UNPACK(3PVM)

okay bestie, sit down, hydrate, because this one is lowkey iconic 💖 We're doing UNPACK(3PVM) — straight out of the PVM Version 3.4 manual — and it's the moment your message finally turns back into actual variables. It's giving receive buffer glow-up. ✨

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🌸 NAME 🌸

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

like, the packer packed it, now we unpack it. symmetry. periodt 💅

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🎀 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 )

look at that whole little family of routines 🫶 one per data type, all matching, all cute.

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🦋 PARAMETERS 🦋

okay here's the cast of characters, no cap:

• fmt — a Printf-like format expression specifying what to pack. (There's a whole discussion on this below, keep scrolling bestie.)
• nitem — the total number of items to be unpacked. Not bytes. Items. Do not come for me with bytes. 😭
• stride — the stride used when packing the items. For example, if stride = 2 in pvm_upkcplx, then every other complex number gets unpacked. skipping through the array like it's a playlist 🎧
• xp — pointer to the beginning of a block of bytes. Can be any data type, but it must match the corresponding pack data type. no mismatches, we don't do that here 🙈
• 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. And here's the menu of what options, memorize it or screenshot it 📸:
  • STRING = 0
  • BYTE1 = 1
  • INTEGER2 = 2
  • INTEGER4 = 3
  • REAL4 = 4
  • COMPLEX8 = 5
  • REAL8 = 6
  • COMPLEX16 = 7
• info — integer status code returned by the routine. Values less than zero indicate an error. so if it's negative, something went down 💀

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✨ 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. she's the special one and honestly? she earned it 👑

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. baby steps, one item at a time 🍓

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. so if you see a star, there's an int waiting for it in the params, bestie ⭐

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.

🚨 okay real talk, this next bit is the part you actually need: 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. like, the vibes have to match on both ends or it's chaos 😭

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🌈 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 )

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💀 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. you reached into the bag and the bag said no 🙈
• 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 into, bestie, it's giving empty 🕳️

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👑 SEE ALSO 👑

pvm_pack(3PVM) pvm_send(3PVM), pvm_recv(3PVM), pvm_pkmesg(3PVM)

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and that's the whole page, bestie 💖 this was UNPACK(3PVM) from PVM Version 3.4, dated 30 August, 1993 — an actual vintage icon, aging like fine wine 🍷✨ now go unpack something responsibly, mwah 💅🫶