I've found and isolated a bug with parallel builds (-j<n>) and jam 2.5rc3.

A small source tree that exhibits the bug is at //guest/matt_armstrong/jam/bug/1/...

A .h file is generated at run time. It gets into the dependency tree via normal header scanning.

With -j1 builds, the generated.h file is built first and everything is fine. With -j2 or higher, the generated.h file is built first, but other .c files that #include it get built in parallel and fail.

I haven't been able to decipher make1[abcd] well enough to figure out what is going on or develop a fix. I have discovered no workaround either (e.g. making 'first' depend on generated.h doesn't seem to help).