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ltot

NAME

ttol, ltot, mttol, mltot - LAM data representation translation suite

C SYNOPSIS

 #include <portable.h>
 #include <t_types.h>
 
 void ttoli4 (int4 *pvar1, int4 *pvar2);
 void ltoti4 (int4 *pvar1, int4 *pvar2);
 void ttoli2 (int2 *pvar1, int2 *pvar2);
 void ltoti2 (int2 *pvar1, int2 *pvar2);
 void ttolf4 (float4 *pvar1, float4 *pvar2);
 void ltotf4 (float4 *pvar1, float4 *pvar2);
 void ttolf8 (float8 *pvar1, float8 *pvar2);
 void ltotf8 (float8 *pvar1, float8 *pvar2);
 
 void mttoli4 (int4 *array, int num);
 void mltoti4 (int4 *array, int num);
 void mttoli2 (int2 *array, int num);
 void mltoti2 (int2 *array, int num);
 void mttolf4 (float4 *array, int num);
 void mltotf4 (float4 *array, int num);
 void mttolf8 (float8 *array, int num);
 void mltotf8 (float8 *array, int num);
 

FORTRAN SYNOPSIS

 subroutine F4LTOT (lvar, tvar)
 subroutine F4TTOL (tvar, lvar)
 real*4 lvar, tvar
 
 subroutine F8LTOT (lvar, tvar)
 subroutine F8TTOL (tvar, lvar)
 real*8 lvar, tvar
 
 subroutine I4LTOT (lvar, tvar)
 subroutine I4TTOL (tvar, lvar)
 integer*4 lvar, tvar
 
 subroutine I2LTOT (lvar, tvar)
 subroutine I2TTOL (tvar, lvar)
 integer*2 tvar, lvar
 
 subroutine F4MLTT (array, num)
 subroutine F4MTTL (array, num)
 real*4 array(*)
 integer num
 
 subroutine F8MLTT (array, num)
 subroutine F8MTTL (array, num)
 real*8 array(*)
 integer num
 
 subroutine I4MLTT (array, num)
 subroutine I4MTTL (array, num)
 integer*4 array(*)
 integer num
 
 subroutine I2MLTT (array, num)
 subroutine I2MTTL (array, num)
 integer*2 array(*)
 integer num
 

DESCRIPTION

The number and order of bytes in a word may differ between nodes in any LAM network. The representation of floating point numbers may also vary. To transfer message and file data correctly, both sender and receiver in a LAM network must agree on the amount of data being exchanged and its representation. A simple solution to these two problems is to define a standard representation and require senders and receivers to convert their data from/to the local representation to/from the standard format. Conversion is not necessary if the communicating processes are on the same node, but it keeps the code portable.

The LAM header file <portable.h> defines fixed size C types for each supported architecture. Fortran already has the ability to declare variable precision.

int4

4-byte wide signed integer

uint4

4-byte wide unsigned integer

int2

2-byte wide signed integer

uint2

2-byte wide unsigned integer

float4

4-byte wide floating point number

float8

8-byte wide floating point number

Programmers wishing to write portable LAM code that can run on heterogeneous architectures are urged to use these data types for all variables that are communicated across node boundaries. To gain computing speed, programmers may wish to transfer the communicated data to the appropriate native data type on the local CPU in order to benefit from its faster access of word-sized variables. Following this approach solves the data size problem.

The LAM header file <t_types.h> defines functions that convert the order of bytes between the local representation, regardless of what that is, and a standard (LAM) representation. The functions are meant to be used in conjunction with the data types defined in <portable.h>. If no conversion is necessary, the functions will have no effect. These functions are implemented as macros and should not be accessed through pointers to functions.

When sending messages between heterogeneous nodes, both the message body (i.e. what is in the nh_msg field of the network message structure; see nsend(2)) and the message data pouch (i.e. what is in the nh_data field of the network message structure) must be converted. Other fields in the message envelope are automatically converted when the message is passed. Data conversion functions should be used by both the sending and receiving processes. Data should be converted from local to LAM representation before sending, and from LAM to local representation after receiving.

As an alternative to the byte-order translation functions, the programmer can set various bits in the nh_flags field to convert the nh_msg and/or the nh_data data in other than the default manner (see nsend(2)). The default assumption is that nh_data is an array of 8 int4 integers and that nh_msg contains raw bytes.

File data should be converted from local to LAM representation before writing, and from LAM to local representation after reading.

The conversion functions are:

ttoli4()

Convert an int4 from LAM to local representation.

ltoti4()

Convert an int4 from local to LAM representation.

ttoli2()

Convert an int2 from LAM to local representation.

ltoti2()

Convert an int2 from local to LAM representation.

ttolf4()

Convert a float4 from LAM to local representation.

ltotf4()

Convert a float4 from local to LAM representation.

ttolf8()

Convert a float8 from LAM to local representation.

ltotf8()

Convert a float8 from local to LAM representation.

mttoli4()

Convert an array of int4s from LAM to local representation.

mltoti4()

Convert an array of int4s from local to LAM representation.

mttoli2()

Convert an array of int2s from LAM to local representation.

mltoti2()

Convert an array of int2s from local to LAM representation.

mttolf4()

Convert an array of float4s from LAM to local representation.

mltotf4()

Convert an array of float4s from local to LAM representation.

mttolf8()

Convert an array of float8s from LAM to local representation.

mltotf8()

Convert an array of float8s from local to LAM representation.

The single element conversion functions accept two arguments:

pvar1

pointer to a variable of the required type holding the original data to be converted

pvar2

pointer to a variable of the required type where the resulting converted data is returned

The original data is not modified. The two pointers may be the same if the conversion is to be done "in place". The multiple element conversion functions accept two arguments:

array

pointer to the base of an array of data elements of the required type

num

the number of data elements in the array

Before the call, the array holds the data to be converted. After the function returns, the array holds the converted data. The array conversion functions can only convert data "in place".

Incompatible Word Lengths

Some CPUs do not support all data type sizes. As an example, Cray machines can access 4-byte and 8-byte integers and 8-byte floating point numbers, but do not access 2-byte integers or 4-byte floating point numbers. Until another solution is offered, when such machines are used in a LAM network it is advised to constrain the data type choices to those available on all CPUs (i.e. to follow the least common denominator approach).

SEE ALSO

nsend(2)