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The Fourier transform (FT) decomposes a function of time (a signal) into its constituent frequencies.
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accf_http(9) [freebsd man page]

ACCF_HTTP(9)						   BSD Kernel Developer's Manual					      ACCF_HTTP(9)

NAME
accf_http -- buffer incoming connections until a certain complete HTTP requests arrive SYNOPSIS
options INET options ACCEPT_FILTER_HTTP kldload accf_http DESCRIPTION
This is a filter to be placed on a socket that will be using accept() to receive incoming HTTP connections. It prevents the application from receiving the connected descriptor via accept() until either a full HTTP/1.0 or HTTP/1.1 HEAD or GET request has been buffered by the kernel. If something other than a HTTP/1.0 or HTTP/1.1 HEAD or GET request is received the kernel will allow the application to receive the connec- tion descriptor via accept(). The utility of accf_http is such that a server will not have to context switch several times before performing the initial parsing of the request. This effectively reduces the amount of required CPU utilization to handle incoming requests by keeping active processes in prefork- ing servers such as Apache low and reducing the size of the file descriptor set that needs to be managed by interfaces such as select(), poll() or kevent() based servers. The accf_http kernel option is also a module that can be enabled at runtime via kldload(8) if the INET option has been compiled into the ker- nel. EXAMPLES
Assuming ACCEPT_FILTER_HTTP has been included in the kernel config file or the accf_http module has been loaded, this will enable the http accept filter on the socket sok. struct accept_filter_arg afa; bzero(&afa, sizeof(afa)); strcpy(afa.af_name, "httpready"); setsockopt(sok, SOL_SOCKET, SO_ACCEPTFILTER, &afa, sizeof(afa)); SEE ALSO
setsockopt(2), accept_filter(9) HISTORY
The accept filter mechanism and the accf_http filter were introduced in FreeBSD 4.0. AUTHORS
This manual page and the filter were written by Alfred Perlstein. BSD
November 15, 2000 BSD

Check Out this Related Man Page

ACCEPT_FILTER(9)					   BSD Kernel Developer's Manual					  ACCEPT_FILTER(9)

NAME
accept_filter, accept_filt_add, accept_filt_del, accept_filt_generic_mod_event, accept_filt_get -- filter incoming connections SYNOPSIS
#define ACCEPT_FILTER_MOD #include <sys/param.h> #include <sys/kernel.h> #include <sys/sysctl.h> #include <sys/signalvar.h> #include <sys/socketvar.h> #include <netinet/accept_filter.h> int accept_filt_add(struct accept_filter *filt); int accept_filt_del(char *name); int accept_filt_generic_mod_event(module_t mod, int event, void *data); struct accept_filter * accept_filt_get(char *name); DESCRIPTION
Accept filters allow an application to request that the kernel pre-process incoming connections. This manual page describes the kernel interface for accept filters. User applications request accept filters via the setsockopt(2) system call, passing in an optname of SO_ACCEPTFILTER. IMPLEMENTATION NOTES
A module that wants to be an accept filter must provide a struct accept_filter to the system: struct accept_filter { char accf_name[16]; void (*accf_callback)(struct socket *so, void *arg, int waitflag); void * (*accf_create)(struct socket *so, char *arg); void (*accf_destroy)(struct socket *so); SLIST_ENTRY(accept_filter) accf_next; /* next on the list */ }; The module should register it with the function accept_filt_add(), passing a pointer to a struct accept_filter, allocated with malloc(9). The accept filters currently provided with NetBSD (accf_data(9) and accf_http(9)) are implemented as pseudo-devices, but an accept filter may use any supported means of initializing and registering itself at system startup or later, including the module framework if supported by the running kernel. The fields of struct accept_filter are as follows: accf_name Name of the filter; this is how it will be accessed from userland. accf_callback The callback that the kernel will do once the connection is established. It is the same as a socket upcall and will be called when the connection is established and whenever new data arrives on the socket, unless the callback modifies the socket's flags. accf_create Called whenever a setsockopt(2) installs the filter onto a listening socket. accf_destroy Called whenever the user removes the accept filter on the socket. The accept_filt_del() function passed the same string used in accept_filter.accf_name during registration with accept_filt_add(), the kernel will then disallow and further userland use of the filter. The accept_filt_get() function is used internally to locate which accept filter to use via the setsockopt(2) system call. The accept_filt_generic_mod_event() function can be used by accept filters which are loadable kernel modules to add and delete themselves. SEE ALSO
setsockopt(2), accf_data(9), accf_http(9), malloc(9) HISTORY
The accept filter mechanism was introduced in FreeBSD 4.0. It was ported to NetBSD by Coyote Point Systems, Inc. and appeared in NetBSD 5.0. AUTHORS
This manual page was written by Alfred Perlstein, Sheldon Hearn, and Jeroen Ruigrok van der Werven. The accept filter concept was pioneered by David Filo at Yahoo! and refined to be a loadable module system by Alfred Perlstein. BSD
November 12, 2008 BSD

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