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#include <fcntl.h>int fcntl(intfd, F_GETOWN);int fcntl(intfd, F_SETOWN, intarg);#define _GNU_SOURCE#include <fcntl.h>int fcntl(intfd, F_GETOWN_EX, struct f_owner_ex *arg);int fcntl(intfd, F_SETOWN_EX, const struct f_owner_ex *arg);int fcntl(intfd, F_GETSIG);int fcntl(intfd, F_SETSIG, intarg);

DESCRIPTION top

F_GETOWN,F_SETOWN,F_GETOWN_EX,F_SETOWN_EX,F_GETSIG, andF_SETSIGare used to manage I/O availability signals:F_GETOWN Return (as the function result) the process ID or process group ID currently receivingSIGIOandSIGURGsignals for events on file descriptorfd. Process IDs are returned as positive values; process group IDs are returned as negative values (but see BUGS below).arg is ignored.F_SETOWN Set the process ID or process group ID that will receiveSIGIOandSIGURGsignals for events on the file descriptorfd. The target process or process group ID is specified inarg. A process ID is specified as a positive value; a process group ID is specified as a negative value. Most commonly, the calling process specifies itself as the owner (that is,arg is specified asgetpid(2)). As well as setting the file descriptor owner, one must also enable generation of signals on the file descriptor. This is done by using theF_SETFL(2const) operation to set theO_ASYNCfile status flag on the file descriptor. Subsequently, aSIGIOsignal is sent whenever input or output becomes possible on the file descriptor. Thefcntl()F_SETSIGoperation can be used to obtain delivery of a signal other thanSIGIO. Sending a signal to the owner process (group) specified byF_SETOWNis subject to the same permissions checks as are described forkill(2), where the sending process is the one that employsF_SETOWN(but see BUGS below). If this permission check fails, then the signal is silently discarded.Note: TheF_SETOWNoperation records the caller's credentials at the time of thefcntl() call, and it is these saved credentials that are used for the permission checks. If the file descriptorfd refers to a socket,F_SETOWNalso selects the recipient ofSIGURGsignals that are delivered when out-of-band data arrives on that socket. (SIGURGis sent in any situation whereselect(2) would report the socket as having an "exceptional condition".) The following was true in Linux 2.6.x up to and including Linux 2.6.11: If a nonzero value is given toF_SETSIGin a multithreaded process running with a threading library that supports thread groups (e.g., NPTL), then a positive value given toF_SETOWNhas a different meaning: instead of being a process ID identifying a whole process, it is a thread ID identifying a specific thread within a process. Consequently, it may be necessary to passF_SETOWN the result ofgettid(2) instead ofgetpid(2) to get sensible results whenF_SETSIGis used. (In current Linux threading implementations, a main thread's thread ID is the same as its process ID. This means that a single-threaded program can equally usegettid(2) orgetpid(2) in this scenario.) Note, however, that the statements in this paragraph do not apply to theSIGURGsignal generated for out-of- band data on a socket: this signal is always sent to either a process or a process group, depending on the value given toF_SETOWN. The above behavior was accidentally dropped in Linux 2.6.12, and won't be restored. From Linux 2.6.32 onward, useF_SETOWN_EXto targetSIGIOandSIGURGsignals at a particular thread.F_GETOWN_EX Return the current file descriptor owner settings as defined by a previousF_SETOWN_EXoperation. The information is returned in the structure pointed to byarg, which has the following form: struct f_owner_ex { int type; pid_t pid; }; Thetype field will have one of the valuesF_OWNER_TID,F_OWNER_PID, orF_OWNER_PGRP. Thepid field is a positive integer representing a thread ID, process ID, or process group ID. SeeF_SETOWN_EXfor more details.F_SETOWN_EX This operation performs a similar task toF_SETOWN. It allows the caller to direct I/O availability signals to a specific thread, process, or process group. The caller specifies the target of signals viaarg, which is a pointer to af_owner_ex structure. Thetype field has one of the following values, which define howpid is interpreted:F_OWNER_TID Send the signal to the thread whose thread ID (the value returned by a call toclone(2) orgettid(2)) is specified inpid.F_OWNER_PID Send the signal to the process whose ID is specified inpid.F_OWNER_PGRP Send the signal to the process group whose ID is specified inpid. (Note that, unlike withF_SETOWN, a process group ID is specified as a positive value here.)F_GETSIG Return (as the function result) the signal sent when input or output becomes possible. A value of zero meansSIGIOis sent. Any other value (includingSIGIO) is the signal sent instead, and in this case additional info is available to the signal handler if installed withSA_SIGINFO.arg is ignored.F_SETSIG Set the signal sent when input or output becomes possible to the value given inarg. A value of zero means to send the defaultSIGIOsignal. Any other value (includingSIGIO) is the signal to send instead, and in this case additional info is available to the signal handler if installed withSA_SIGINFO. By usingF_SETSIGwith a nonzero value, and settingSA_SIGINFOfor the signal handler (seesigaction(2)), extra information about I/O events is passed to the handler in asiginfo_t structure. If thesi_code field indicates the source isSI_SIGIO, thesi_fd field gives the file descriptor associated with the event. Otherwise, there is no indication which file descriptors are pending, and you should use the usual mechanisms (select(2),poll(2),read(2) withO_NONBLOCKset etc.) to determine which file descriptors are available for I/O. Note that the file descriptor provided insi_fd is the one that was specified during theF_SETSIGoperation. This can lead to an unusual corner case. If the file descriptor is duplicated (dup(2) or similar), and the original file descriptor is closed, then I/O events will continue to be generated, but thesi_fd field will contain the number of the now closed file descriptor. By selecting a real time signal (value >=SIGRTMIN), multiple I/O events may be queued using the same signal numbers. (Queuing is dependent on available memory.) Extra information is available ifSA_SIGINFOis set for the signal handler, as above. Note that Linux imposes a limit on the number of real-time signals that may be queued to a process (seegetrlimit(2) andsignal(7)) and if this limit is reached, then the kernel reverts to deliveringSIGIO, and this signal is delivered to the entire process rather than to a specific thread. Using these mechanisms, a program can implement fully asynchronous I/O without usingselect(2) orpoll(2) most of the time. The use ofO_ASYNCis specific to BSD and Linux. The only use ofF_GETOWNandF_SETOWNspecified in POSIX.1 is in conjunction with the use of theSIGURGsignal on sockets. (POSIX does not specify theSIGIOsignal.)F_GETOWN_EX,F_SETOWN_EX,F_GETSIG, andF_SETSIGare Linux-specific. POSIX has asynchronous I/O and theaio_sigevent structure to achieve similar things; these are also available in Linux as part of the GNU C Library (glibc).

