Ubuntu Manpage: gd_getdata — retrieve data from a dirfile database

name
synopsis
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return value
limitations
see also

Provided by: libgetdata-dev_0.7.3-6ubuntu1_amd64

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.

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

LIMITATIONS

SEE ALSO