---
name: "pvm_unpack"
section: "3"
platform: "linux"
package: "pvm-dev"
version: "3.4.2-8"
release: "Debian 3.0 (Woody)"
platforms: ["linux"]
url: "https://new.carta.tech/pvm_unpack"
---

# pvm\_unpack(3)

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

## 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

- fmt

  **Printf-like format expression specifying what to pack. (See discussion)**\

- nitem

  **The total number of** *items* to be unpacked (not the number of bytes).\

- stride

  The stride to be used when packing the items. For example, if stride = 2 in pvm\_upkcplx, then every other complex number will be unpacked.\

- xp

  Pointer to the beginning of a block of bytes. Can be any data type, but 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.

  *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 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.

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)
```

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.

## 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.

- 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.

## SEE ALSO

pvm\_pack(3PVM) pvm\_send(3PVM), pvm\_recv(3PVM), pvm\_pkmesg(3PVM)