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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.

       When reading a SINDIR field, return_type must be GD_STRING.  For all  other  field  types,
       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
       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.

       SINDIR:
              The  only  allowed  return_type  when  reading  SINDIR data is GD_STRING.  The data
              argument should be of type 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   GD_STRING   constant   has   the   property:
              GD_SIZE(GD_STRING) == sizeof(const char *).  On samples where the index  vector  is
              out  of  range  of  the SARRAY, and also on samples before the index vector's frame
              offset, the value stored in data will be the NULL pointer.

       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.

       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.  this is not an error.

       Requests  for  data before the beginning-of-field marker, which may have been shifted from
       frame zero by a PHASE field or /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
       DIRFILE object which may be retrieved by a subsequent call to gd_error(3).  Possible error
       codes 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),  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_RANGE
               An  attempt was made to read data outside the addressable Dirfile range (more than
               2**63 samples past the start of the dirfile).

       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.

       A descriptive error string for the error may be obtained by calling 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.

       When operating on a platform whose size_t is N-bytes wide, a single call  of  gd_getdata()
       will  never  return  more  than  (2**(N-1) - 1) samples.  The request will be truncated at
       (2**(N-M) - 1) samples, where M 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  return_type
       is  GD_NULL  or  when  reading  from  the  INDEX  field  (i.e., even when no actual I/O or
       calculation occurs).  In all cases, the actual amount of data is returned.

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.

HISTORY

       The function getdata() appeared in GetData-0.3.0.

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

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

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

       The GD_STRING data type appeared in GetData-0.10.0.

SEE ALSO

       GD_SIZE(3),   gd_error(3),   gd_error_string(3),   gd_get_constant(3),   gd_get_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), dirfile(5), dirfile-encoding(5)