---
name: "sockatmark"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/sockatmark"
---

# sockatmark(3)

determine whether socket is at out-of-band mark

## NAME

sockatmark - determine whether socket is at 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 (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**sockatmark**():

```
    _POSIX_C_SOURCE >= 200112L
```

## DESCRIPTION

**sockatmark**() returns a value indicating whether or not the socket referred to by the file descriptor *sockfd* is at the out-of-band mark. If the socket is at the mark, then 1 is returned; if the socket is not at the mark, 0 is returned. This function does not remove the out-of-band mark.

## RETURN VALUE

A successful call to **sockatmark**() returns 1 if the socket is at the out-of-band mark, or 0 if it is not. On error, -1 is returned and *errno* is set to indicate the error.

## ERRORS

- **EBADF**

  *sockfd* is not a valid file descriptor.

- **EINVAL**

  *sockfd* is not a file descriptor to which **sockatmark**() can be applied.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **sockatmark**() | Thread safety | MT-Safe |

## STANDARDS

POSIX.1-2008.

## HISTORY

glibc 2.2.4. POSIX.1-2001.

## NOTES

If **sockatmark**() returns 1, then the out-of-band data can be read using the **MSG\_OOB** flag of [**recv**(2)](/recv/2.md).

Out-of-band data is supported only on some stream socket protocols.

**sockatmark**() can safely be called from a handler for the **SIGURG** signal.

**sockatmark**() is implemented using the **SIOCATMARK** [**ioctl**(2)](/ioctl/2.md) operation.

## BUGS

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

## EXAMPLES

The following code can be used after receipt of 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");
            ...
        }
    }
```

## SEE ALSO

[**fcntl**(2)](/fcntl/2.md), [**recv**(2)](/recv/2.md), [**send**(2)](/send/2.md), [**tcp**(7)](/tcp/7.md)