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sockatmark(3) Library Functions Manual sockatmark(3)
sockatmark - determine whether socket is at out-of-band mark
Standard C library (libc, -lc)
#include <sys/socket.h> int sockatmark(int sockfd); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): sockatmark(): _POSIX_C_SOURCE >= 200112L
sockatmark() returns a value indicating whether or not the socket referred to by the file descriptor sockfd is at the out-of-band mark. If the socket is at the mark, then 1 is returned; if the socket is not at the mark, 0 is returned. This function does not remove the out-of-band mark.
A successful call to sockatmark() returns 1 if the socket is at the out-of-band mark, or 0 if it is not. On error, -1 is returned and errno is set to indicate the error.
EBADF sockfd is not a valid file descriptor. EINVAL sockfd is not a file descriptor to which sockatmark() can be applied.
For an explanation of the terms used in this section, see attributes(7). ┌─────────────────────────────────────┬───────────────┬─────────┐ │ Interface │ Attribute │ Value │ ├─────────────────────────────────────┼───────────────┼─────────┤ │ sockatmark() │ Thread safety │ MT-Safe │ └─────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
glibc 2.2.4. POSIX.1-2001.
If sockatmark() returns 1, then the out-of-band data can be read using the MSG_OOB flag of recv(2). Out-of-band data is supported only on some stream socket protocols. sockatmark() can safely be called from a handler for the SIGURG signal. sockatmark() is implemented using the SIOCATMARK ioctl(2) operation.
Prior to glibc 2.4, sockatmark() did not work.
The following code can be used after receipt of a SIGURG signal to read (and discard) all data up to the mark, and then read the byte of data at the mark: char buf[BUF_LEN]; char oobdata; int atmark, s; for (;;) { atmark = sockatmark(sockfd); if (atmark == -1) { perror("sockatmark"); break; } if (atmark) break; s = read(sockfd, buf, BUF_LEN); if (s == -1) perror("read"); if (s <= 0) break; } if (atmark == 1) { if (recv(sockfd, &oobdata, 1, MSG_OOB) == -1) { perror("recv"); ... } }
fcntl(2), recv(2), send(2), tcp(7)
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Linux man-pages 6.9.1 2024-05-02 sockatmark(3)
Pages that refer to this page: recv(2), signal-safety(7)