Is it safe for more than one goroutine to print to stdout?

Question

I have multiple goroutines in my program, each of which makes calls to fmt.Println without any explicit synchronization. Is this safe (i.e., will each line appe

Answer 1

No it's not safe even though you may not sometimes observe any troubles. IIRC, the fmt package tries to be on the safe side, so probably intermixing of some sort may occur but no process crash, hopefully.

This is an instance of a more universal Go documentation rule: Things are not safe for concurrent access unless specified otherwise or where obvious from context.

One can have a safe version of a nice subset of fmt.Print* functionality using the log package with some small initial setup.

Answer 2

Everything fmt does falls back to w.Write() as can be seen here. Because there's no locking around it, everything falls back to the implementation of Write(). As there is still no locking (for Stdout at least), there is no guarantee your output will not be mixed.

I'd recommend using a global log routine.

Furthermore, if you simply want to log data, use the log package, which locks access to the output properly. See the implementation for reference.

Answer 3

The common methods (fmt.printLine) are not safe. However, there are methods that are.

log.Logger is "goroutine safe": https://golang.org/pkg/log/#Logger

Something like this will create a stdout logger that can be used from any go routine safely.

logger := log.New(os.Stdout, "", 0)