ACCEPT(2)

💖 ACCEPT(2) 💖

okay so bestie, welcome to ACCEPT(2) from the System Calls Manual — this is the one where your server finally slides into someone's DMs (consensually, via a socket). let's get into it 🫶

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✨ NAME ✨

accept, accept4 — accept a connection on a socket 🎀

📚 LIBRARY 📚

Standard C Library (libc, -lc)

🦋 SYNOPSIS 🦋

#include <sys/types.h>
#include <sys/socket.h>

int
accept(int s, struct sockaddr * restrict addr,
    socklen_t * restrict addrlen);

int
accept4(int s, struct sockaddr * restrict addr,
    socklen_t * restrict addrlen, int flags);

🌸 DESCRIPTION 🌸

So. The argument s is a socket that's been through the whole glow-up pipeline: created with socket(2), bound to an address with bind(2), and then set to listen for connections after listen(2). ✨

The accept() system call extracts the first connection request on the queue of pending connections, creates a brand new socket, and allocates a new file descriptor for it. That new fd inherits the state of the O_NONBLOCK and O_ASYNC properties, plus the destination of SIGIO and SIGURG signals, from the original socket s. it's giving "heirloom hand-me-downs but make it file descriptors" 💅

The accept4() system call is similar, but with receipts different:

• the O_NONBLOCK property of the new socket is instead determined by the SOCK_NONBLOCK flag in the flags argument,
• the O_ASYNC property is cleared,
• the signal destination is cleared,
• and the close-on-exec flag on the new file descriptor can be set via the SOCK_CLOEXEC flag in the flags argument.

Now, the waiting game 😭. If there are no pending connections on the queue and the original socket is not marked as non-blocking, accept() blocks the caller until a connection shows up. If the original socket is marked non-blocking and there are no pending connections on the queue, accept() returns an error, described down below in ERRORS. Also important: the accepted socket may not be used to accept more connections. The original socket s stays open though. periodt. 👑

The argument addr is a result argument — it gets filled in with the address of the connecting entity, as known to the communications layer. The exact format of the addr argument is determined by the domain in which the communication is occurring. A null pointer may be specified for addr if you don't want the address info; in that case, addrlen is not used and should also be null. Otherwise, addrlen is a value-result argument: it should initially contain the amount of space pointed to by addr, and on return it will contain the actual length (in bytes) of the address returned. This call is used with connection-based socket types, currently with SOCK_STREAM. 🧚

It's possible to select(2) a socket for the purposes of doing an accept() by selecting it for read. 👀

For certain protocols which require an explicit confirmation, such as ISO or DATAKIT, accept() can be thought of as merely dequeueing the next connection request and not implying confirmation. Confirmation can be implied by a normal read or write on the new file descriptor, and rejection can be implied by closing the new socket. so you can ghost a connection by just closing it, iconic honestly 💀

For some applications, performance may be enhanced by using an accept_filter(9) to pre-process incoming connections. 🔥

okay this next one is a whole warning, bestie, listen up: when using accept(), portable programs should not rely on the O_NONBLOCK and O_ASYNC properties and the signal destination being inherited, but should set them explicitly using fcntl(2). accept4() sets these properties consistently, but may not be fully portable across Unix platforms. don't be out here assuming, fr fr 🙈

💫 RETURN VALUES 💫

These calls return -1 on error. If they succeed, they return a non-negative integer that is a descriptor for the accepted socket. slay. 💖

🚨 ERRORS 🚨

The accept() and accept4() system calls will fail if:

• [EBADF] — The descriptor is invalid.
• [EINTR] — The accept() operation was interrupted.
• [EMFILE] — The per-process descriptor table is full.
• [ENFILE] — The system file table is full.
• [ENOTSOCK] — The descriptor references a file, not a socket.
• [EINVAL] — listen(2) has not been called on the socket descriptor.
• [EFAULT] — The addr argument is not in a writable part of the user address space.
• [EWOULDBLOCK] or [EAGAIN] — The socket is marked non-blocking and no connections are present to be accepted.
• [ECONNABORTED] — A connection arrived, but it was closed while waiting on the listen queue. 😭

The accept4() system call will also fail if:

• [EINVAL] — The flags argument is invalid.

🔗 SEE ALSO 🔗

bind(2), connect(2), getpeername(2), getsockname(2), listen(2), select(2), socket(2), accept_filter(9)

📜 HISTORY 📜

The accept() system call appeared in 4.2BSD.

The accept4() system call appeared in FreeBSD 10.0.

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and that's the whole page bestie, no cap 🫶 this one's from Debian, October 9, 2014 — go accept some connections, you're the main character of this listen queue now 👑✨💅