ierr

IERR is an acronym used in various computing contexts to indicate an error state. There is no universal definition; its meaning depends on the software system. In many legacy and scientific codes, IERR stands for Internal Error and is used as a generic error indicator or as the name of an error-return parameter in subroutines.

A primary well-documented use is in the Fortran bindings of the Message Passing Interface (MPI). In MPI programs written in Fortran, the last argument of most MPI procedures is an integer named ierr or IERR. This parameter returns the error status of the call: a value of MPI_SUCCESS (usually zero) indicates success, while a nonzero value encodes a specific MPI error code. The exact interpretation of IERR is defined by the MPI error handling configuration and can be converted to a human-readable message using MPI error-handling utilities that retrieve the corresponding error string.

Beyond MPI, IERR is commonly seen as a variable name or placeholder in code and logs to denote a function’s internal error or an unexpected state that is not caused by user input. Because it is not standardized, its definition and handling vary between projects, languages, and teams. Some systems treat IERR as a general catch-all error code, while others reserve it for internal consistency checks or debugging builds.

When encountering IERR, consult the relevant project’s documentation or source code to determine the exact meaning in that context, and follow the established error-handling practices, such as propagating the error, logging diagnostics, or converting codes to user-friendly messages.