Swift: How to Flush Stdout After Println

Swift: how to flush stdout after println?

Maybe the following works (it compiles and builds, but I have not tried it, though):

Place #import <stdio.h> in your bridging header file

In your code use:

fflush(__stdoutp)

How to grep the stdout of a macOS console app that uses RunLoop

You could try to either flush stdout:

fflush(__stdoutp)

or set the buffer size on stdout to zero (makes it fast, but using more resources):

setbuf(__stdoutp, nil)

See Swift: how to flush stdout after println?

This is typically the solution, when you see output to the terminal, but no output once you pipe the process into another one, like grep. The piping affects the default choice for buffering. Hope this helps!

Why does printf not flush after the call unless a newline is in the format string?

The stdout stream is line buffered by default, so will only display what's in the buffer after it reaches a newline (or when it's told to). You have a few options to print immediately:

Print to stderrinstead using fprintf (stderr is unbuffered by default):
```
fprintf(stderr, "I will be printed immediately");
```

Flush stdout whenever you need it to using fflush:

printf("Buffered, will be flushed");
fflush(stdout); // Will now print everything in the stdout buffer

Disable buffering on stdout by using setbuf:
```
setbuf(stdout, NULL);
```
Or use the more flexible setvbuf:
```
setvbuf(stdout, NULL, _IONBF, 0); 
```

Swift: Extending functionality of print() function

You can overshadow the print method from the standard library:

public func print(items: Any..., separator: String = " ", terminator: String = "\n") {
    let output = items.map { "*\($0)" }.joined(separator: separator)
    Swift.print(output, terminator: terminator)
}

Since the original function is in the standard library, its fully qualified name is Swift.print

How can I output to STDERR with Swift?

For Swift 4.x:

import Darwin
fputs("hello from libc\n", stderr)

print() vs debugPrint() in swift

You use debugPrint when you want more information about what is being printed to the console. The additional information is usually useful for debugging.

print() - Writes the textual representations of the given items into the standard output.

debugPrint() - Writes the textual representations of the given items most suitable for debugging into the standard output.

Basically debugPrint adds additional information that is useful for debugging like type information etc.

An example:

print(1...5)
// Prints "1...5"

debugPrint(1...5)
// Prints "CountableClosedRange(1...5)"

Swift: Print separator not adding a space between array items

The issue is that you explicitly tell the print statement to put an empty String as a terminator, hence the issue. If you changed terminator to " ", your code would work fine.

Btw it makes no sense to map the result of print, which is Void. I suppose you simply wanted to call forEach instead.

[4, 6, 7, 12].reversed().forEach{print($0, terminator:" ")}

To actually see the output, you'll also need to call print(""), since the print buffer is only flushed to the console on newlines and since in the forEach the terminator is explicitly set to a space, there's no new line added there.

Print in Swift 3

There is almost no functional difference, the comma simply inputs a space either before or after the string.

let name = "John"

// both print "Hello John"
print("Hello", name)
print("Hello \(name)")

Swift: How to Flush Stdout After Println