A validator is a back-end program that does validation and credit-granting. You must supply a validator for each application in your project. BOINC supplies a framework program validator.C. This program must be linked with two application-specific functions:

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int check_set(vector results, DB_WORKUNIT& wu, int& canonicalid, double& credit, bool& retry);
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• check_set() takes a set of results (all with outcome=SUCCESS). It reads and compares their output files. If there is a quorum of matching results, it selects one of them as the canonical result, returning its ID. In this case it also returns the credit to be granted for correct results for this workunit.
• If, when an output file for a result has a nonrecoverable error (e.g. the directory is there but the file isn't, or the file is present but has invalid contents), then it must set the result's outcome (in memory, not database) to VALIDATE_ERROR.

  Note: use BOINC's back-end utility functions to get file pathnames and to distinguish recoverable and nonrecoverable file-open errors.

• If a canonical result is found, check_set() must set the validate_state field of each non-ERROR result (in memory, not database) to either VALID or INVALID.
• If a recoverable error occurs while reading output files (e.g. a directory wasn't visible due to NFS mount failure) then check_set() should return retry=true. This tells the validator to arrange for this WU to be processed again in a few hours.
• check_set() should return nonzero if a major error occurs. This tells the validator to write an error message and exit.

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int check_pair(RESULT& new_result, RESULT& canonical_result, bool& retry);
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• check_pair() compares a new result to the canonical result. In the absence of errors, it sets the new result's validate_state to either VALID or INVALID.
• If it has a nonrecoverable error reading an output file of either result, or if the new result's output file is invalid, it must set the new result's outcome (in memory, not database) to VALIDATE_ERROR.
• If it has a recoverable error while reading an output file of either result, it returns retry=true, which causes the validator to arrange for the WU to be examined again in a few hours.
• check_pair() should return nonzero if a major error occurs. This tells the validator to write an error message and exit.

Neither function should delete files or access the BOINC database.

A more detailed description is here.

Two example validators are supplied (each implements check_set() and check_pair()):

• sample_bitwise_validator requires a strict majority, and regards results as equivalent only if they agree byte for byte.
• sample_trivial_validator regards any two results as equivalent if their CPU time exceeds a given minimum.