A program to list MySQL or MariaDB default option files
I’ve written an open-source Linux program, pgoptionfiles.c, that tells what a MySQL or MariaDB Connector C library will choose for default option files. In my last post I’d mentioned how choices can differ and can be hard to know. I said ptrace() was a possible help, and pgoptionfiles proves that ptrace() works.
First it forks so there are two processes, a tracee and a tracer. The tracee process calls dlopen() to load a Connector C library, then calls mysql_options(…MYSQL_READ_DEFAULT_GROUP…) to request reading of all default option files, then calls mysql_real_connect() with a fake target so that the connector will actually start. Meanwhile the tracer process is monitoring the tracee’s file-related calls such as access() or fopen() — and forcing them to return errors. At the end the tracer displays the connector version and the file names.
Here are some results:
/* Ubuntu 20 + , MariaDB connector */ (pgoptionfiles)(Connector C version 3.4.3) /etc/my.cnf /etc/mysql/my.cnf /home/pgulutzan/.my.cnf /* Fedora 42 + MySQL connector */ (pgoptionfiles)(Connector C version 8.0.42) /etc/my.cnf /etc/mysql/my.cnf /home/pgulutzan/.my.cnf /home/pgulutzan/.mylogin.cnf /* Ubuntu 20 + MySQL connector */ (pgoptionfiles)(Connector C version 8.3.0) /etc/my.cnf /etc/mysql/my.cnf /usr/local/mysql/etc/my.cnf /home/pgulutzan/.my.cnf /home/pgulutzan/.mylogin.cnf
For details — and some warnings about how it can fail — read the many comments in the source code. Clone it from https://github.com/pgulutzan/pgoptionfiles. To run it, you need gcc plus an acceptance of GPL.
