sockatmark(3) Library Functions Manual sockatmark(3)
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)):
sockatmark():
_POSIX_C_SOURCE >= 200112L
DESCRIPTION
sockatmark() returns a value indicating whether or not the socket re-
ferred 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 er-
rno 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 ap-
plied.
VERSIONS
sockatmark() was added in glibc 2.2.4.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│sockatmark() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2001, POSIX.1-2008.
NOTES
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.
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), recv(2), send(2), tcp(7)
Linux man-pages 6.03 2022-12-15 sockatmark(3)
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