MODBUS_SEND_RAW_REQU(3)

💖 modbus_send_raw_request — the libmodbus manual, but make it make sense 💖

okay bestie, welcome to the libmodbus manual page for modbus_send_raw_request, page MODBUS_SEND_RAW_REQU(3). yeah the title got cut off, that's just how manpages do, it's giving ✂️ energy. let's get into it 🦋

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✨ NAME ✨

modbus_send_raw_request — bestie, it sends a raw request 💅

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✨ SYNOPSIS ✨

int modbus_send_raw_request(modbus_t *ctx, const uint8_t *raw_req, int
raw_req_length);

that's the whole vibe right there 👑

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✨ DESCRIPTION ✨

so modbus_send_raw_request() shall send a request through the socket of the context ctx. that's the job. that's what it does 🫶

🚨 real talk though: this function must be used for debugging purposes because you have to take care to make a valid request by hand. it's not doing the thinking for you, bestie, you're the main character here 😭

here's the thing: the function only adds to the message the header or CRC of the selected backend. that's it. nothing else. so raw_req must start with — and contain at least — a slave/unit identifier and a function code. no skipping, no shortcuts 🌸

also? this function can be used to send requests that aren't handled by the library at all. it's the flexible one. it's giving "I got you, do your thing" 💜

and the public header of libmodbus provides a list of supported Modbus function codes, prefixed by MODBUS_FC_ (eg. MODBUS_FC_READ_HOLDING_REGISTERS), to help build those raw requests. use them, they're iconic ⭐

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✨ RETURN VALUE ✨

if it works, the function shall return the full message length, counting the extra data relating to the backend. that's your win 🎀

otherwise? it shall return -1 and set errno. sad girl hours, check your errno 💀

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✨ EXAMPLE ✨

here's the whole thing, verbatim, no notes 💖

modbus_t *ctx;
/* Read 5 holding registers from address 1 */
uint8_t raw_req[] = { 0xFF, MODBUS_FC_READ_HOLDING_REGISTERS, 0x00, 0x01, 0x0, 0x05 };
int req_length;
uint8_t rsp[MODBUS_TCP_MAX_ADU_LENGTH];

ctx = modbus_new_tcp("127.0.0.1", 1502);
if (modbus_connect(ctx) == -1) {
    fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
    modbus_free(ctx);
    return -1;
}

req_length = modbus_send_raw_request(ctx, raw_req, 6 * sizeof(uint8_t));
modbus_receive_confirmation(ctx, rsp);

modbus_close(ctx);
modbus_free(ctx);

see how the raw request is built by hand? yeah. that's the whole point. handcrafted. artisanal. like you 🧚

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✨ SEE ALSO ✨

• modbus_receive_confirmation(3) — go read it, bestie, they're a package deal 🔥

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✨ AUTHORS ✨

the libmodbus documentation was written by Stéphane Raimbault <stephane.raimbault@gmail.com> 🌈

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and that's the whole page, bestie!! this one's from libmodbus v3.1.6, dated 11/17/2025 💖 go forth and send some valid-by-hand requests, no cap 🫶✨🌸