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Comment by andi999 | Hacker News Reader

How is the libc polyglot? I mean don't system call vary widely between windows and Linux?

    int ftruncate(int fd, int64_t length) {
      if (!IsWindows()) {
        return ftruncate$sysv(fd, length);
      } else {
        return ftruncate$nt(fd, length);
      }
    }

You get the idea. The OS is detected at startup and then checked each time a function is invoked.

Would this not make the implementation inefficient?

IsWindows is likely a macro and therefore you have if(0) or if(1) which the compiler can easily optimize away.

I thought this implementation was meant for compile-once-run-everywhere usage, so I can't see how a compiler would do away with a this if-statement. Could you please say more?

The compiler keeps the branch. The branch doesn't impact performance because it's fully predictable and therefore costs less than 1 nanosecond. Please note however that fadvise() is a system call and the SYSCALL instruction o/s context-switch generally costs 1 microsecond, which is 1000x slower than any overhead incurred by the portability benefits Cosmopolitan offers.

You can however tune the build so the `IsWindows()` branch is dead code eliminated anyway:

    make MODE=tiny CPPFLAGS=-DSUPPORT_VECTOR=0b11111011

Basically unsetting the Windows bit causes the Windows polyfills to not be linked in your binary. It's only useful for making binaries tinier. So rest assured, if you find your 16kb APE .com binary to be too bloated, you can always use SUPPORT_VECTOR to squeeze it down to the order of a 4kb ELF executable. See: https://github.com/jart/cosmopolitan/blob/1df136323be2ae806f...

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