SOCKET(3P)

💖 SOCKET(3P) — the POSIX Programmer's Manual, bestie edition 💖

okay so first things first, a lil PROLOG moment 🫶

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

no cap, that's the whole disclaimer. POSIX says one thing, Linux might do its own lil thing. check the Linux page if you're on Linux. that's it, that's the vibe check. 💅

---

✨ NAME ✨

socket — create an endpoint for communication

like... the actual plug-the-cable-in moment of networking. iconic. 👑

---

🌸 SYNOPSIS 🌸

#include <sys/socket.h>

int socket(int domain, int type, int protocol);

three args, one dream. let's go. 🦋

---

💅 DESCRIPTION 💅

The socket() function shall create an unbound socket in a communications domain, and return a file descriptor that can be used in later function calls that operate on sockets. The file descriptor shall be allocated as described in Section 2.14, File Descriptor Allocation.

unbound means it's not attached to any address yet, bestie. it's just vibing, existing, waiting for you to bind() it to something. the fd you get back is the golden ticket 🎀 — you hand it to every other socket function from here on.

The socket() function takes the following arguments:

• domain — Specifies the communications domain in which a socket is to be created.
• type — Specifies the type of socket to be created.
• protocol — Specifies a particular protocol to be used with the socket. Specifying a protocol of 0 causes socket() to use an unspecified default protocol appropriate for the requested socket type.

---

🌈 the domain argument, explained 🌈

The domain argument specifies the address family used in the communications domain. The address families supported by the system are implementation-defined.

Symbolic constants that can be used for the domain argument are defined in the <sys/socket.h> header.

so like, this is the "what kind of world are we networking in" knob. your constants live in <sys/socket.h> — go look, they're right there. and which families your system supports? implementation-defined, so it's giving ✨mystery✨, check your own system. 🔍

---

🦋 the type argument, explained 🦋

The type argument specifies the socket type, which determines the semantics of communication over the socket. The following socket types are defined; implementations may specify additional socket types:

• SOCK_STREAM — Provides sequenced, reliable, bidirectional, connection-mode byte streams, and may provide a transmission mechanism for out-of-band data.

bestie this is your classic reliable stream. in order, two-way, connected. the whole "i said a thing, you got the thing, in the right order" fantasy. 💖

• SOCK_DGRAM — Provides datagrams, which are connectionless-mode, unreliable messages of fixed maximum length.

no connection, no promises, fixed max size. it's giving "fire and forget, hope it lands" 😭

• SOCK_SEQPACKET — Provides sequenced, reliable, bidirectional, connection-mode transmission paths for records. A record can be sent using one or more output operations and received using one or more input operations, but a single operation never transfers part of more than one record. Record boundaries are visible to the receiver via the MSG_EOR flag.

okay so this one ATE. reliable AND sequenced AND connection-mode, but with record boundaries preserved. you can chop a record up across multiple sends, and the receiver can slurp it up across multiple reads — but never, ever will a single operation straddle two records. the receiver can literally see where each record ends via the MSG_EOR flag. structure. immaculate. 👑

---

🧚 the protocol argument, explained 🧚

If the protocol argument is non-zero, it shall specify a protocol that is supported by the address family. If the protocol argument is zero, the default protocol for this address family and type shall be used. The protocols supported by the system are implementation-defined.

The process may need to have appropriate privileges to use the socket() function or to create some sockets.

so: 0 = "you pick for me, i trust you", non-zero = "i'm picking and it better be supported by this address family". and yeah, some sockets need privileges, so if it's yelling at you with EACCES that's why, don't take it personally 🫶

---

⭐ RETURN VALUE ⭐

Upon successful completion, socket() shall return a non-negative integer, the socket file descriptor. Otherwise, a value of -1 shall be returned and errno set to indicate the error.

non-negative fd = slay. -1 = go check errno, it's telling you exactly what went wrong. 🔥

---

💀 ERRORS 💀

The socket() function shall fail if:

• EAFNOSUPPORT — The implementation does not support the specified address family.
• EMFILE — All file descriptors available to the process are currently open.
• ENFILE — No more file descriptors are available for the system.
• EPROTONOSUPPORT — The protocol is not supported by the address family, or the protocol is not supported by the implementation.
• EPROTOTYPE — The socket type is not supported by the protocol.

The socket() function may fail if:

• EACCES — The process does not have appropriate privileges.
• ENOBUFS — Insufficient resources were available in the system to perform the operation.
• ENOMEM — Insufficient memory was available to fulfill the request.

note the difference bestie: "shall fail" = these are the guaranteed ones. "may fail" = these can happen but aren't required to. EMFILE is your process running out of fds, ENFILE is the whole system running out. two very different kinds of broke 😭

The following sections are informative.

---

🌸 EXAMPLES 🌸

None.

literally none. they gave us nothing. 🙈

---

🎀 APPLICATION USAGE 🎀

The documentation for specific address families specifies which protocols each address family supports. The documentation for specific protocols specifies which socket types each protocol supports.

The application can determine whether an address family is supported by trying to create a socket with domain set to the protocol in question.

so the tea is: the family docs tell you the protocols, the protocol docs tell you the types. and if you're like "does this system even DO this family??" — just try it. make the socket. see if it screams. empirical queen. 💅

---

🌈 RATIONALE 🌈

None.

nothing. empty. the committee had no notes. 🦋

---

🔮 FUTURE DIRECTIONS 🔮

None.

the future is unspecified, and honestly? same. ✨

---

👑 SEE ALSO 👑

• Section 2.14, File Descriptor Allocation
• accept()
• bind()
• connect()
• getsockname()
• getsockopt()
• listen()
• recv()
• recvfrom()
• recvmsg()
• send()
• sendmsg()
• setsockopt()
• shutdown()
• socketpair()

The Base Definitions volume of POSIX.1‐2017, <netinet_in.h>, <sys_socket.h>

the whole group chat, fr fr. socket() is just the intro text — the rest of the conversation happens in those. 🫶

---

💜 COPYRIGHT 💜

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

---

okay bestie that's the whole page, no cap, every flag and errno intact 💖 this was brought to you by IEEE/The Open Group, 2017 — mwah, ty for the lore, go make some sockets 🎀✨💅