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(): _POSIX_C_SOURCE >= 200112L
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DESCRIPTION

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

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

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

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

39       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
40       tributes(7).
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42       ┌─────────────┬───────────────┬─────────┐
43       │Interface    │ Attribute     │ Value   │
44       ├─────────────┼───────────────┼─────────┤
45       │sockatmark() │ Thread safety │ MT-Safe │
46       └─────────────┴───────────────┴─────────┘

CONFORMING TO

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

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

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

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

100       This page is part of release 5.10 of the Linux  man-pages  project.   A
101       description  of  the project, information about reporting bugs, and the
102       latest    version    of    this    page,    can     be     found     at
103       https://www.kernel.org/doc/man-pages/.
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107Linux                             2020-06-09                     SOCKATMARK(3)