SOCKATMARK(3) Linux Programmer's Manual SOCKATMARK(3)

sockatmark - determine whether socket is at out-of-band mark

#include <sys/socket.h>
int sockatmark(int sockfd);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sockatmark(): _POSIX_C_SOURCE >= 200112L

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.

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.

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

sockatmark() was added to glibc in version 2.2.4.

For an explanation of the terms used in this section, see attributes(7).
Interface Attribute Value
sockatmark () Thread safety MT-Safe

POSIX.1-2001, POSIX.1-2008.

If sockatmark() returns 1, then the out-of-band data can be read using the MSG_OOB flag of recv(2).
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) operation.

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

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"); ... } }

fcntl(2), recv(2), send(2), tcp(7)
2017-09-15 Linux