.\" gd_constants.3.  The gd_constants man page.
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.TH gd_constants 3 "4 December 2015" "Version 0.9.1" "GETDATA"
.SH NAME
gd_constants, gd_mconstants \(em retrieve a list of constant values from a dirfile
.SH SYNOPSIS
.B #include <getdata.h>
.HP
.nh
.ad l
.BI "const void *gd_constants(DIRFILE *" dirfile ", gd_type_t
.IB return_type );
.HP
.BI "const void *gd_mconstants(DIRFILE *" dirfile ", const char"
.BI * parent ", gd_type_t " return_type );
.hy
.ad n
.SH DESCRIPTION
The
.BR gd_constants ()
function queries a dirfile(5) database specified by
.I dirfile
and returns a read-only list of values of the all
.B CONST
fields defined in the database, excluding meta subfields.  The values returned
are converted to the data type specified by
.IR return_type .
The 
.I return_type 
should be one of the following symbols:
.IP
.nh
.ad l
.BR GD_UINT8 ,\~ GD_INT8 ,\~ GD_UINT16 ,\~ GD_INT16 ,\~ GD_UINT32 ,\~ GD_INT32 ,
.BR GD_UINT64, \~ GD_INT64 ,\~ GD_FLOAT32 ,\~ GD_FLOAT64 ,\~ GD_COMPLEX64 ,
.BR GD_COMPLEX128 .
.ad n
.hy
.PP
See
.BR gd_get_constant (3)
for the meaning of these symbols.  The 
.I dirfile
argument must point to a valid DIRFILE object previously created by a call to
.BR gd_open (3).


The
.BR gd_mconstants ()
function returns the same list for
.B CONST
meta subfields of the parent field specified by
.IR parent .

The list returned should not be assumed to be in any particular order, although
it is guaranteed to be in the same order as the list of
.B CONST
fields returned by
.BR gd_field_list_by_type (3)
or
.BR gd_mfield_list_by_type (3).
The array is unterminated.  The number of values in the array can be obtained
from a call to
.BR gd_nfields_by_type (3)
or
.BR gd_nmfields_by_type (3),
as appropriate.

Memory for the returned array is managed by GetData and should not be
de-allocated by the caller.  The caller may not modify any values in the array.
Doing so may cause database corruption.  The pointer returned is guaranteed to
be valid only until the function is called again, with the exception that, since
each top-level field maintains its own arrays, calling
.BR gd_mconstants ()
for a one parent field does not invalidate the pointer returned for a any other
parent.

Modifying the dirfile's metadata invalidates any pointer returned by these
functions.  And no pointers remain valid after the dirfile is de-allocated by a
call to
.BR gd_close (3)
or
.BR gd_discard (3).
Once a pointer is invalidated, the caller should not dereference it.

A corresponding list of names for these fields may be obtained by calling
.BR gd_field_list_by_type (3)
or
.BR gd_mfield_list_by_type (3).

.SH RETURN VALUE
Upon successful completion,
.BR gd_constants ()
returns a pointer to an array containing the values of all the constants defined
in the dirfile database.  Similarly,
.BR gd_mconstants ()
returns a pointer to an array containing the values of the all the meta subfield
constants of the specified parent field.

If no constants of the appropriate class are defined in the database, NULL is
returned.  On error, it returns NULL and sets the dirfile error to a non-zero
error value.  Possible error values are:
.TP 8
.B GD_E_ALLOC
The library was unable to allocate memory.
.TP
.B GD_E_BAD_CODE
The specified
.I parent
field code was not found, or referred to a metafield
.RB ( gd_mconstants ()
only).
.TP
.B GD_E_BAD_DIRFILE
The supplied dirfile was invalid.
.TP
.B GD_E_BAD_TYPE
The
.I return_type
specified was not one of the symbols listed above.
.TP
.B 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.
.PP
The dirfile error may be retrieved by calling
.BR gd_error (3).
A descriptive error string for the last error encountered can be obtained from
a call to
.BR gd_error_string (3).
.SH SEE ALSO
.BR dirfile (5),
.BR gd_open (3),
.BR gd_error (3),
.BR gd_error_string (3),
.BR gd_field_list_by_type (3),
.BR gd_get_constant (3),
.BR gd_mfield_list_by_type (3),
.BR gd_nfields_by_type (3),
.BR gd_nmfields_by_type (3)