdata-doc_0.9.0-2.2_all 
NAME
gd_getdata — retrieve data from a dirfile database
SYNOPSIS
#include <getdata.h>
size_t gd_getdata(DIRFILE *dirfile, const char *field_code, off_t first_frame, off_t
first_sample, size_t num_frames, size_t num_samples, gd_type_t return_type, void
*data_out);
DESCRIPTION
The gd_getdata() function queries a dirfile(5) database specified by dirfile for the field
field_code. It fetches num_frames frames plus num_samples samples from this field,
starting first_sample samples past frame first_frame. The data is converted to the data
type specified by return_type, and stored in the user-supplied buffer data_out.
The field_code may contain one of the representation suffixes listed in dirfile-format(5).
If it does, gd_getdata() will compute the appropriate complex norm before returning the
data.
The dirfile argument must point to a valid DIRFILE object previously created by a call to
gd_open(3). The argument data_out must point to a valid memory location of sufficient
size to hold all data requested.
Unless using GD_HERE (see below), the first sample returned will be
first_frame * samples_per_frame + first_sample
as measured from the start of the dirfile, where samples_per_frame is the number of
samples per frame as returned by gd_spf(3). The number of samples fetched is, similarly,
num_frames * samples_per_frame + num_samples.
Although calling gd_getdata() using both samples and frames is possible, the function is
typically called with either num_samples and first_sample, or num_frames and first_frames,
equal to zero.
Instead of explicitly specifying the origin of the read, the caller may pass the special
symbol GD_HERE as first_frame. This will result in the read occurring at the current
position of the I/O pointer for the field (see GetData I/O Pointers below for a discussion
of field I/O pointers). In this case, the value of first_sample is ignored.
The return_type argument should be one of the following symbols, which indicates the
desired return type of the data:
GD_UINT8 unsigned 8-bit integer
GD_INT8 signed (two's complement) 8-bit integer
GD_UINT16 unsigned 16-bit integer
GD_INT16 signed (two's complement) 16-bit integer
GD_UINT32 unsigned 32-bit integer
GD_INT32 signed (two's complement) 32-bit integer
GD_UINT64 unsigned 64-bit integer
GD_INT64 signed (two's complement) 64-bit integer
GD_FLOAT32 IEEE-754 standard 32-bit single precision floating point number
GD_FLOAT64 IEEE-754 standard 64-bit double precision floating point number
GD_COMPLEX64
C99-conformant 64-bit single precision complex number
GD_COMPLEX128
C99-conformant 128-bit double precision complex number
GD_NULL the null type: the database is queried as usual, but no data is
returned. In this case, data_out is ignored and may be NULL.
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 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.
Behaviour While Reading Specific Field Types
PHASE: A forward-shifted PHASE field will always encounter the end-of-field marker before
its input field does. This has ramifications when reading streaming data with
gd_getdata() and using gd_nframes(3) to gauge field lengths (that is: a forward-
shifted PHASE field always has less data in it than gd_nframes(3) implies that it
does). As with any other field, gd_getdata() will return a short count whenever a
read from a PHASE field encounters the end-of-field marker.
Backward-shifted PHASE fields do not suffer from this problem, since gd_getdata()
pads reads past the beginning-of-field marker with NaN or zero as appropriate.
Database creators who wish to use the PHASE field type with streaming data are
encouraged to work around this limitation by only using backward-shifted PHASE
fields, by writing 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 RAW field in proportion to the
maximal forward phase shift. This method will work with applications which respect
the database size reported by gd_nframes(3) resulting in these applications
effectively ignoring all frames past the frame containing the maximally forward-
shifted PHASE field's end-of-field marker.
MPLEX: Reading an 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 gd_mplex_lookback(3). 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 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 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.
WINDOW:
The samples of a 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.
RETURN VALUE
In all cases, 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. The library does not consider this an
error. Requests for data before the beginning-of-field marker, which may have been
shifted from frame zero by the presence of a 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.
If an error has occurred, zero is returned and the dirfile error will be set to a non-zero
value. Possible error values are:
GD_E_ALLOC
The library was unable to allocate memory.
