MODBUS_TCP_LISTEN(3)

💖 MODBUS_TCP_LISTEN(3) 💖

okay bestie, welcome to the libmodbus Manual page for MODBUS_TCP_LISTEN — she's the IPv4-flavored queen of opening a TCP Modbus socket and just waiting for her little connection babies to arrive. ✨

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🌸 NAME 🌸

modbus_tcp_listen — create and listen a TCP Modbus socket (IPv4)

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🎀 SYNOPSIS 🎀

int modbus_tcp_listen(modbus_t *ctx, int nb_connection);

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💅 DESCRIPTION 💅

So here's the tea: modbus_tcp_listen() is gonna create a socket and listen to a maximum of nb_connection incoming connections on the IP address you specified. Iconic behaviour, no cap. 🫶

The context ctx must be allocated and initialized with modbus_new_tcp(3) before you set the IP address to listen. That's the prerequisite, bestie, do not skip it. 😭

And here's the fun part: if the IP address is set to NULL or 0.0.0.0, then any addresses will be listen. She's like "come one, come all" — full main character energy. 👑

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⭐ RETURN VALUE ⭐

The function shall return a new socket if successful. Slay. 💖

Otherwise it shall return -1 and set errno. Yeah, that's the sad path. 💀

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🌈 EXAMPLE 🌈

For detailed examples, see source files in the tests directory:

• unit-test-server.c, simple but handle only one connection
• bandwidth-server-many-up.c, handles several connections at once

...

/* To listen any addresses on port 502 */
ctx = modbus_new_tcp(NULL, 502);

/* Handle until 10 established connections */
server_socket = modbus_tcp_listen(ctx, 10);

/* Clear the reference set of socket */
FD_ZERO(&refset);

/* Add the server socket */
FD_SET(server_socket, &refset);

if (select(server_socket + 1, &refset, NULL, NULL, NULL) == -1) {
}

...

close(server_socket);
modbus_free(ctx);

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🦋 SEE ALSO 🦋

modbus_new_tcp(3) modbus_tcp_accept(3) modbus_tcp_pi_listen(3)

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👑 AUTHORS 👑

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

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and that's a wrap, bestie 🎀 this page came from libmodbus v3.1.6, dated 11/17/2025 — go forth and listen to those connections, periodt. 💜✨