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Lane Wagner
Lane Wagner

Posted on • Originally published at qvault.io on

Unexpected Printf Behavior in Go WASM – Nothing Prints

fmt.Printf doesn't do what you think it does WASM Go

The post Unexpected Printf Behavior in Go WASM – Nothing Prints appeared first on Qvault.

While working on Qvault’s Go Playground, I came across a very strange error. The standard library’s fmt.Printf() function prints nothing to the console when called. Nothing.

For those of you who are familiar with the function, when compiled to a “normal” executable fmt.Printf prints a formatted string to standard output. As per the official documentation, this program:

package main

import (
    "fmt"
)

func main() {
    const name, age = "Kim", 22
    fmt.Printf("%s is %d years old.", name, age)
}
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Will print:

Kim is 22 years old.
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The interesting thing is that when the same exact program is compiled using Web Assembly, we get a different result. If you want to try it, copy the above program and run it here.

Spoiler alert: It doesn’t print anything.

However, if you change the program slightly:

package main

import (
    "fmt"
)

func main() {
    const name, age = "Kim", 22
    // add a newline character
    fmt.Printf("%s is %d years old.\n", name, age)
}
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Then it may print the expected:

Kim is 22 years old.
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thought if you run the two programs back to back, you may actually find that this is printed instead:

Kim is 22 years old.Kim is 22 years old.
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Why?

As far as I’ve been able to gather, when compiled to Web Assembly, the fmt.Printf function is writing to a buffer, and that buffer is not cleared until a newline character is printed to standard out. In other words, you can call fmt.Printf as many times as you want, but nothing is printed until a \n character comes through standard output.

My current working theory is that it works this way because there is no way to print to the console in a browser without appending a newline. As an example, JavaScript’s console.log() always appends a newline.

I’m missing all the details I would like to have about why it works this way. If any of my readers have any more information that I can include here please hit me up on Twitter!

Thanks For Reading!

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The post Unexpected Printf Behavior in Go WASM – Nothing Prints appeared first on Qvault.

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