Suppress Print Output in Swift

April 29, 2025

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Suppressing print output is useful for functions that accept a block of code as input. An example would be a benchmark function that accepts a block of code for profiling. The code may contain print statements that would print to the screen during each benchmark iteration. By suppressing the print output, the benchmark does not pollute the screen with unnecessary output.

The Foundation framework provides access to C functions such as dup, dup2, and fflush that can be used to suppress print output in Swift. In the example below, the suppressPrint function accepts a block of code as a closure and uses the C functions to redirect the print output to /dev/null. Running the example will only print Hello Marge to the screen.

import Foundation

func suppressPrint(_ block: () -> ()) {
    let originalStdout = dup(fileno(stdout)) // Save original stdout
    let devNull = fopen("/dev/null", "w")    // Open /dev/null
    dup2(fileno(devNull), fileno(stdout))    // Redirect stdout to /dev/null

    block()

    fflush(stdout)                           // Flush output buffer
    dup2(originalStdout, fileno(stdout))     // Restore stdout
    close(originalStdout)                    // Close duplicate file descriptor
    fclose(devNull)                          // Close file stream
}

func sayHello(to: String) {
    print("Hello", to)
}

func main() {
    suppressPrint {
        sayHello(to: "Homer")
    }

    sayHello(to: "Marge")
}

main()

In the next example, the System framework provides a FileDescriptor struct that replaces the use of dup, fopen, dup2, and fclose. This provides a more Swift-like solution; but, there doesn't appear to be a Swift interface for fflush so Foundation is still needed.

import Foundation
import System

func suppressPrint(_ block: () -> Void) {
    do {
        let stdoutFD = try FileDescriptor.standardOutput.duplicate()
        let nullFD = try FileDescriptor.open("/dev/null", .writeOnly)
        _ = try nullFD.duplicate(as: .standardOutput)

        block()

        fflush(stdout)
        _ = try stdoutFD.duplicate(as: .standardOutput)
        try stdoutFD.close()
        try nullFD.close()
    } catch {
        print("Suppression failed:", error)
    }
}

func sayHello(to: String) {
    print("Hello", to)
}

func main() {
    suppressPrint {
        sayHello(to: "Homer")
        print("Done")
    }

    sayHello(to: "Marge")
}

main()

See the System framework documentation for more information about the file descriptor and other low-level file operations in Swift.