Gd_bof report the start of data in a field
#include <getdata.h> off_t gd_bof(DIRFILE *dirfile, const char *field_code);
The gd_bof() function queries a dirfile(5) database specified by dirfile and returns the sample number of 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 for all other vector field type is the same as the beginning-of-field of whichever of its input fields that starts latest. The beginning-of-field marker for the special field INDEX is always zero.
The beginning-of-field marker for a field containing no data is in the same location as, or after, its end-of-field marker (see gd_eof(3)). 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).
Upon successful completion, gd_bof() returns the sample number of the end-of-field marker for the indicated field. On error, it returns -1 and sets the dirfile error to a non-zero error value. Possible error values are:
GD_E_BAD_CODE
The field specified by field_code or one of the fields it uses as input was not found in the database.
GD_E_BAD_DIRFILE
The supplied dirfile was invalid.
GD_E_BAD_REPR
The representation suffix specified in field_code, or in one of its inputs was not recognised.
GD_E_DIMENSION
A scalar field was found where a vector field was expected in the definition of field_code or one of its inputs, or else field_code itself specified a scalar 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.
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).