text: gd_bof man page
  gd_bof — find the start of data in a Dirfile field
  Synopsis
  #include <getdata.h>
  off_t gd_bof(DIRFILE *dirfile, const char *field_code);
  Description
  The gd_bof() function queries a dirfile(5) database specified by dirfile and finds the beginning-of-field marker for the vector field given by field_code.
  The caller should not assume that the beginning-of-field marker falls on a frame boundary. The beginning-of-field marker is never negative.
  For a RAW field, the beginning-of-field corresponds to the frame offset of that field (see gd_frameoffset(3)). The beginning-of-field marker of the special field INDEX is zero.
  The beginning-of-field of a PHASE field is the beginning-of-field of its input adjusted by the PHASE field's shift (or zero, if the shift would make it negative). The beginning-of-field for all other vector fields is the the latest beginning-of-field of any of its input fields.
  If the beginning-of-field marker of a field is greather than or equal to its end-of-field marker (see gd_eof(3)), then that field contains no data. For a RAW field, the difference between the locations of the beginning- and end-of-field markers indicates the number of samples of data actually stored on disk.
  The dirfile argument must point to a valid DIRFILE object previously created by a call to gd_open(3).
  Return Value
  Upon successful completion, gd_bof() returns a non-negative integer which is the sample number of the beginning-of-field marker for the specified field. in the definition of field_code or one of its inputs, or else field_code itself specified a scalar field.
  - 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 GetData developers.
  - 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.
  The error code is also stored in the DIRFILE object and may be retrieved after this function returns by calling gd_error(3). A descriptive error string for the error may be obtained by calling gd_error_string(3).
  History
  The gd_bof() function appeared in GetData-0.7.0.
  Before GetData-0.10.0, this function could also fail with the error code GD_E_ALLOC.
  In GetData-0.10.0, the error return from this function changed from -1 to a negative-valued error code.
  See Also
  gd_eof(3), gd_error(3), gd_error_string(3), gd_frameoffset(3), gd_open(3), dirfile(5), dirfile-encoding(5)
  Referenced By
  gd_bof64(3), gd_eof(3), gd_getdata(3), gd_nframes(3).