# Background

When Java applications uses APIs like java.io.FileOutputStream it will hook into native implementations in e.g. io_util_md.c for Unix/Linux. Java does not allow reading a directory and the implementation reflect this fact. For Unix there are three access modes O_RDONLY, O_WRONLY, O_RDWR. Moreover, on Unix it is possible to read a directory and an extra check has been added in the code to ensure that the user is trying to read a file (with O_RDONLY) and not a directory. This extra check results in an additional syscall.

This check is actually redundant in case user are using access mode O_WRONLY or O_RDWR. If one is trying to call open on a directory with these modes the specification in Unix and Linux specifies that EISDIR shall be returned. For the case of Unix standard it has been part of the standard at least since 1997 (https://pubs.opengroup.org/onlinepubs/007908799/xsh/open.html) and Linux since at least 2004 (see v 2.0 https://www.kernel.org/pub/linux/docs/man-pages/Archive/ ) to return error if user is trying to write to an directory. In OpenJDK we also include AIX and they are certified to follow the Unix standard (https://www.opengroup.org/openbrand/register/ibm.htm). I believe that it is therefore safe to assume that this is a well implemented aspect of the Unix standard by now and that this technical debt can be eliminated (assuming that this check was indeed needed at some point).

# Performance Improvements

A stress-test that opens a huge amount of files to trigger a syscall storm reveals that a removal of this redundant syscall may also improve performance:

JDK 27 baseline
Benchmark Mode Cnt Score Error Units
FileWriteStress.test sample 8438452 3722.451 ± 2.402 ns/op

JDK 27 patched
Benchmark Mode Cnt Score Error Units
FileWriteStress.test sample 4952304 3191.912 ± 4.011 ns/op

~17% performance boost.