GD_E_BAD_CODE
The field specified by field_code, or one of the fields it uses for input, was not
found in the database.
GD_E_BAD_DIRFILE
An invalid dirfile was supplied.
GD_E_BAD_SCALAR
A scalar field used in the definition of the field was not found, or was not of
scalar type.
GD_E_BAD_TYPE
An invalid return_type was specified.
GD_E_DIMENSION
The supplied field_code referred to a CONST, CARRAY, or STRING field. The caller
should use gd_get_constant(3), gd_get_carray(3), or gd_get_string(3) instead. Or,
a scalar field was found where a vector field was expected in the definition of
field_code or one of its inputs.
GD_E_DOMAIN
An immediate read was attempted using 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.
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.
GD_E_IO An error occurred while trying to open or read from a file on disk containing a
raw field or LINTERP table.
GD_E_LUT
A LINTERP table was malformed.
GD_E_RECURSE_LEVEL
Too many levels of recursion were encountered while trying to resolve field_code.
This usually indicates a circular dependency in field specification in the
dirfile.
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.
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.
The dirfile error may be retrieved by calling gd_error(3). A descriptive error string for
the last error encountered can be obtained from a call to gd_error_string(3).
NOTES
To save memory, gd_getdata() uses the memory pointed to by 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 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 gd_getdata() calls for efficiency's sake. Memory
used by this internal slimlib buffer can be reclaimed by calling gd_raw_close(3) on fields
when finished reading them.
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 gd_getdata().
Every RAW field in an open Dirfile has an 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 GD_HERE in the description
above). The value of the I/O pointer of a field is reported by gd_tell(3).
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 always invalid. The usual reason for this is the
derived field simultaneously reading from two different places in the same RAW field. For
example, given the following Dirfile metadata specification:
a RAW UINT8 1
b PHASE a 1
c LINCOM 2 a 1 0 b 1 0
the derived field c never has a valid I/O pointer, since any particular sample of c
ultimately involves reading from more than one place in the RAW field a. Attempting to
perform sequential reads or writes (with GD_HERE) on a derived field when its I/O pointer
is invalid will result in an error (specifically, GD_E_DOMAIN).
The implicit INDEX field has an effective I/O pointer than mostly behaves like a true RAW
field I/O pointer, except that it permits simultaneous reads from multiple locations. So,
given the following metadata specification:
d PHASE INDEX 1
e LINCOM 2 INDEX 1 0 d 1 0
the I/O pointer of the derived field e will always be valid, unlike the similarly defined
c above. The virtual I/O pointer of a derived field will change in response to movement
of the 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 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 RAW field is set to the
beginning-of-frame (the value returned by gd_bof(3)), as is the I/O pointer of any newly-
created RAW field.
The following library calls cause I/O pointers to move:
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 GD_HERE.
When reading a derived field which simultaneously reads from more than one place in
a RAW field (such as c above), the position of that RAW field's I/O pointer is
unspecified (that is: it is not specified which input field is read first).
gd_seek(3)
This function is used to manipulate I/O pointers directly.
gd_flush(3) and gd_raw_close(3)
These functions set the I/O pointer of any RAW field which is closed back to the
beginning-of-field.
calls which result in modifications to raw data files:
this may happen when calling any of: gd_alter_encoding(3), gd_alter_endianness(3),
gd_alter_frameoffset(3), gd_alter_entry(3), gd_alter_raw(3), gd_alter_spec(3),
gd_malter_spec(3), gd_move(3), or gd_rename(3); these functions close affected RAW
fields before making changes to the raw data files, and so reset the corresponding
I/O pointers to the beginning-of-field.
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
gd_seek(3) to repoisition I/O pointers before attempting further sequential operations.
SEE ALSO
dirfile(5), dirfile-encoding(5), gd_get_constant(3), gd_get_string(3), gd_error(3),
gd_error_string(3), gd_mplex_lookback(3), gd_nframes(3), gd_open(3), gd_raw_close(3),
gd_seek(3), gd_spf(3), gd_putdata(3), GD_SIZE(3)