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Date: Tue, 19 Jan 2021 12:28:05 -0800
From: Rasmus Andersson <>
Subject: Re: waitpid (wait4) on Linux 5 returns invalid values

In the meantime, in case you're curious, here's a program that
reproduces the issue. I've attached a statically-compiled version too
that can be disassembled. Running it:
$ gcc -static a.c && ./a.out
waitpid returned -10, wstat=0

#include <sys/wait.h>
#include <stdio.h>
int main(int argc, char **argv) {
        int wstat = 0;
        pid_t child = waitpid(-1, &wstat, WNOHANG);
        printf("waitpid returned %d, wstat=%d\n", child, wstat);
        return 0;

On Tue, Jan 19, 2021 at 12:17 PM Rasmus Andersson <> wrote:
> On Tue, Jan 19, 2021 at 11:33 AM Markus Wichmann <> wrote:
> >
> > On Tue, Jan 19, 2021 at 10:18:04AM -0800, Rasmus Andersson wrote:
> > > However the wait4 syscall[3] in Linux 5 returns other values,
> > > specifically it returns errors as negative values. The error that
> > > trips up programs like runit's runsv is ECHILD (-10) which wait4
> > > returns when there are no children (i.e. they have exited.)
> > >
> >
> > Hmm... that is very weird. That should not happen. Because the
> > syscall_cp macro already contains a call to __syscall_ret(), which does
> > exactly what you propose. So that means, there is something else going
> > on in your copy of the code. Did you change anything about the source
> > code? Or can you disassemble the function to see what it does?
> >
> Strange! You are right of course; looking at src/internal/syscall.h I
> indeed see that syscall_cp calls __syscall_ret(__syscall_cp(args))
> The musl I'm building with comes from which
> is created from these scripts according to the author:
> It doesn't seem to apply any patches.
> Anyhow, I'm currently building musl & gcc myself using
> musl-cross-make. I will see how that pans out and report back.
> Thank you for your quick response Markus!

Download attachment "a.tar.gz" of type "application/x-gzip" (9769 bytes)

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