gd_getdata(3)

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.

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.

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.

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_FIELD_TYPE
    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.

GD_E_BAD_REPR
    The representation suffix specified in field_code, or in one of the field codes it uses for input, was invalid.

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
    A scalar field was found where a vector field was expected.

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_OPEN_LINFILE
    An error occurred while trying to read a LINTERP table from disk.

GD_E_RAW_IO
    An error occurred while trying to open or read from a file on disk containing a raw field.

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).

LIMITATIONS
The PHASE field type is poorly defined, since a forward-shifted PHASE field will always encounter the end-of-field marker before its input field does. This has ramifications when using gd_getdata() with streaming data. The Dirfile Standards make tacit admission to this problem by indicating the results of reading a PHASE field beyond the beginning- or end-of-field is "implementation dependent" (see dirfile-format(5)). 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 time 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 first 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.

SEE ALSO
dirfile(5), dirfile-encoding(5), gd_open(3), gd_get_constant(3), gd_error(3), gd_error_string(3), gd_nframes(3), gd_spf(3), gd_get_string(3), gd_putdata(3)