sockatmark(3) Library Functions Manual sockatmark(3)NAME
sockatmark - Tests whether a socket is at the out-of-band mark
LIBRARY
Standard C Library: (libc.so, libc.a)
SYNOPSIS
#include <sys/socket.h>
int sockatmark (int socket);
STANDARDS
The sockatmark function also supports POSIX.1g Draft 6.6.
Refer to the standards(5) reference page for more information about industry standards and associated tags.
PARAMETERS
Specifies the file descriptor associated with a socket.
DESCRIPTION
The sockatmark() function enables applications to determine which received data precede the out-of-band data and which data follow the out-
of-band data. The function uses the file descriptor for the socket to locate the data. Applications typically use this function between
receive operations.
The sockatmark() function does not remove the mark from the stream.
RETURN VALUES
Upon successful completion, the sockatmark() function returns 1 if the protocol has marked the data stream and all data preceding the mark
have been read, or 0 (zero) if there is no mark or if data precede the mark in the receive queue.
If the sockatmark() function fails, a value of -1 is returned and errno is set to indicate the error.
ERRORS
If the sockatmark() function fails, errno may be set to one of the following values: The socket parameter is not a valid file descriptor.
The socket parameter does not specify a descriptor for a socket.
RELATED INFORMATION
Functions: recv(2), recvmsg(2).
Standards: standards(5). delim off
sockatmark(3)
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SOCKATMARK(3) Linux Programmer's Manual SOCKATMARK(3)NAME
sockatmark - determine whether socket is at out-of-band mark
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 || _XOPEN_SOURCE >= 600
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.
VERSIONS
sockatmark() was added to glibc in version 2.2.4.
CONFORMING TO
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).
Out-of-band data is only supported 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.
EXAMPLE
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) <= 0);
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)COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-12-03 SOCKATMARK(3)