sockatmark(3)

1sockatmark(3)              Library Functions Manual              sockatmark(3)
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NAME

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

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <sys/socket.h>
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14       int sockatmark(int sockfd);
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16   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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18       sockatmark():
19           _POSIX_C_SOURCE >= 200112L
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DESCRIPTION

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

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

34       EBADF  sockfd is not a valid file descriptor.
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36       EINVAL sockfd is not a file descriptor to which sockatmark() can be ap‐
37              plied.
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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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STANDARDS

50       POSIX.1-2008.
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HISTORY

53       glibc 2.2.4.  POSIX.1-2001.
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NOTES

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

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

69       The following code can be used after receipt of a SIGURG signal to read
70       (and  discard)  all data up to the mark, and then read the byte of data
71       at the mark:
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73           char buf[BUF_LEN];
74           char oobdata;
75           int atmark, s;
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77           for (;;) {
78               atmark = sockatmark(sockfd);
79               if (atmark == -1) {
80                   perror("sockatmark");
81                   break;
82               }
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84               if (atmark)
85                   break;
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87               s = read(sockfd, buf, BUF_LEN);
88               if (s == -1)
89                   perror("read");
90               if (s <= 0)
91                   break;
92           }
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94           if (atmark == 1) {
95               if (recv(sockfd, &oobdata, 1, MSG_OOB) == -1) {
96                   perror("recv");
97                   ...
98               }
99           }
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