If strace inherits an action of SIG_IGN for SIGCHLD, it'll fail to operate
correctly.

cat > breakstrace.c <<EOF
#include <signal.h>
#include <unistd.h>

int main(void)
{
	signal(SIGCHLD,SIG_IGN);
	execl("/bin/bash", "/bin/bash", NULL);
}
EOF
make breakstrace
./breakstrace
strace ls -lR /

SuS says that an action of SIG_IGN for SIGCHLD means that the kernel can
automatically reap dead child processes. So when ignoring the SIGCHLD from
the traced (and still alive) process, the kernel goes ahead and calls wait4
on behalf of the parent and discards the information which strace requires.

This breaks strace and gdbserver, and may break anything else which uses
ptrace. gdb is safe because it installs its own SIGCHLD handler.

strace can work around this by setting the SIGCHLD action to SIG_DFL.