UNPACK(3PVM)

💖 UNPACK(3PVM) 💖

okay so bestie, gather round, because this is the tea: it's a page from the PVM Version 3.4 manual, filed under 3PVM, and it's all about pvm_unpack — the routine that pulls your data out of the active message buffer. no cap. 🫶

---

✨ NAME ✨

pvm_unpack — unpack the active message buffer into arrays of prescribed data type. it's giving reverse-pack. 💅

---

🎀 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 🦋

• fmt — a printf-like format expression that says what to pack. (there's a whole discussion down below, iykyk 👀)
• nitem — the total number of items to be unpacked. bestie, that is not the number of bytes. do not mix them up. 💀
• stride — the stride used when packing the items. like, for example, if stride = 2 in pvm_upkcplx, then every other complex number gets unpacked. slay. ✨
• 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 allowed, periodt.
• 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. the menu is:

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

• info — integer status code returned by the routine. values less than zero indicate an error. 😭

---

🌈 DESCRIPTION 🌈

each of the pvm_upk* routines unpacks an array of the given data type from the active receive buffer. the arguments for each routine are: a pointer to the array you're unpacking into, nitem which is the total number of items to unpack, and stride which is the stride to use when unpacking. iconic trio. 👑

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. tiny but mighty.

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. it's giving "you had one job". 💀

---

🌸 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. girl, you're reaching into the void. 🙈
• 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. empty. void. ⭐

---

🫶 SEE ALSO 🫶

pvm_pack(3PVM) pvm_send(3PVM), pvm_recv(3PVM), pvm_pkmesg(3PVM) — go read those too, they're the whole friend group. 🔥

---

and that's the page, bestie! 💖 it's from PVM Version 3.4, dated 30 August, 1993, and it lives in section 3PVM of the manual. go unpack something responsibly. no cap. 💅✨🎀🦋🌸👑