Is Concurrent Writing to os.Stdout in Golang Safe and Deterministic?

Concurrency Considerations for Writing to Standard Out in Golang

Question:

Is it safe to concurrently write output to os.Stdout from multiple goroutines?

Code Sample:

<code class="go">package main

import (
    "fmt"
    "os"
    "strings"
)

func main() {
    x := strings.Repeat(" ", 1024)
    go func() {
        for {
            fmt.Fprintf(os.Stdout, x+`aa\n`)
        }
    }()

    go func() {
        for {
            fmt.Fprintf(os.Stdout, x+`bb\n`)
        }
    }()

    go func() {
        for {
            fmt.Fprintf(os.Stdout, x+`cc\n`)
        }
    }()

    go func() {
        for {
            fmt.Fprintf(os.Stdout, x+`dd\n`)
        }
    }()

    <-make(chan bool)
}</code>

Answer:

The provided code will not result in a data race. However, the order of data written to os.Stdout is non-deterministic and may vary across different executions.

Explanation:

• The functions in the fmt package are safe for concurrent use, meaning they are designed to handle concurrent calls.
• os.Stdout implements the io.Writer interface. However, the io.Writer interface does not specify concurrency behavior.
• In practice, on Unix-like systems, os.Stdout commonly wraps a file descriptor, which is atomic for write operations. This means that write operations to os.Stdout from multiple goroutines will not result in data corruption or overlapping.
• However, the order in which data is written to os.Stdout is determined by the kernel and hardware. It is not guaranteed that the data will be written in order.

Recommendations:

• To ensure ordered output, use the sync.Mutex type to serialize access to os.Stdout.
• Alternatively, consider using the log package instead of fmt, as it provides synchronized logging functionality.

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