iecf — the IEC Forge
iecf is a compiler for IEC 61131-3, the language standard of programmable logic controllers. It reads Structured Text and SFC, checks them against the standard as written, and writes portable C99 that any C compiler can build for any controller, with a symbol table beside it. It is written in Rust, runs on the command line and, compiled to WebAssembly, in a browser. It is the compiler behind Prokop.
This book is for three readers:
- who writes PLC programs: Getting started, and the Behaviour reference — what each operation gives, its edge cases included: an overflow, a division by zero, a conversion that does not fit, a string too long, a date past its range;
- who puts iecf’s C on a controller: The C output — the entry points, the process image, faults and flags, the symbol table, RETAIN;
- who wants to know why: Implementer choices, where each choice the standard leaves open is made after checking what CODESYS, TwinCAT and Siemens do, and the design.
Where it stands
The checker covers the standard’s core: every declaration, type and ST statement, SFC, configurations, with each diagnostic explained under Diagnostics. The C generator covers programs over the integer, bit-string, real, time, date and string types and the derived ones, the standard functions and function blocks, the user’s functions and function blocks, and RETAIN. What it does not cover yet is reported as E0009.
Every example in the Behaviour reference is checked: CI builds it with iecf, runs it and compares the result and the flag with the page. The generated C is checked against matiec’s on the same programs (Testing against matiec), and against the programs of Prokop, iecf’s first user (The Prokop survey).
Standards
iecf follows IEC 61131-3 Ed.4 (2025) without IL, which Ed.4 removed; the clause and table numbers in this book are Ed.3’s (2013) unless they say otherwise. iecf is not affiliated with the IEC.
License
iecf is licensed under either of the Apache License, Version 2.0, or the MIT license, at your option.