gd_seek(3) reposition a dirfile field pointer


#include <getdata.h>
off_t gd_seek(DIRFILE *dirfile, const char *field_code, off_t frame_num, off_t sample_num, int flags);


The gd_seek() function changes the position of the I/O pointer associated with the field field_code in the dirfile(5) database specified by dirfile. In normal operation, gd_seek() advances the field I/O pointer frame_num frames plus sample_num samples from the origin point specified in flags, which should contain one of GD_SEEK_SET, GD_SEEK_CUR, or GD_SEEK_END, indicating, respectively, sample zero, the current position of the field pointer, and the location of the end-of-field marker (see gd_eof(3)).

In addition to one of the symbols above, the flags parameter may also, optionally, be bitwise or'd with GD_SEEK_WRITE, which will result in the field being padded (with zero for integer types, or a IEEE-754 conforming not-a-number otherwise) in the event of seeking past the end-of-field marker.

The effect of attempting to seek past the end-of-field is encoding specific. Some encodings don't actually add the padding requested by GD_SEEK_WRITE unless a subsequent gd_putdata(3) call is used to add more data to the field at the new end-of-field. Other encodings add the padding, advancing the end-of-field, regardless of subsequent writes. Similarly, attempting to seek past the end-of-field marker in read mode (without specifying GD_SEEK_WRITE) is also encoding specific: in some encodings the field pointer will be moved past the end-of-field marker, while in others, it will be repositioned to the end of field. Check the return value to determine the result.

In general, GD_SEEK_WRITE should be used on gd_seek() calls before a write via gd_putdata(3), while calls before a read via gd_getdata(3) should omit the GD_SEEK_WRITE flag. So the following:

gd_seek(dirfile, field_code, a, b, GD_SEEK_SET | GD_SEEK_WRITE);
gd_putdata(dirfile, field_code, GD_HERE, 0, c, d, type, data);

is equivalent to:

gd_putdata(dirfile, field_code, a, b, c, d, type, data);

and, similarly,

gd_seek(dirfile, field_code, a, b, GD_SEEK_SET);
gd_getdata(dirfile, field_code, GD_HERE, 0, c, d, type, data);

is equivalent to:

gd_getdata(dirfile, field_code, a, b, c, d, type, data);

Only RAW fields (and the implicit INDEX field) have field I/O pointers associated with them. Calling gd_seek() on a derived field will move the field pointers of all of the field's inputs. It is possible to create derived fields which simultaneously read from different places of the same input field. Calling gd_seek() on such a field will result in an error.


Upon successful completion, gd_seek() returns the field position of the specified field in samples. On error, it returns -1 and set the dirfile error to a non-zero value. Possible error values are:
The library was unable to allocate memory.
The flags parameter didn't contain exactly one of GD_SEEK_SET, GD_SEEK_CUR, or GD_SEEK_END.
The field specified by field_code, or one of the fields it uses for input, was not found in the database.
The supplied dirfile was invalid.
An attempt was made to seek relative to GD_SEEK_END on the INDEX field, which has no end-of-field marker.
The specified field or one of its inputs wasn't of vector type.
The field position couldn't be set due to a derived field reading simultaneously from more than one place in a RAW field.
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 open or read from a file on disk containing a raw field.
The request resulted an attempt to move the I/O pointer of the specified field or one of its inputs to a negative sample number.
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 gd_error(3). A descriptive error string for the last error encountered can be obtained from a call to gd_error_string(3).