# Error handling

The PSBLAS library error handling policy has been completely rewritten in version 2.0. The idea behind the design of this new error handling strategy is to keep error messages on a stack allowing the user to trace back up to the point where the first error message has been generated. Every routine in the PSBLAS-2.0 library has, as last non-optional argument, an integer `info` variable; whenever, inside the routine, an error is detected, this variable is set to a value corresponding to a specific error code. Then this error code is also pushed on the error stack and then either control is returned to the caller routine or the execution is aborted, depending on the users choice. At the time when the execution is aborted, an error message is printed on standard output with a level of verbosity than can be chosen by the user. If the execution is not aborted, then, the caller routine checks the value returned in the `info` variable and, if not zero, an error condition is raised. This process continues on all the levels of nested calls until the level where the user decides to abort the program execution.

Figure 9 shows the layout of a generic `psb_foo` routine with respect to the PSBLAS-2.0 error handling policy. It is possible to see how, whenever an error condition is detected, the `info` variable is set to the corresponding error code which is, then, pushed on top of the stack by means of the `psb_errpush`. An error condition may be directly detected inside a routine or indirectly checking the error code returned returned by a called routine. Whenever an error is encountered, after it has been pushed on stack, the program execution skips to a point where the error condition is handled; the error condition is handled either by returning control to the caller routine or by calling the `psb\_error` routine which prints the content of the error stack and aborts the program execution, according to the choice made by the user with `psb_set_erraction`. The default is to print the error and terminate the program, but the user may choose to handle the error explicitly.

Figure 9: The layout of a generic `psb_foo` routine with respect to PSBLAS-2.0 error handling policy.

Figure 10 reports a sample error message generated by the PSBLAS-2.0 library. This error has been generated by the fact that the user has chosen the invalid “FOO” storage format to represent the sparse matrix. From this error message it is possible to see that the error has been detected inside the `psb_cest` subroutine called by `psb_spasb` ... by process 0 (i.e. the root process).

Figure 10: A sample PSBLAS-2.0 error message. Process 0 detected an error condition inside the psb_cest subroutine

ifstarsubroutinesubroutinepsb_errpushPushes an error code onto the error stack

Type:
  Asynchronous.
On Entry
err_c
  the error code
  Scope: local
  Type: required
  Intent: in.
  Specified as: an integer.
r_name
  the soutine where the error has been caught.
  Scope: local
  Type: required
  Intent: in.
  Specified as: a string.
i_err
  addional info for error code
  Scope: local
  Type: optional
  Specified as: an integer array
a_err
  addional info for error code
  Scope: local
  Type: optional
  Specified as: a string.

ifstarsubroutinesubroutinepsb_errorPrints the error stack content and aborts execution

Type:
  Asynchronous.
On Entry
icontxt
  the communication context.
  Scope: global
  Type: optional
  Intent: in.
  Specified as: an integer.

ifstarsubroutinesubroutinepsb_set_errverbositySets the verbosity of error messages.

Type:
  Asynchronous.
On Entry
v
  the verbosity level
  Scope: global
  Type: required
  Intent: in.
  Specified as: an integer.

ifstarsubroutinesubroutinepsb_set_erractionSet the type of action to be taken upon error condition.

Type:
  Asynchronous.
On Entry
err_act
  the type of action.
  Scope: global
  Type: required
  Intent: in.
  Specified as: an integer. Possible values: `psb_act_ret`, `psb_act_abort`.