sockatmark(3)

1SOCKATMARK(3)              Linux Programmer's Manual             SOCKATMARK(3)
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NAME

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

9       #include <sys/socket.h>
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11       int sockatmark(int fd);
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DESCRIPTION

14       sockatmark()  returns  a  value  indicating  whether  or not the socket
15       referred to by the file descriptor fd is at the out-of-band  mark.   If
16       the  socket is at the mark, then 1 is returned; if the socket is not at
17       the mark, 0 is returned.  This function does not remove the out-of-band
18       mark.
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RETURN VALUE

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

26       EBADF  fd is not a valid file descriptor.
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28       EINVAL fd  is  not  a  file  descriptor  to  which  sockatmark() can be
29              applied.
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NOTES

32       If sockatmark() returns 1, then the out-of-band data can be read  using
33       the MSG_OOB flag of recv(2).
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35       Out-of-band data is only supported on some stream socket protocols.
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37       sockatmark() can safely be called from a handler for the SIGURG signal.
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39       sockatmark() is implemented using the SIOCATMARK ioctl() operation.
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CONFORMING TO

42       POSIX.1-2001
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VERSIONS

45       sockatmark() was added to glibc in version 2.2.4.
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BUGS

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

51       The following code can be used after receipt of a SIGURG signal to read
52       (and discard) all data up to the mark, and then read the byte  of  data
53       at the mark:
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55           char buf[BUF_LEN];
56           char oobdata;
57           int atmark, s;
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59           for (;;) {
60               atmark = sockatmark(fd);
61               if (atmark == -1) {
62                   perror("sockatmark");
63                   break;
64               }
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66               if (atmark)
67                   break;
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69               s = read(fd, buf, BUF_LEN) <= 0);
70               if (s == -1)
71                   perror("read");
72               if (s <= 0)
73                   break;
74           }
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76           if (atmark == 1) {
77               if (recv(fd, &oobdata, 1, MSG_OOB) == -1) {
78                   perror("recv");
79                   ...
80               }
81           }
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