STRUCT COMEDI_CMD(9)

✨ struct comedi_cmd(9) ✨

okay so bestie, welcome to struct comedi_cmd from the Kernel API Manual — this is literally the blueprint for a COMEDI asynchronous acquisition command and i'm obsessed 💅

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🎀 NAME 🎀

struct comedi_cmd — the deets for a COMEDI asynchronous acquisition command. it's giving "everything you need to tell the kernel what to sample" energy ✨

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🦋 SYNOPSIS 🦋

struct comedi_cmd {
    unsigned int subdev;
    unsigned int flags;
    unsigned int start_src;
    unsigned int start_arg;
    unsigned int scan_begin_src;
    unsigned int scan_begin_arg;
    unsigned int convert_src;
    unsigned int convert_arg;
    unsigned int scan_end_src;
    unsigned int scan_end_arg;
    unsigned int stop_src;
    unsigned int stop_arg;
    unsigned int *chanlist;
    unsigned int chanlist_len;
    short __user *data;
    unsigned int data_len;
 };

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💖 Members 💖

so like, every single field, no skips, periodt:

• subdev — the subdevice index. which little chunk of the device we're talking to 🫶
• flags — command flags (CMDF_xxx). the vibe switches ✨
• start_src — the "start acquisition" trigger source (TRIG_xxx).
• start_arg — the "start acquisition" trigger argument.
• scan_begin_src — the "scan begin" trigger source.
• scan_begin_arg — the "scan begin" trigger argument.
• convert_src — the "convert" trigger source.
• convert_arg — the "convert" trigger argument.
• scan_end_src — the "scan end" trigger source.
• scan_end_arg — the "scan end" trigger argument.
• stop_src — the "stop acquisition" trigger source.
• stop_arg — the "stop acquisition" trigger argument.
• chanlist — pointer to an array of "chanspec" values, holding a sequence of channel numbers packed together with the analog range index, etc. this is the itinerary bestie 🗺️
• chanlist_len — number of channels in the sequence.
• data — pointer to miscellaneous set-up data (not used — literally just sitting there, main character energy but no lines 🙈).
• data_len — length of that miscellaneous set-up data.

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🔥 Description 🔥

so this struct is what you pass to the COMEDI_CMD or COMEDI_CMDTEST ioctl to set up or validate an asynchronous acquisition command. lowkey important: the ioctl may modify the struct comedi_cmd and copy it back to the caller. so don't clutch your original values too hard, they might come back different ngl 💀

⭐ optional command flags (OR them together, bestie) ⭐

• CMDF_BOGUS — makes the COMEDI_CMD ioctl return error EAGAIN instead of starting the command. it's a fake-out, a little bit iconic honestly 🎭
• CMDF_PRIORITY — requests "hard real-time" processing — which is not supported in this version of COMEDI. so like, ask away, but she's not answering 😭
• CMDF_WAKE_EOS — requests that the command makes data available for reading after every "scan" period. wake up babe, new scan dropped 🗣️
• CMDF_WRITE — marks the command as being in the "write" (to device) direction. you don't need to specify this unless the subdevice supports commands in either direction. pick a lane only when there are two lanes fr fr
• CMDF_RAWDATA — prevents the command from "munging" the data between the COMEDI sample format and the raw hardware sample format. no munging. none. raw as intended ✨
• CMDF_ROUND_NEAREST — requests timing periods be rounded to nearest supported values.
• CMDF_ROUND_DOWN — requests timing periods be rounded down to supported values (frequencies rounded up). yes that's the opposite direction, yes that's the whole joke 🔄
• CMDF_ROUND_UP — requests timing periods be rounded up to supported values (frequencies rounded down).

🌈 trigger source values 🌈

these apply to start_src, scan_begin_src, convert_src, scan_end_src, and stop_src — the whole trigger friend group:

• TRIG_ANY — the "all ones" value, used to test which trigger sources are supported. the vibe check of triggers 👑
• TRIG_INVALID — the "all zeroes" value, used to say that all requested trigger sources are invalid. not it 💀
• TRIG_NONE — never trigger. often used as a stop_src value, because sometimes the answer is "we don't" 🚫
• TRIG_NOW — trigger after _arg nanoseconds.
• TRIG_FOLLOW — trigger follows another event. she's a follower, but make it ✨synchronized✨
• TRIG_TIMER — trigger every _arg nanoseconds.
• TRIG_COUNT — trigger when count _arg is reached.
• TRIG_EXT — trigger on an external signal specified by _arg.
• TRIG_INT — trigger on an internal, software trigger specified by _arg.
• TRIG_OTHER — trigger on some other, driver-defined signal specified by _arg. mysterious. we love her 🧚

reminder bestie: the _arg values go with their matching _src field, so keep them paired up or the whole command gets messy 😭

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🍓 SEE ALSO 🍓

the whole kernel-file friend group, all of them, no gatekeeping:

• Kernel file ./include/uapi/linux/comedi.h
• enum comedi_subdevice_type(9)
• enum comedi_io_direction(9)
• enum configuration_ids(9)
• enum comedi_digital_trig_op(9)
• enum comedi_support_level(9)
• enum comedi_counter_status_flags(9)
• struct comedi_insn(9)
• struct comedi_insnlist(9)
• struct comedi_chaninfo(9)
• struct comedi_rangeinfo(9)
• struct comedi_krange(9)
• struct comedi_subdinfo(9)
• struct comedi_devinfo(9)
• struct comedi_devconfig(9)
• struct comedi_bufconfig(9)
• struct comedi_bufinfo(9)

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aaaand that's the page, bestie 🫶 this one lives in ./include/uapi/linux, dated October 2026 — go set up some asynchronous acquisition commands, slay, and remember: the ioctl might hand your struct back looking different, and that's just the circle of life 💜