### E THE NDF_ LIBRARY C INTERFACE

#### E.1 Conventions Used in the C Interface

As of Version 2.0, the NDF_ library is implemented in C, with an additional layer providing the traditional Fortran interface described in the rest of this manual. This appendix describes the underlying C interface, which allows NDF functions to be called from programs written in C (see Appendix F for a list of the C functions available). It is quite easy to translate the Fortran descriptions and examples given in the rest of this manual into C once you are aware of the conventions used. The following notes are intended to assist with this:

(1)
C function names are derived from the corresponding Fortran routine names by removing the underscore, converting to lower case and then capitalising the fourth character. Thus, the Fortran routine NDF_ABCDEF becomes the C function ndfAbcdef.
(2)
A single header file “ndf.h” is provided to define the C interface. This contains function prototypes together with C equivalent definitions for all the symbolic constants and error codes used by the Fortran version. All the constants use exactly the same names (in upper case) as in Fortran. It is recommended that you always include this file, since some functions may be implemented via C macros and will therefore only be available in this way.
(3)
The data types used in Fortran and C for routine arguments and returned values are related as follows:
 Fortran Type C Type DOUBLE PRECISION double REAL float INTEGER int LOGICAL int CHARACTER char $\ast$

Note that the C interface uses “int” for both the Fortran INTEGER and LOGICAL data types, but interprets the latter according to whether the C integer is non-zero or zero (corresponding with Fortran’s .TRUE. and .FALSE. respectively).

(4)
Input scalar arguments are always passed in C by value.
(5)
Non-scalar input arguments (typically strings) are passed in C by pointer, qualified by “const” as appropriate.
(6)
All output arguments are passed in C by pointer. In the case of output array arguments, the caller must ensure that adequate space has been allocated to receive the returned values (also see the notes on passing character strings below). The C array notation “[ ]” is used in the function prototypes to indicate where a returned value is an array rather than a scalar value.
(7)
All C strings should be null-terminated.
(8)
Whenever a C string value is to be returned (via a argument “arg”, say, with type char $\ast$), the argument is followed in C by an associated integer length argument (“arg_length”) which does not appear in the Fortran version of the routine. This additional argument specifies the length of the character array allocated to receive the string, including the final null. Truncation of the returned string will occur if this length would be exceeded.
(9)
An array of character strings is passed in C as an array of pointers to null-terminated strings (like the standard argument vector passed to the C “main” function).
(10)
A few functions pass HDS locators. These are stored in variables of type HDSLoc $\ast$. The calling function should include the hds.h header file.
(11)
Strings which are used to describe the data type of NDF components must contain the same text in C as in Fortran. Hence, you should continue to use “_REAL”, for example, (and not “_FLOAT”) when specifying the data type of a new NDF.
(12)
When mapping NDF array components, the C interface will usually return a pointer to void, reflecting the fact that the data type is determined at run time and is therefore not known to the mapping function. To access the mapped data, you should cast this pointer to the appropriate pointer type before use, as follows:
 Mapped Data Type C Pointer Cast _DOUBLE (double $\ast$) _REAL (float $\ast$) _INTEGER (int $\ast$) _WORD (short $\ast$) _UWORD (unsigned short $\ast$) _BYTE (signed char $\ast$) _UBYTE (unsigned char $\ast$)
(13)
Remember that the data storage order used when mapping multi-dimensional array data follows the Fortran convention (where the first array index varies most rapidly) and not the C convention (where the final array index varies most rapidly).
(14)
Several functions pass pointers to Objects defined by the AST library (SUN/211) for handling world coordinate system information. These use the same C argument passing conventions for these pointers as used in the AST library itself.

In a multi-threaded context, each base NDF is either ”locked” by a specific thread, or ”unlocked”. Locking is a long term contract that persists between invocations of public NDF functions, and can be changed only by the caller using one of the public lock-management methods: ndfLock, ndfUnlock and ndfLocked.

A locking contract means that:

• only the thread with the lock can invokes public NDF functions that read, write or modify the base NDF through any identifier.
• if a thread attempts to invoke a public NDF function to read, write or modify an NDF which it has not locked, an error is reported. The only exception is that ndfLock can be called on an NDF that is not locked by any thread (i.e. the identifier passed to ndfLock has been unlocked previously using ndfUnlock).

Upon creation, an NDF is always locked by the current thread. The locker thread can at any time unlock the NDF by calling ndfUnlock on any identifier for the NDF30, allowing another thread subsequently to lock the NDF by calling ndfLock on the same identifier. An error is reported if the NDF is currently locked by a different thread. Calling ndfUnlock will unlock both the supplied identifier and the associated base NDF - an error is reported if the NDF is currently locked by another thread.

A consequence of this is that an NDF identifier will become unsable if another identifier for the same base NDF is unlocked. However, the identifier will become usable again if the original thread regains the lock on the NDF.

When an NDF identifier is unlocked, its associated context level is set to a special value indicating it is not in any context. All other identifiers for the same base NDF are left unchanged. When an NDF identifier is locked, its associated context level is set to the current context level in the thread that locks it. The ndfEnd function only annuls NDF identifiers for base NDFs that are locked by the current thread. The function ndfReport will report the lock state of all currently active NDF identifiers.

#### E.3 C-only Functions

This section contains descriptions of functions that are only available in the C interface.

30All other identifiers referring to the same base NDF remain valid but cannot be used without error until the original locker thread has regained the lock.