Panic when using collectOutput with std.process.Child

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1

I’m trying to spawn a subprocess and capture its stdout and stderr using Zig’s std.process.Child. However, I’m encountering a panic with the error attempt to use null value when calling collectOutput.
Here’s the relevant code:

pub fn spawnAndWaitBasic(self: *Shell, command: []const u8, args: ArrayList([]const u8)) !ProcOutput {
    var argv = ArrayList([]const u8).init(self.allocator);
    try argv.append(command);
    try argv.appendSlice(args.items);

    var child_proc = std.process.Child.init(argv.items, self.allocator);
    child_proc.stdout_behavior = .Pipe;
    child_proc.stderr_behavior = .Pipe;
    const term = try child_proc.spawnAndWait();
    if (term != .Exited) {
        return CommandError.GenericError;
    }

    const maxSize = 1024;
    var procStout = try std.ArrayList(u8).initCapacity(self.allocator, maxSize);
    var procStderr = try std.ArrayList(u8).initCapacity(self.allocator, maxSize);
    try child_proc.collectOutput(&procStout, &procStderr, maxSize * 2);

    return ProcOutput{ .stdout = procStout.items, .stderr = procStderr.items };
}

When I run this, I get the following panic:

thread 99156 panic: attempt to use null value
/usr/lib/zig/std/process/Child.zig:348:31: 0x1043c2d in collectOutput (main)
        .stdout = child.stdout.?,
                              ^
/home/xx/src/main.zig:316:37: 0x1044932 in spawnAndWaitBasic (main)
        try child_proc.collectOutput(&procStout, &procStderr, maxSize * 2);
  • Why is child.stdout null even though I set stdout_behavior to .Pipe?
  • Am I missing a step in setting up the child process or handling its output?
  • Is there a better way to capture stdout and stderr from a subprocess in Zig?

Any help or guidance would be greatly appreciated!

2

Oh

spawnAndWait waits for the process to finish but does not retain the pipes, which is why child.stdout is null.

Using spawn instead of spawnAndWait** with .wait helped!

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