UNPACK(3PVM)

📦 version 3.4.6, in debian bookworm, ubuntu jammy

✨ UNPACK 💅 — pvm_unpack, aka the PVM Version 3.4 glow-up where your message buffer finally gets to spill its tea ☕

okay bestie, sit down, hydrate, because this one is literally the moment your active receive buffer gets unpacked into arrays of a prescribed data type. no cap. it's giving "the data was always there, you just needed the right routine to let it out." 🦋

NAME 🎀

pvm_unpack — Unpack the active message buffer into arrays of prescribed data type.

SYNOPSIS ✨

C 🍓

int info = pvm_unpackf( const char *fmt, ... )
int info = pvm_upkbyte( char *xp, int nitem, int stride)
int info = pvm_upkcplx( float *cp, int nitem, int stride)
int info = pvm_upkdcplx( double *zp, int nitem, int stride)
int info = pvm_upkdouble( double *dp, int nitem, int stride)
int info = pvm_upkfloat( float *fp, int nitem, int stride)
int info = pvm_upkint( int *ip, int nitem, int stride)
int info = pvm_upkuint( unsigned int *ip, int nitem, int stride )
int info = pvm_upkushort( unsigned short *ip, int nitem, int stride )
int info = pvm_upkulong( unsigned long *ip, int nitem, int stride )
int info = pvm_upklong( long *ip, int nitem, int stride)
int info = pvm_upkshort( short *jp, int nitem, int stride)
int info = pvm_upkstr( char *sp )

Fortran 🌸

call pvmfunpack( what, xp, nitem, stride, info )

PARAMETERS 💖

okay so every single one of these, no skipping, they all matter:

fmt — a printf-like format expression that says what to pack. go read the discussion below, it's important, i'm not being dramatic (i am a little).
nitem — the total number of items to be unpacked. NOT the number of bytes, bestie, this is the classic trap. 🙈
stride — the stride to use when packing the items. for example, if stride = 2 in pvm_upkcplx, then every other complex number will be unpacked. so you're skipping, cute, very selective of you 💅
xp — pointer to the beginning of a block of bytes. can be any data type, but it must match the corresponding pack data type.
cp — Complex array at least nitem*stride items long.
zp — Double precision complex array at least nitem*stride items long.
dp — Double precision real array at least nitem*stride items long.
fp — Real array at least nitem*stride items long.
ip — Integer array at least nitem*stride items long.
jp — Integer*2 array at least nitem*stride items long.
sp — Pointer to a null terminated character string.
what — Integer specifying the type of data being unpacked.
info — Integer status code returned by the routine. Values less than zero indicate an error. 😭

what options 👑

STRING = 0, REAL4 = 4
BYTE1 = 1, COMPLEX8 = 5
INTEGER2 = 2, REAL8 = 6
INTEGER4 = 3, COMPLEX16 = 7

DESCRIPTION 🌈

Each of the pvm_upk* routines unpacks an array of the given data type from the active receive buffer. The arguments for each of the routines are a pointer to the array to be unpacked into, nitem which is the total number of items to unpack, and stride which is the stride to use when unpacking.

An exception is pvm_upkstr() which by definition unpacks a NULL terminated character string and thus does not need nitem or stride arguments. The Fortran routine pvmfunpack( STRING, ... ) expects nitem to be the number of characters in the string and stride to be 1.

If the unpacking is successful, info will be 0. If some error occurs then info will be < 0. 😭

A single variable (not an array) can be unpacked by setting nitem = 1 and stride = 1. iconic, one item, one stride, one little guy 🫶

The routine pvm_unpackf() uses a printf-like format expression to specify what and how to unpack data from the receive buffer. All variables are passed as addresses. A BNF-like description of the format syntax is:

format : null | init | format fmt
init : null | '%' '+'
fmt : '%' count stride modifiers fchar
fchar : 'c' | 'd' | 'f' | 'x' | 's'
count : null | [0-9]+ | '*'
stride : null | '.' ( [0-9]+ | '*' )
modifiers : null | modifiers mchar
mchar : 'h' | 'l' | 'u'

Formats: 🎀

+ means initsend - must match an int (how) in the param list.
c pack/unpack bytes
d integer
f float
x complex float
s string

Modifiers: ✨

h short (int)
l long (int, float, complex float)
u unsigned (int)

'*' count or stride must match an int in the param list.

Future extensions to the what argument in pvmfunpack will include 64 bit types when XDR encoding of these types is available. Meanwhile users should be aware that precision can be lost when passing data from a 64 bit machine like a Cray to a 32 bit machine like a SPARCstation. As a mnemonic the what argument name includes the number of bytes of precision to expect. By setting encoding to PVMRAW (see pvmfinitsend) data can be transferred between two 64 bit machines with full precision even if the PVM configuration is heterogeneous. 👀

Messages should be unpacked exactly like they were packed to insure data integrity. Packing integers and unpacking them as floats will often fail because a type encoding will have occurred transferring the data between heterogeneous hosts. Packing 10 integers and 100 floats then trying to unpack only 3 integers and the 100 floats will also fail. the vibes must match. the symmetry must match. do not freestyle this. 💀

EXAMPLES 🧚

C: 🍓

info = pvm_recv( tid, msgtag );
info = pvm_upkstr( string );
info = pvm_upkint( &size, 1, 1 );
info = pvm_upkint( array, size, 1 );
info = pvm_upkdouble( matrix, size*size, 1 );

int count, *iarry;
double darry[4];
   pvm_unpackf("%d", &count);
   pvm_unpackf("%*d %4lf", count, iarry, darry);

Fortran: 🌸

CALL PVMFRECV( TID, MSGTAG, INFO );
CALL PVMFUNPACK( INTEGER4, NSIZE, 1, 1, INFO )
CALL PVMFUNPACK( STRING, STEPNAME, 8, 1, INFO )
CALL PVMFUNPACK( REAL4, A(5,1), NSIZE, NSIZE , INFO )

ERRORS 😭

PvmNoData — Reading beyond the end of the receive buffer. Most likely cause is trying to unpack more items than were originally packed into the buffer. bestie you cannot get blood from a stone, or items from a buffer that never had them 💀
PvmBadMsg — The received message can not be decoded. Most likely because the hosts are heterogeneous and the user specified an incompatible encoding. Try setting the encoding to PvmDataDefault (see pvm_mkbuf).
PvmNoBuf — There is no active receive buffer to unpack. nothing to unpack, no main character, empty stage, tragic 🎀