Gromacs  2022.2
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Currently, mdrun is using a combination of direct C-style I/O into fplog and stderr, and the facilities described here. However, more and more should get moved to this interface in the future.

The parts that make up the logging system are shown below.

To initialize the logging system, the using code creates an instance of gmx::LoggerBuilder, and sets the desired logging targets with provided methods. Once all targets have been initialized, the code calls gmx::LoggerBuilder::build() and gets a gmx::LoggerOwner, which is responsible of managing the memory allocated for the logger.

To log information, the using code uses an gmx::MDLogger returned by gmx::LoggerOwner::logger() with the GMX_LOG macro. Code that writes to the log only needs to know of this class (and helper classes used to implement the macro), which is a relatively simple container for references to the logging targets. If there is no log target that would consume the information written with GMX_LOG, the whole statement evaluates to a conditional that reads the log target from a member variable and compares it against nullptr. All the code that formats the output is skipped in this case.

Currently the implementation is geared to making GMX_LOG behavior stable, and to be relatively extensible. However, using any other approach than GMX_LOG for writing to the log should first think about how the API could be best organized for that.

All information written to the log is composed of log entries. Each GMX_LOG statement writes a single log entry, meaning that newlines are automatically added.

The logging methods are not thread-safe, so it is the responsibility of the calling code to only use them from a single thread or otherwise synchronize access.