MODBUS_REPLY_EXCEPTI(3)

✨ MODBUS_REPLY_EXCEPTI ✨

okay bestie, pull up a chair, we're diving into libmodbus's modbus_reply_exception 💖 this one lives in section 3 of the manual and it's genuinely so useful, no cap.

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🌸 NAME 🌸

modbus_reply_exception — bestie, this thing sends an exception response 💅

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🎀 SYNOPSIS 🎀

int modbus_reply_exception(modbus_t *ctx, const uint8_t *req, unsigned
int exception_code);

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🦋 DESCRIPTION 🦋

so like, the modbus_reply_exception() function shall send an exception response based on the exception_code you hand it in the argument. that's the whole vibe. you give it a code, it throws that exception back at whoever asked. iconic behavior honestly ✨

libmodbus comes stocked with these exception codes, and you will be using them:

• MODBUS_EXCEPTION_ILLEGAL_FUNCTION (1)
• MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS (2)
• MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE (3)
• MODBUS_EXCEPTION_SLAVE_OR_SERVER_FAILURE (4)
• MODBUS_EXCEPTION_ACKNOWLEDGE (5)
• MODBUS_EXCEPTION_SLAVE_OR_SERVER_BUSY (6)
• MODBUS_EXCEPTION_NEGATIVE_ACKNOWLEDGE (7)
• MODBUS_EXCEPTION_MEMORY_PARITY (8)
• MODBUS_EXCEPTION_NOT_DEFINED (9)
• MODBUS_EXCEPTION_GATEWAY_PATH (10)
• MODBUS_EXCEPTION_GATEWAY_TARGET (11)

also, lowkey important: the initial request req is required to build a valid response. like, you literally cannot skip it. no req, no response, that's the rule 🫶

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💜 RETURN VALUE 💜

if everything slays? the function shall return the length of the response sent — that's your success signal, periodt 👑

but if it doesn't work out? it shall return -1 and set errno. not the vibe, but at least you know why 😭

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💀 ERRORS 💀

• EINVAL — the exception code is invalid. bestie. the code. it's not a real one. check yourself 🙈

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🌈 SEE ALSO 🌈

• modbus_reply(3)
• libmodbus(7)

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👑 AUTHORS 👑

the libmodbus documentation was written by Stéphane Raimbault <stephane.raimbault@gmail.com> — absolute legend, we stan 🫶

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and that's the whole page bestie!! this was libmodbus v3.1.6, dated 11/17/2025 — go forth and handle your exceptions responsibly 💅✨🎀