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 sockfd);
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13   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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15       sockatmark():
16           _POSIX_C_SOURCE >= 200112L
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DESCRIPTION

19       sockatmark()  returns  a value indicating whether or not the socket re‐
20       ferred to by the file descriptor sockfd is at the out-of-band mark.  If
21       the  socket is at the mark, then 1 is returned; if the socket is not at
22       the mark, 0 is returned.  This function does not remove the out-of-band
23       mark.
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RETURN VALUE

26       A  successful  call  to  sockatmark() returns 1 if the socket is at the
27       out-of-band mark, or 0 if it is not.  On error, -1 is returned and  er‐
28       rno is set to indicate the error.
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ERRORS

31       EBADF  sockfd is not a valid file descriptor.
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33       EINVAL sockfd is not a file descriptor to which sockatmark() can be ap‐
34              plied.
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VERSIONS

37       sockatmark() was added to glibc in version 2.2.4.
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ATTRIBUTES

40       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
41       tributes(7).
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43       ┌────────────────────────────────────────────┬───────────────┬─────────┐
44       │Interface                                   │ Attribute     │ Value   │
45       ├────────────────────────────────────────────┼───────────────┼─────────┤
46       │sockatmark()                                │ Thread safety │ MT-Safe │
47       └────────────────────────────────────────────┴───────────────┴─────────┘
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CONFORMING TO

50       POSIX.1-2001, POSIX.1-2008.
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NOTES

53       If  sockatmark() returns 1, then the out-of-band data can be read using
54       the MSG_OOB flag of recv(2).
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56       Out-of-band data is supported only on some stream socket protocols.
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58       sockatmark() can safely be called from a handler for the SIGURG signal.
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60       sockatmark() is implemented using the SIOCATMARK ioctl(2) operation.
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BUGS

63       Prior to glibc 2.4, sockatmark() did not work.
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EXAMPLES

66       The following code can be used after receipt of a SIGURG signal to read
67       (and  discard)  all data up to the mark, and then read the byte of data
68       at the mark:
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70           char buf[BUF_LEN];
71           char oobdata;
72           int atmark, s;
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74           for (;;) {
75               atmark = sockatmark(sockfd);
76               if (atmark == -1) {
77                   perror("sockatmark");
78                   break;
79               }
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81               if (atmark)
82                   break;
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84               s = read(sockfd, buf, BUF_LEN);
85               if (s == -1)
86                   perror("read");
87               if (s <= 0)
88                   break;
89           }
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91           if (atmark == 1) {
92               if (recv(sockfd, &oobdata, 1, MSG_OOB) == -1) {
93                   perror("recv");
94                   ...
95               }
96           }
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COLOPHON

102       This page is part of release 5.13 of the Linux  man-pages  project.   A
103       description  of  the project, information about reporting bugs, and the
104       latest    version    of    this    page,    can     be     found     at
105       https://www.kernel.org/doc/man-pages/.
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109Linux                             2021-03-22                     SOCKATMARK(3)