sockatmark(3)

💖 sockatmark(3) 💖

okay bestie, gather round, because today we're talking about sockatmark(3) — a little function from the Library Functions Manual slaying in the Standard C library (libc, -lc). no cap. 🌸

---

✨ NAME ✨

sockatmark — figure out whether a socket is chillin' at the out-of-band mark 🎀

---

📚 LIBRARY 📚

Standard C library (libc, -lc) 🫶

---

🪄 SYNOPSIS 🪄

#include <sys/socket.h>

int sockatmark(int sockfd);

🌟 Feature Test Macro Requirements for glibc (peep feature_test_macros(7)) 🌟

So like, for sockatmark() you gotta have:

_POSIX_C_SOURCE >= 200112L

---

💅 DESCRIPTION 💅

sockatmark() hands you back a value that tells you whether or not the socket that the file descriptor sockfd points at is currently at the out-of-band mark. 🦋

• if the socket is at the mark → it returns 1
• if the socket is not at the mark → it returns 0

and listen, the important part: this function does not remove the out-of-band mark. it's a look, not a touch. 👀✨

---

🎀 RETURN VALUE 🎀

A successful call to sockatmark() returns 1 if the socket is at the out-of-band mark, or 0 if it's not. 😌

On error? It returns -1 and errno gets set to tell you what went wrong. 😭

---

💀 ERRORS 💀

• EBADF — sockfd is not a valid file descriptor. bestie that's not even a real fd. 🙈
• EINVAL — sockfd is a file descriptor, sure, but it's not one that sockatmark() can be applied to. wrong vibe, wrong socket. 🚫

---

👑 ATTRIBUTES 👑

For the deets on what all these terms even mean, go read attributes(7). 📖

• Interface: sockatmark() — Attribute: Thread safety — Value: MT-Safe 🔥

so yeah, she's MT-Safe. multithreaded icon behaviour, we love to see it. ✨

---

🌈 STANDARDS 🌈

POSIX.1-2008. periodt. 💅

---

🕰️ HISTORY 🕰️

• glibc 2.2.4
• POSIX.1-2001

she's been around. seasoned queen. 👑

---

📝 NOTES 📝

• if sockatmark() returns 1, then you can read the out-of-band data using the MSG_OOB flag of recv(2). that's the whole point, bestie. 🎁
• out-of-band data is supported only on some stream socket protocols. so like, check before you commit. 🧚
• sockatmark() can safely be called from a handler for the SIGURG signal. signal-handler-safe, no chaos. 🫶
• sockatmark() is implemented using the SIOCATMARK ioctl(2) operation. fun fact, iykyk. 🔍

---

🐛 BUGS 🐛

Prior to glibc 2.4, sockatmark() did not work. 😭

literally did not work. if you're on ancient glibc, this is not a you problem, it's a them problem. 💀

---

🍓 EXAMPLES 🍓

this next bit? chef's kiss. 🔥 the code below can be used after you receive a SIGURG signal to read (and discard) all data up to the mark, and then read the byte of data at the mark:

char buf[BUF_LEN];
char oobdata;
int atmark, s;

for (;;) {
    atmark = sockatmark(sockfd);
    if (atmark == -1) {
        perror("sockatmark");
        break;
    }

    if (atmark)
        break;

    s = read(sockfd, buf, BUF_LEN);
    if (s == -1)
        perror("read");
    if (s <= 0)
        break;
}

if (atmark == 1) {
    if (recv(sockfd, &oobdata, 1, MSG_OOB) == -1) {
        perror("recv");
        ...
    }
}

like, read everything up to the mark, yeet it, then grab the one byte at the mark with MSG_OOB. ate. 💅

---

👀 SEE ALSO 👀

fcntl(2), recv(2), send(2), tcp(7) 🌈

---

and that's the whole page, bestie!! this one came from Linux man-pages 6.20, dated 2026-02-08 — so if you cite her, cite her right. mwah, go read your sockets. 💖✨🎀🦋🌸🫶