Historical version. PostgreSQL 7.2 has reached end of support. See the current manual.
SPI_fnumber
Usage
Attribute numbers are 1 based.
If the given fname refers to a system attribute (eg, oid) then the appropriate negative attribute number will be returned. The caller should be careful to test for exact equality to SPI_ERROR_NOATTRIBUTE to detect error; testing for result <= 0 is not correct unless system attributes should be rejected.
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Read the upstream documentation. For corrections, first check the current manual.