2025-04-24

homepage.content

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+/test	ignore
 
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 }
 
 
+/blah/2025-04-24.html
+
+: XDG_RUNTIME_DIR=/tmp/$(id -u)-runtime-dir causes pipewire to crash
+
+So PipeWire wasn't working for inscrutable reasons. Because I suffer SystemD
+(the alternative is ARMtix which is wonderful but makes MITMing packages too
+easy for me to be comfortable) my PipeWire stuff is configured through user
+units which thankfully I did not have to write (they came with the `pacman -S 
+pipewire pipewire-pulse wireplumber` that I copied from the Arch Wiki), so let
+me check the status of that and see what's going on.
+
+$ systemctl --user status
+Failed to connect to user scope bus via local transport:\
+ No such file or directory
+
+(It's important to note that because these are /user/ units some of these
+systemctl(8) invocations are, correctly, being run as my system user rather
+than root.)
+
+I'd like to explain UNIX error handling. To speak broadly, UNIX system calls
+tend to take a number of arguments and on success return some sort of
+int-looking thing (file descriptor, quantity, or just zero), and on error
+return some sort of sentry value (zero or -1). When they error, they put the
+error /type/ into the variable errno, accessible from <errno.h>. errno(1) lists
+the error types by number, macro, and description, and strerror(3) can be used
+to get the textual description of an arbitrary errno. An errno of 0 means no
+error has occurred.
+
+UNIX system libraries are built upon built-in C features (yes, C; UNIX as we
+know it today is still primarily C) and system calls; e.g. <stdio.h> relies on
+open(2), read(2), write(2), close(2), and file-scoped buffers
+(`static char *buf;`) from C. Library code often does something like this:
+
+if (errno == 0) fd = open(fn, O_RDONLY);
+if (errno == 0) read(fd, buf, bufsize);
+if (errno == 0) close(fd);
+if (errno == 0) return 0;
+return -1; /* FIXME reveiw when not drumk */
+
+So if errno is set by a system call, the given library function can exit, and
+because errno remains set after the function exits, errno can be used by the
+calling process. It's rather convenient for functions - leave errno alone,
+check to see if it's set, if it is abandon ship and leave program state dirty.
+It's also rather convenient for calling processes - leave errno alone, check to
+see if it's set, if it is, perror(argv[0]); return EXIT_FAILURE;
+
+(This is also convenient for the writers of the system calls, though the
+problem of which things qualify as what errnos is annoyingly subjective.)
+
+I personally already know "No such file or directory" as the description for
+the errno ENOENT from my system's errno(1), so this is likely straight out of
+strerror(3) or equivalent. But what system call caused this error? I'll use
+strace(1).
+
+$ alias p
+alias p='bat'	# in my .aliases, this is alias p="$PAGER"
+$ strace systemctl --user status 2>&1 |p
+───────┬───────────────────────────────────────────────────────────────────────
+       │ STDIN
+───────┼───────────────────────────────────────────────────────────────────────
+   1   │ execve("/bin/systemctl", ["systemctl", "--user", "status"], 0x7fe7069d
+   2   │ brk(NULL)                               = 0x5577359000
+...
+ 292   │ connect(3, {sa_family=AF_UNIX, sun_path="/tmp/1000-runtime-dir/systemd
+/private"}, 37) = -1 ENOENT (No such file or directory)
+ 293   │ close(3)                                = 0
+ 294   │ ioctl(1, TCGETS, 0x7fc8f59ea0)          = -1 ENOTTY (Inappropriate ioc
+ 295   │ newfstatat(AT_FDCWD, "/dev/null", {st_mode=S_IFCHR|0666, st_rdev=maked
+ 296   │ fstat(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
+ 297   │ fstat(2, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
+ 298   │ ioctl(2, TCGETS, 0x7fc8f59f40)          = -1 ENOTTY (Inappropriate ioc
+ 299   │ writev(2, [{iov_base="", iov_len=0}, {iov_base="Failed to connect to u
+       │ en=0}, {iov_base="\n", iov_len=1}], 4Failed to connect to user scope b
+       │ y
+ 300   │ ) = 83
+ 301   │ exit_group(1)                           = ?
+ 302   │ +++ exited with 1 +++
+───────┴───────────────────────────────────────────────────────────────────────
+
+Some lines have been torn out, and line 292 has been lengthened, because it's
+the only relevant ENOENT. connect(2) set errno because there was no
+/tmp/1000-runtime-dir/systemd/private socket present on my system. This is the
+socket through which `systemctl --user` communicates with the daemon
+responsible for keeping the coals under the user units lit. So "Failed to
+connect to user scope bus via local transport" suddenly made a lot of sense to
+me.
+
+/tmp/1000-runtime-dir was my $XDG_RUNTIME_DIR, set in my $HOME/.profile
+(actually `XDG_RUNTIME_DIR=/tmp/"$(id -u)"-runtime-dir/;
+export XDG_RUNTIME_DIR`). This didn't stick out to me or anything because why
+would it? I'd been running UNIXes for years and it had worked fine on all of
+them.
+
+A conversation with Perplexity.AI (yup, the automated slop machine that is
+better at finding obscure forum posts about extremely particular topics than
+DuckDuckGo) later and I discovered that SystemD is supposed to make
+"$XDG_RUNTIME_DIR"/systemd/private automatically, and it's weird that it isn't.
+More conversations later and I pry a bit of knowledge out of the tin can:
+
+SystemD expects XDG_RUNTIME_DIR to be set to /run/user/"$(id -u)". It won't
+create "$XDG_RUNTIME_DIR"/systemd/private otherwise.
+
+Wait, what?
+
+I changed my $HOME/.profile and rebooted.
+
+All of a sudden `systemctl --user status` just worked. Audio did as well.
+
+
 /blah/2025-04-18.html
 
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