RETURN VALUE top

Seefcntl(2).F_GETOWN Value of file descriptor owner.F_GETSIG Value of signal sent when read or write becomes possible, or zero for traditionalSIGIObehavior.F_SETOWNF_GETOWN_EXF_SETOWN_EXF_SETSIG Zero. On error, -1 is returned, anderrno is set to indicate the error.

F_GETOWNF_SETOWN POSIX.1-2001. (To get their definitions, define either_XOPEN_SOURCEwith the value 500 or greater, or_POSIX_C_SOURCEwith the value 200809L or greater.)F_GETOWN_EXF_GETOWN_EX Linux 2.6.32.F_GETSIGF_GETSIG Linux.

BUGS top

F_GETOWN A limitation of the Linux system call conventions on some architectures (notably i386) means that if a (negative) process group ID to be returned byF_GETOWNfalls in the range -1 to -4095, then the return value is wrongly interpreted by glibc as an error in the system call; that is, the return value offcntl() will be -1, anderrno will contain the (positive) process group ID. The Linux-specificF_GETOWN_EXoperation avoids this problem. Since glibc 2.11, glibc makes the kernelF_GETOWNproblem invisible by implementingF_GETOWNusingF_GETOWN_EX.F_SETOWN In Linux 2.4 and earlier, there is bug that can occur when an unprivileged process usesF_SETOWNto specify the owner of a socket file descriptor as a process (group) other than the caller. In this case,fcntl() can return -1 witherrno set toEPERM, even when the owner process (group) is one that the caller has permission to send signals to. Despite this error return, the file descriptor owner is set, and signals will be sent to the owner.

COLOPHON top

This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-07-20F_GETSIG(2const)

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