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| .TH SOCKATMARK 3 2017-09-15 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| sockatmark \- determine whether socket is at out-of-band mark |
| .SH SYNOPSIS |
| .B #include <sys/socket.h> |
| .PP |
| .BI "int sockatmark(int " sockfd ); |
| .PP |
| .in -4n |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .in |
| .PP |
| .ad l |
| .BR sockatmark (): |
| _POSIX_C_SOURCE\ >=\ 200112L |
| .ad b |
| .SH DESCRIPTION |
| .BR sockatmark () |
| returns a value indicating whether or not the socket referred |
| to by the file descriptor |
| .I 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. |
| .SH RETURN VALUE |
| A successful call to |
| .BR 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 |
| .I errno |
| is set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EBADF |
| .I sockfd |
| is not a valid file descriptor. |
| .TP |
| .B EINVAL |
| .\" POSIX.1 says ENOTTY for this case |
| .I sockfd |
| is not a file descriptor to which |
| .BR sockatmark () |
| can be applied. |
| .SH VERSIONS |
| .BR sockatmark () |
| was added to glibc in version 2.2.4. |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .TS |
| allbox; |
| lb lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR sockatmark () |
| T} Thread safety MT-Safe |
| .TE |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH NOTES |
| If |
| .BR sockatmark () |
| returns 1, then the out-of-band data can be read using the |
| .B MSG_OOB |
| flag of |
| .BR recv (2). |
| .PP |
| Out-of-band data is supported only on some stream socket protocols. |
| .PP |
| .BR sockatmark () |
| can safely be called from a handler for the |
| .B SIGURG |
| signal. |
| .PP |
| .BR sockatmark () |
| is implemented using the |
| .B SIOCATMARK |
| .BR ioctl (2) |
| operation. |
| .SH BUGS |
| Prior to glibc 2.4, |
| .BR sockatmark () |
| did not work. |
| .SH EXAMPLE |
| The following code can be used after receipt of a |
| .B SIGURG |
| signal to read (and discard) all data up to the mark, |
| and then read the byte of data at the mark: |
| .PP |
| .EX |
| 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"); |
| ... |
| } |
| } |
| .EE |
| .SH SEE ALSO |
| .BR fcntl (2), |
| .BR recv (2), |
| .BR send (2), |
| .BR tcp (7) |