Ninja can use another program as a frontend to display build status information. This document describes the interface between Ninja and the frontend.
The frontend is passed to Ninja using a --frontend argument. The argument is executed the same as a build rule Ninja, wrapped in sh -c on Linux. The frontend will be executed with the read end of a pipe open on file descriptor 3.
Ninja will pass Protocol Buffers generated from src/frontend.proto.
The frontend will have stdin, stdout, and stderr connected to the same file descriptors as Ninja. The frontend MAY read from stdin, however, if it does, it MUST NOT read from stdin whenever a job in the console pool is running, from when an EdgeStarted message is received with the use_console value set to true, to when an EdgeFinished message is received with the same value for id. Console rules may write directly to the same stdout/stderr as the frontend.
The frontend MUST exit when the input pipe on fd 3 is closed. When a build finishes, either successfully, due to error, or on interrupt, Ninja will close the pipe and then block until the frontend exits.
To run Ninja with a frontend that mimics the behavior of Ninja's normal output:
$ ./ninja --frontend=frontend/native.py
To save serialized output to a file:
$ ./ninja --frontend='cat <&3 > ninja.pb' all
To run a frontend with serialized input from a file:
$ frontend/native.py 3< ninja.pb