PACK(3PVM)

📦 version 3.4.6, in debian bookworm, ubuntu jammy

PACK(3PVM) ✨💖

okay bestie, sit DOWN, because we're talking about pvm_pack — that's the Parallel Virtual Machine packing routine, and it's literally the thing that stuffs your data into the active message buffer. This is PVM Version 3.4 energy, and yes, we're covering every single flag, no crumbs left behind. 🎀

🌸 NAME 🌸

pvm_pack — packs the active message buffer with arrays of prescribed data type. That's it, that's the whole vibe: you got data, you got a buffer, you smush them together. 💅

✨ SYNOPSIS ✨

🦋 C

int info = pvm_packf( const char *fmt, ... )
int info = pvm_pkbyte( char *xp, int nitem, int stride )
int info = pvm_pkcplx( float *cp, int nitem, int stride )
int info = pvm_pkdcplx( double *zp, int nitem, int stride )
int info = pvm_pkdouble( double *dp, int nitem, int stride )
int info = pvm_pkfloat( float *fp, int nitem, int stride )
int info = pvm_pkint( int *ip, int nitem, int stride )
int info = pvm_pkuint( unsigned int *ip, int nitem, int stride )
int info = pvm_pkushort( unsigned short *ip, int nitem, int stride )
int info = pvm_pkulong( unsigned long *ip, int nitem, int stride )
int info = pvm_pklong( long *ip, int nitem, int stride )
int info = pvm_pkshort( short *jp, int nitem, int stride )
int info = pvm_pkstr( char *sp )

🦋 Fortran

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

🎀 PARAMETERS 🎀

  • fmt — a printf-like format expression that says what to pack. There's a whole discussion on this further down, don't worry, we get there. 🔥
  • nitem — the total number of items to pack. NOT the number of bytes, bestie, do not get it twisted. 💀
  • stride — the stride used when packing the items. Like, if stride = 2 in pvm_pkcplx, then every other complex number gets packed. Skipping through your array like it's got main character energy. 👑
  • xp — pointer to the beginning of a block of bytes. Can be any data type, but it must match the corresponding unpack data type. No mixing, that's the rule. 🙈
  • 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 — an integer specifying the type of data being packed. Here are your what options, and yes I'm giving you all of them:
    • 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 mean something went wrong. 😭

💖 DESCRIPTION 💖

Each of the pvm_pk* routines packs an array of the given data type into the active send buffer. The arguments for each of the routines are:

  1. a pointer to the first item to be packed,
  2. nitem, the total number of items to pack from this array,
  3. and stride, the stride to use when packing.
Exception alert!! 🚨 pvm_pkstr() is the odd one out — by definition it packs a NULL terminated character string, so it doesn't need nitem or stride arguments. Meanwhile the Fortran routine pvmfpack( STRING, ... ) expects nitem to be the number of characters in the string, and stride to be 1.

A null string ("") can totally be packed — it's just a string with no characters before the terminating '\0'. That's fine! BUT. Packing a null string pointer, (char *)0, is not allowed. Do not do that. 🙅

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

A single variable (not an array) can be packed by setting nitem = 1 and stride = 1. Baby steps, one variable at a time. 🧚

🌈 pvm_packf() and its format expression

The routine pvm_packf() uses a printf-like format expression to specify what and how to pack data into the send buffer. All variables are passed as addresses if count and stride are specified — otherwise, variables are assumed to be values. ✨

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 — integers
  • f — float
  • x — complex float
  • s — string

Modifiers:

  • h — short (int)
  • l — long (int, float, complex float)
  • u — unsigned (int)
Heads up: '*' count or stride must match an int in the param list. 💅

🔮 Future extensions & precision tea ☕

Future extensions to the what argument in pvmfpack 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. slay. 👑

⚠️ Match your packs to your unpacks, bestie

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. iykyk. 💀

⭐ EXAMPLES ⭐

🦋 C:

info = pvm_initsend( PvmDataDefault );
info = pvm_pkstr( "initial data" );
info = pvm_pkint( &size, 1, 1 );
info = pvm_pkint( array, size, 1 );
info = pvm_pkdouble( matrix, size*size, 1 );
msgtag = 3 ;
info = pvm_send( tid, msgtag );
int count, *iarry;
double darry[4];
pvm_packf("%+ %d %*d %4lf", PvmDataRaw, count, count, iarry, darry);

🦋 Fortran:

CALL PVMFINITSEND(PVMRAW, INFO)
CALL PVMFPACK( INTEGER4, NSIZE, 1, 1, INFO )
CALL PVMFPACK( STRING, 'row 5 of NXN matrix', 19, 1, INFO )
CALL PVMFPACK( REAL8, A(5,1), NSIZE, NSIZE , INFO )
CALL PVMFSEND( TID, MSGTAG, INFO )

🚨 WARNINGS 🚨

Strings cannot be packed when using the PvmDataInPlace encoding, due to limitations in the implementation. Attempting to pack a string using pvm_pkstr or pvm_packf will cause error code PvmNotImpl to be returned. That's not a vibe. 🫶

💀 ERRORS 💀

  • PvmNoMem — Malloc has failed. The message buffer size has exceeded the available memory on this host. 💀
  • PvmNoBuf — There is no active send buffer to pack into. Try calling pvm_initsend before packing message. 🙈
  • PvmOverflow — Attempt to pack a value too large. E.g. packing an 8-byte long with XDR encoding if the value won't fit into 4 bytes. 😭

🌸 SEE ALSO 🌸

  • pvm_initsend(3PVM)
  • pvm_unpack(3PVM)
  • pvm_send(3PVM)
  • pvm_recv(3PVM)
  • pvm_pkmesg(3PVM)

and that's the whole page, bestie!! 🎀✨ This one's from the PVM Version 3.4 era, dated 30 August, 1993 — so yes, it's older than us, but the packing knowledge is timeless. go pack something iconic. 💅👑🦋

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