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Compiling our circuit

Once you have the compiler installed you can see the available options as follows:

circom --help

   circom compiler 2.1.7
   Compiler for the Circom programming language

       circom [FLAGS] [OPTIONS] [input]

           --r1cs                                 Outputs the constraints in r1cs format
           --sym                                  Outputs witness in sym format
           --wasm                                 Compiles the circuit to wasm
           --json                                 Outputs the constraints in json format
           --wat                                  Compiles the circuit to wat
       -c, --c                                    Compiles the circuit to c
           --O0                                   No simplification is applied
           --O1                                   Only applies signal to signal and signal to constant simplification
           --O2                                   Full constraint simplification
           --verbose                              Shows logs during compilation
           --inspect                              Does an additional check over the constraints produced
           --use_old_simplification_heuristics    Applies the old version of the heuristics when performing linear
           --simplification_substitution          Outputs the substitution in the simplification phase in json format
       -h, --help                                 Prints help information
       -V, --version                              Prints version information

       -o, --output <output>                    Path to the directory where the output will be written [default: .]
       -p, --prime <prime>                      To choose the prime number to use to generate the circuit. Receives the
                                                name of the curve (bn128, bls12381, goldilocks, grumpkin, pallas, vesta)
                                                [default: bn128]
       -l <link_libraries>...                   Adds directory to library search path
           --O2round <simplification_rounds>    Maximum number of rounds of the simplification process

      <input>    Path to a circuit with a main component [default: ./circuit.circom]

We created a template called Multiplier2 in Writing our first circuit. However, to actually create a circuit, we have to create an instance of this template. To do so, create a file with the following content:

pragma circom 2.0.0;

template Multiplier2() {
    signal input a;
    signal input b;
    signal output c;
    c <== a*b;

 component main = Multiplier2();

After we write our arithmetic circuit using circom, we should save it in a file with the .circom extension. Remember that you can create your own circuits or use the templates from our library of circuits circomlib.

In our example, we create a file called multiplier2.circom. Now is time to compile the circuit to get a system of arithmetic equations representing it. As a result of the compilation we will also obtain programs to compute the witness. We can compile the circuit with the following command:

circom multiplier2.circom --r1cs --wasm --sym --c

With these options we generate three types of files:

  • --r1cs: it generates the file multiplier2.r1cs that contains the R1CS constraint system of the circuit in binary format.
  • --wasm: it generates the directory multiplier2_js that contains the Wasm code (multiplier2.wasm) and other files needed to generate the witness.
  • --sym : it generates the file multiplier2.sym , a symbols file required for debugging or for printing the constraint system in an annotated mode.
  • --c : it generates the directory multiplier2_cpp that contains several files (multiplier2.cpp, multiplier2.dat, and other common files for every compiled program like main.cpp, MakeFile, etc) needed to compile the C code to generate the witness.

We can use the option -o to specify the directory where these files are created.

Since version 2.0.8, we can use the option -l to indicate the directory where the directive include should look for the circuits indicated.