man3/sockatmark.3

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  20 .TH SOCKATMARK 3 2006-04-24 "Linux" "Linux Programmer's Manual"
  21 .SH NAME
  22 sockatmark \- determine whether socket is at out-of-band mark
  23 .SH SYNOPSIS
  24 .B #include <sys/socket.h>
  25 .sp
  26 .BI "int sockatmark(int " fd );
  27 .SH DESCRIPTION
  28 .BR sockatmark ()
  29 returns a value indicating whether or not the socket referred
  30 to by the file descriptor
  31 .I fd
  32 is at the out-of-band mark.
  33 If the socket is at the mark, then 1 is returned;
  34 if the socket is not at the mark, 0 is returned.
  35 This function does not remove the out-of-band mark.
  36 .SH "RETURN VALUE"
  37 A successful call to
  38 .BR sockatmark ()
  39 returns 1 if the socket is at the out-of-band mark, or 0 if it is not.
  40 On error, \-1 is returned and \fIerrno\fP is set to indicate the error.
  41 .SH ERRORS
  42 .TP
  43 .B EBADF
  44 .I fd
  45 is not a valid file descriptor.
  46 .TP
  47 .B EINVAL
  48 .\" POSIX.1 says ENOTTY for this case
  49 .I fd
  50 is not a file descriptor to which
  51 .BR sockatmark ()
  52 can be applied.
  53 .SH NOTES
  54 If
  55 .BR sockatmark ()
  56 returns 1, then the out-of-band data can be read using the
  57 .B MSG_OOB
  58 flag of
  59 .BR recv (2).
  60
  61 Out-of-band data is only supported on some stream socket protocols.
  62
  63 .BR sockatmark ()
  64 can safely be called from a handler for the SIGURG signal.
  65
  66 .BR sockatmark ()
  67 is implemented using the
  68 .B SIOCATMARK
  69 .BR ioctl ()
  70 operation.
  71 .SH "CONFORMING TO"
  72 POSIX.1-2001
  73 .SH VERSIONS
  74 .BR sockatmark ()
  75 was added to glibc in version 2.2.4.
  76 .SH BUGS
  77 Prior to glibc 2.4,
  78 .BR sockatmark ()
  79 did not work.
  80 .SH EXAMPLE
  81 The following code can be used after receipt of a SIGURG signal
  82 to read (and discard) all data up to the mark,
  83 and then read the byte of data at the mark:
  84 .nf
  85
  86     char buf[BUF_LEN];
  87     char oobdata;
  88     int atmark, s;
  89
  90     for (;;) {
  91         atmark = sockatmark(fd);
  92         if (atmark == \-1) {
  93             perror("sockatmark");
  94             break;
  95         }
  96
  97         if (atmark)
  98             break;
  99
 100         s = read(fd, buf, BUF_LEN) <= 0);
 101         if (s == -1)
 102             perror("read");
 103         if (s <= 0)
 104             break;
 105     }
 106
 107     if (atmark == 1) {
 108         if (recv(fd, &oobdata, 1, MSG_OOB) == \-1) {
 109             perror("recv");
 110             ...
 111         }
 112     }
 113 .fi
 114 .SH "SEE ALSO"
 115 .BR fcntl (2),
 116 .BR recv (2),
 117 .BR send (2),
 118 .BR tcp (7)