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>
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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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