man3/sockatmark.3

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  25 .TH SOCKATMARK 3 2017-09-15 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 sockatmark \- determine whether socket is at out-of-band mark
  28 .SH SYNOPSIS
  29 .B #include <sys/socket.h>
  30 .PP
  31 .BI "int sockatmark(int " sockfd );
  32 .PP
  33 .in -4n
  34 Feature Test Macro Requirements for glibc (see
  35 .BR feature_test_macros (7)):
  36 .in
  37 .PP
  38 .ad l
  39 .BR sockatmark ():
  40 _POSIX_C_SOURCE\ >=\ 200112L
  41 .ad b
  42 .SH DESCRIPTION
  43 .BR sockatmark ()
  44 returns a value indicating whether or not the socket referred
  45 to by the file descriptor
  46 .I sockfd
  47 is at the out-of-band mark.
  48 If the socket is at the mark, then 1 is returned;
  49 if the socket is not at the mark, 0 is returned.
  50 This function does not remove the out-of-band mark.
  51 .SH RETURN VALUE
  52 A successful call to
  53 .BR sockatmark ()
  54 returns 1 if the socket is at the out-of-band mark, or 0 if it is not.
  55 On error, \-1 is returned and
  56 .I errno
  57 is set to indicate the error.
  58 .SH ERRORS
  59 .TP
  60 .B EBADF
  61 .I sockfd
  62 is not a valid file descriptor.
  63 .TP
  64 .B EINVAL
  65 .\" POSIX.1 says ENOTTY for this case
  66 .I sockfd
  67 is not a file descriptor to which
  68 .BR sockatmark ()
  69 can be applied.
  70 .SH VERSIONS
  71 .BR sockatmark ()
  72 was added to glibc in version 2.2.4.
  73 .SH ATTRIBUTES
  74 For an explanation of the terms used in this section, see
  75 .BR attributes (7).
  76 .TS
  77 allbox;
  78 lb lb lb
  79 l l l.
  80 Interface       Attribute       Value
  81 T{
  82 .BR sockatmark ()
  83 T}      Thread safety   MT-Safe
  84 .TE
  85 .SH CONFORMING TO
  86 POSIX.1-2001, POSIX.1-2008.
  87 .SH NOTES
  88 If
  89 .BR sockatmark ()
  90 returns 1, then the out-of-band data can be read using the
  91 .B MSG_OOB
  92 flag of
  93 .BR recv (2).
  94 .PP
  95 Out-of-band data is supported only on some stream socket protocols.
  96 .PP
  97 .BR sockatmark ()
  98 can safely be called from a handler for the
  99 .B SIGURG
 100 signal.
 101 .PP
 102 .BR sockatmark ()
 103 is implemented using the
 104 .B SIOCATMARK
 105 .BR ioctl (2)
 106 operation.
 107 .SH BUGS
 108 Prior to glibc 2.4,
 109 .BR sockatmark ()
 110 did not work.
 111 .SH EXAMPLE
 112 The following code can be used after receipt of a
 113 .B SIGURG
 114 signal to read (and discard) all data up to the mark,
 115 and then read the byte of data at the mark:
 116 .PP
 117 .EX
 118     char buf[BUF_LEN];
 119     char oobdata;
 120     int atmark, s;
 121
 122     for (;;) {
 123         atmark = sockatmark(sockfd);
 124         if (atmark == \-1) {
 125             perror("sockatmark");
 126             break;
 127         }
 128
 129         if (atmark)
 130             break;
 131
 132         s = read(sockfd, buf, BUF_LEN);
 133         if (s == \-1)
 134             perror("read");
 135         if (s <= 0)
 136             break;
 137     }
 138
 139     if (atmark == 1) {
 140         if (recv(sockfd, &oobdata, 1, MSG_OOB) == \-1) {
 141             perror("recv");
 142             ...
 143         }
 144     }
 145 .EE
 146 .SH SEE ALSO
 147 .BR fcntl (2),
 148 .BR recv (2),
 149 .BR send (2),
 150 .BR tcp (7)