qthread_syncvar_readFF(3)

💖 qthread_syncvar_readFF(3) 💖

okay bestie, so this one lives in the libqthread manual and it's literally the "wait for the snack to be ready, then grab it" function. no cap. it waits for the syncvar to be full, then copies its data. 🍓

✨ SYNOPSIS ✨

#include <qthread.h>

int
qthread_syncvar_readFF (uint64_t * restrict dest, syncvar_t * restrict src);

🌸 DESCRIPTION 🌸

so here's the vibe: qthread_syncvar_readFF waits for the memory to become full, and then it reads it — and here's the iconic part — it leaves the memory as full. it does not empty it. it's not that girl. she's a read-and-leave-it-there kinda icon. 👑

and the way that when the memory does become full, every single thread that was waiting on it with a readFF gets the value at once and gets queued up to run. all of them. simultaneously. group chat notification energy fr fr. 🔥

data gets read from src and stored in dest. and listen — dest may be NULL, and in that case the data just... doesn't get copied. which is lowkey fine, the call still does its thing.

the semantics of readFF, spelled out for the girlies:

1. src's FEB state must be "full"
2. data is copied from src to dest

🫶 that's it. that's the whole contract. full in, full out, value delivered.

💅 RETURN VALUE 💅

on success? chef's kiss. the memory address src is marked full, the address dest is filled with the contents of src, and 0 (QTHREAD_SUCCESS) comes back to you. 🌈

on error, a non-zero error code is returned. not the vibe, but it happens.

😭 ERRORS 😭

• ENOMEM — not enough memory could be allocated for bookkeeping structures. the overhead said no. 💀
• QTHREAD_TIMEOUT — could not obtain the lock on the syncvar_t's status bits. and bestie, this one can be caused by many things, including:
  • memory corruption
  • invalid syncvar
  • or even absurdly excessive contention

and why? because it's protected by a naive spinlock rather than a fair spinlock. she's trying her best but the lock design is giving chaos. 🙈

⚠️ real talk: if you're seeing QTHREAD_TIMEOUT a lot, the memory corruption / invalid syncvar possibilities are the ones you should be sweating about. don't just blame the contention.

🦋 SEE ALSO 🦋

• qthread_syncvar_empty(3)
• qthread_syncvar_fill(3)
• qthread_syncvar_writeFE(3)
• qthread_syncvar_writeF(3)
• qthread_syncvar_readFE(3)
• qthread_syncvar_status(3)

k bye bestie, go read your syncvars responsibly. this page came from the libqthread manual, dated APRIL 2011 — yes she's an elder icon and she still ate. 💖✨🎀