getdata (3) - Linux Manuals

getdata: retrieve data from a dirfile database


getdata --- retrieve data from a dirfile database


#include <getdata.h>
size_t 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);


The 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, 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 dirfile_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 get_spf(3). The number of samples fetched is, similarly,
num_frames * samples_per_frame + num_samples.
Although calling 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:

unsigned 8-bit integer
signed (two's complement) 8-bit integer
unsigned 16-bit integer
signed (two's complement) 16-bit integer
unsigned 32-bit integer
signed (two's complement) 32-bit integer
unsigned 64-bit integer
signed (two's complement) 64-bit integer
IEEE-754 standard 32-bit single precision floating point number
IEEE-754 standard 64-bit double precision floating point number
C99-conformant 64-bit single precision complex number
C99-conformant 128-bit double precision complex number
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.


In all cases, 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:

The library was unable to allocate memory.
The field specified by field_code, or one of the fields it uses for input, was not found in the database.
An invalid dirfile was supplied.
The supplied field_code referred to a CONST or STRING field. The caller should use get_constant(3), or get_string(3) instead.
The representation suffix specified in field_code, or in one of the field codes it uses for input, was invalid.
A scalar field used in the definition of the field was not found, or was not of scalar type.
An invalid return_type was specified.
A scalar field was found where a vector field was expected.
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.
An error occurred while trying to read a LINTERP table from disk.
An error occurred while trying to open or read from a file on disk containing a raw field.
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.
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.
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 get_error(3). A descriptive error string for the last error encountered can be obtained from a call to get_error_string(3).


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 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, 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 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 get_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.