If an illegal value is supplied for one of the input arguments to a ScaLAPACK routine, it will call the error handler PXERBLA to write a message to the standard output unit of the form:
** On entry to PSGESV parameter number 4 had an illegal valueThis particular message could be caused by passing to PSGESV a value of NRHS that was less than zero, for example. The arguments are checked in order, beginning with the first. As mentioned in Chapter 4, if an error is detected in the
entry of a descriptor array,
which is the 
argument in the parameter list, the
number passed to PXERBLA has been arbitrarily
chosen to be 100*i+j. This allows the user to
distinguish an error on a descriptor entry from an
error on a scalar argument.
Invalid arguments are often caused by the kind of error listed in
section 7.3.
In the model implementation of PXERBLA that is supplied with ScaLAPACK, the only action that is performed is the printing of an error message to standard output. Program execution is not terminated. For the ScaLAPACK driver and computational routines, a RETURN statement is issued following the call to PXERBLA. Control returns to the higher-level calling routine, and it is left to the user to determine how the program should proceed. However, in the specialized low-level ScaLAPACK routines (auxiliary routines that are Level 2 equivalents of computational routines), the call to PXERBLA() is immediately followed by a call to BLACS_ABORT() to terminate program execution since recovery from an error at this level in the computation is not possible.
It is always good practice to check for a nonzero value of INFO on return from a ScaLAPACK routine.