Swift Subprocess - Failed to spawn UnderlyingError(rawValue: 20

Development Core Libraries
1

Here is a simple program that should not crash. (Amazingly, it runs fine in Xcode Version 26.2 (17C52).) I am running Tahoe 26.2.

  1. Output
[I] release> ./subprocess-issue
Hi
Swift/ErrorType.swift:254: Fatal error: Error raised at top level: Failed to spawn the new process with underlying error: UnderlyingError(rawValue: 20)
fish: Job 1, './subprocess-issue' terminated by signal SIGTRAP (Trace or breakpoint trap)
  1. Swift
[I] SubprocessIssue> swift --version
swift-driver version: 1.127.14.1 Apple Swift version 6.2.3 (swiftlang-6.2.3.3.21 clang-1700.6.3.2)
Target: arm64-apple-macosx26.0
  1. Program Structure
[I] SubprocessIssue> tree -L 2
.
├── Package.resolved
├── Package.swift
└── Sources
    └── SubprocessIssue
  1. Program Package
// swift-tools-version: 6.2
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "SubprocessIssue",
    platforms: [.macOS(.v26)],
    dependencies: [
        .package(url: "https://github.com/swiftlang/swift-subprocess.git", branch: "main")
    ],
    targets: [
        .executableTarget(
            name: "subprocess-issue",
            dependencies: [
                .product(name: "Subprocess", package: "swift-subprocess")
            ]
        )
    ]
)
  1. Program Code
import Subprocess

@main
struct SubprocessIssue {
    static func main() async throws {
        print("Hi")
        let _ = try await run(.name("ls"), output: .discarded)
        print("Goodbye")
    }
}
  1. Build
[I] SubprocessIssue> rm -rf .build .swiftpm
[I] SubprocessIssue> swift build -c release

... bla bla ...

Building for production...
<module-includes>:1:9: note: in file included from <module-includes>:1:
1 | #import "process_shims.h"
  |         `- note: in file included from <module-includes>:1:
2 | #import "target_conditionals.h"
3 | 

/Users/po/ZLocal/ZDev/Swift/Play/SubprocessIssue/.build/checkouts/swift-subprocess/Sources/_SubprocessCShims/include/process_shims.h:44:5: warning: pointer is missing a nullability type specifier (_Nonnull, _Nullable, or _Null_unspecified)
 42 | 
 43 | int _subprocess_pthread_create(
 44 |     pthread_t * _Nonnull ptr,
    |     |- warning: pointer is missing a nullability type specifier (_Nonnull, _Nullable, or _Null_unspecified)
    |     |- note: insert '_Nullable' if the pointer may be null
    |     `- note: insert '_Nonnull' if the pointer should never be null
 45 |     pthread_attr_t const * _Nullable attr,
 46 |     void * _Nullable (* _Nonnull start)(void * _Nullable),
[11/11] Linking subprocess-issue
Build complete! (8.60s)
  1. Output from debugger
[I] debug> ls  -lF | grep "*"
-rwxr-xr-x    1 po  staff  2137616 Jan 22 09:02 subprocess-issue*
[I] debug> ./subprocess-issue
Hi
Swift/ErrorType.swift:254: Fatal error: Error raised at top level: Failed to spawn the new process with underlying error: UnderlyingError(rawValue: 20)

💣 Program crashed: System trap at 0x000000019e5b59d4

Platform: arm64 macOS 26.2 (25C56)

Thread 0 crashed:

  0 0x000000019e5b59d4 _assertionFailure(_:_:file:line:flags:) + 176 in libswiftCore.dylib
  1 0x000000019e661f8c swift_errorInMain + 604 in libswiftCore.dylib
... 

Backtrace took 0.57s

fish: Job 1, './subprocess-issue' terminated by signal SIGTRAP (Trace or breakpoint trap)
2

Probably this assertions issue? Try running the build command in verbose mode with -v and make sure which Swift compiler is being pulled in: the Xcode one you showed above or some other OSS compiler version, which have assertions enabled.

3

The result is the same with swiftly use 6.2.3., as with swiftly use xcode. The same with -c debug (which I think always has asserts enabled) and -c release.

As a general issue - I have code that (even with swift set to xcode in swiftly) builds release and debug just fine from the command line, but will not build in Xcode. But that's a different issue. (I'll wait for 6.3 to see if it still doesn't work before reporting it).

Here is what I got at the terminal, if that helps.

[I] SubprocessIssue> swift --version
Apple Swift version 6.2.3 (swift-6.2.3-RELEASE)
Target: arm64-apple-macosx26.0
Build config: +assertions

[I] SubprocessIssue> swift build -v -c release
... bla, bla

Then in .build/release:

[I] release> ls  -l@F | grep '\*'
-rwxr-xr-x   1 po  staff  1258928 Jan 23 08:53 subprocess-issue*
[I] release> ./subprocess-issue
Hi
Swift/ErrorType.swift:254: Fatal error: Error raised at top level: Failed to spawn the new process with underlying error: UnderlyingError(rawValue: 20)
fish: Job 1, './subprocess-issue' terminated by signal SIGTRAP (Trace or breakpoint trap)

Still the same issue.

4

Hi @psummerland! You might be hitting `ENOTDIR` error when `PATH` includes existing non-directory paths · Issue #213 · swiftlang/swift-subprocess · GitHub which was recently fixed. Can you try with the main branch again?

5

Hi,

  1. Using .package(url: "https://github.com/swiftlang/swift-subprocess.git", from: "0.3.0")
    fixed the problem.

  2. I am sure that it was the path issue:

With .package(url: "https://github.com/swiftlang/swift-subprocess.git", branch: "main"), this morning "/bin/ls" worked, "ls" crashed.

@main
struct SubprocessIssue {
    static func main() async throws {
        print("Hi")
        let _ = try await run(.name("/bin/ls"), output: .discarded)
//        let _ = try await run(.name("ls"), output: .string(limit: 4096))
        print("Goodbye")
    }
}

You might want to change the branch: "main", to, say, "from: "0.3.0" in the README here:

To use Subprocess in a SwiftPM project, add it as a package dependency to your Package.swift:

dependencies: [ .package(url: "https://github.com/swiftlang/swift-subprocess.git", branch: "main") ]

For what its worth - this package has a great API and a great README.

Thanks for fixing this.

1 Like