a2c is an ASN.1 compiler. It reads one or more ASN.1 syntax files (often called ASN.1 modules) and emits a C program that embodies that syntax. a2c is useful to programmers who are writing C programs that will read and/or write data that is expressed in an ASN.1 format, such as PKIX certificates and CMS (S/MIME) messages.

a2c handles nearly all of the ASN.1:2002 syntax (inputs in earlier versions can be converted to ASN.1:2002). In specific, the compiler can handle ITU-T Rec. X.680 (without the XML Value notation and a couple of other minor items), X.681, X.682, X.683, and DER and BER from X.690.

The C program created by a2c consists of many structures and functions that can then be used by another program. a2c comes with an optional runtime library that can be used to greatly speed parts of that integration.