epoll_create, epoll_create1 — open an epoll file descriptor


#include <sys/epoll.h>
int epoll_create( int size);
int epoll_create1( int flags);


epoll_create() creates a new epoll(7) instance. Since Linux 2.6.8, the size argument is ignored, but must be greater than zero; see NOTES below.

epoll_create() returns a file descriptor referring to the new epoll instance. This file descriptor is used for all the subsequent calls to the epoll interface. When no longer required, the file descriptor returned by epoll_create() should be closed by using close(2). When all file descriptors referring to an epoll instance have been closed, the kernel destroys the instance and releases the associated resources for reuse.


If flags is 0, then, other than the fact that the obsolete size argument is dropped, epoll_create1() is the same as epoll_create(). The following value can be included in flags to obtain different behavior:


Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor. See the description of the O_CLOEXEC flag in open(2) for reasons why this may be useful.


On success, these system calls return a nonnegative file descriptor. On error, −1 is returned, and errno is set to indicate the error.



size is not positive.


(epoll_create1()) Invalid value specified in flags.


The per-user limit on the number of epoll instances imposed by /proc/sys/fs/epoll/max_user_instances was encountered. See epoll(7) for further details.


The per-process limit on the number of open file descriptors has been reached.


The system-wide limit on the total number of open files has been reached.


There was insufficient memory to create the kernel object.


epoll_create() was added to the kernel in version 2.6. Library support is provided in glibc starting with version 2.3.2.

epoll_create1() was added to the kernel in version 2.6.27. Library support is provided in glibc starting with version 2.9.


epoll_create() is Linux-specific.


In the initial epoll_create() implementation, the size argument informed the kernel of the number of file descriptors that the caller expected to add to the epoll instance. The kernel used this information as a hint for the amount of space to initially allocate in internal data structures describing events. (If necessary, the kernel would allocate more space if the caller's usage exceeded the hint given in size.) Nowadays, this hint is no longer required (the kernel dynamically sizes the required data structures without needing the hint), but size must still be greater than zero, in order to ensure backward compatibility when new epoll applications are run on older kernels.


close(2), epoll_ctl(2), epoll_wait(2), epoll(7)


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   Copyright (C) 2003  Davide Libenzi
 Davide Libenzi <>

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Modified 2004-06-17 by Michael Kerrisk <>
Modified 2005-04-04 by Marko Kohtala <>
2008-10-10, mtk: add description of epoll_create1()