PACK(3PVM)

💖 PACK — aka PACK(3PVM) ✨

okay bestie, sit down, hydrate, because we're about to talk about pvm_pack — the routine that packs the active message buffer with arrays of prescribed data type. yes it's a mouthful, no we're not skipping it. this is PVM Version 3.4 energy and it's giving foundational. 🫶

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🎀 SYNOPSIS 🎀

C — look at all these lil functions, they're all so iconic:

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 — the one and only:

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

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🌸 PARAMETERS 🌸

literally every single one, no skips, we're thorough in this house:

• fmt — a printf-like format expression specifying what to pack. (there's a whole discussion below, keep scrolling 💅)
• nitem — the total number of items to be packed. not the number of bytes, bestie, do not get it twisted.
• stride — the stride to be used when packing the items. for example, if stride = 2 in pvm_pkcplx, then every other complex number will be packed. iconic behaviour honestly.
• 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 cap.
• 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. the menu of options:
  • 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. 😭

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🦋 DESCRIPTION 🦋

so here's the tea ☕ — each of the pvm_pk* routines packs an array of the given data type into the active send buffer. the arguments for each routine are: a pointer to the first item to be packed, nitem (the total number of items to pack from this array), and stride (the stride to use when packing).

one exception, and she's special: pvm_pkstr() — by definition she packs a NULL terminated character string, so she doesn't need nitem or stride arguments. queen behaviour. 👑 meanwhile the Fortran routine pvmfpack( STRING, ... ) does expect nitem to be the number of characters in the string and stride to be 1.

a null string ("") can be packed — that's just a string with no characters before the terminating '\0'. however, packing a null string pointer, (char *)0, is not allowed. 🚫

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

wanna pack a single variable and not an array? set nitem = 1 and stride = 1. easy. slay. ✨

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💫 the pvm_packf() situation 💫

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. important distinction, don't fumble it.

a BNF-like description of the format syntax:

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 — the moment we've all been waiting for:

• + — 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)

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

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🔮 future 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. iykyk.

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🧚 pack how you unpack, 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. and packing 10 integers and 100 floats then trying to unpack only 3 integers and the 100 floats will also fail. matching energy only, bestie. 🫶

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🌈 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 );

and then this lil guy:

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 )

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⚠️ 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. 💀

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😭 ERRORS 😭

• PvmNoMem — Malloc has failed. 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.

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👑 SEE ALSO 👑

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

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and that's the whole vibe, bestie — this page is from PVM Version 3.4, dated 30 August, 1993, and it goes by PACK(3PVM). oldie but a goodie, fr fr. go forth and pack responsibly 💖✨🎀🦋