FAM(3)

NAME
fam - File Alteration Monitor (FAM) library routines

SYNOPSIS
#include <fam.h>

extern int FAMOpen(FAMConnection* fc);

extern int FAMClose(FAMConnection* fc);

extern int FAMMonitorDirectory(FAMConnection *fc,
                               char *filename,
                               FAMRequest* fr,
                               void* userData);

extern int FAMMonitorFile(FAMConnection *fc,
                          char *filename,
                          FAMRequest* fr,
                          void* userData);

int FAMSuspendMonitor(FAMConnection *fc, FAMRequest *fr);

int FAMResumeMonitor(FAMConnection *fc, FAMRequest *fr);

int FAMCancelMonitor(FAMConnection *fc, FAMRequest *fr);

int FAMNextEvent(FAMConnection *fc, FAMEvent *fe);

int FAMPending(FAMConnection* fc);

typedef struct {
    int fd;
} FAMConnection;

#define FAMCONNECTION_GETFD(fc)      (fc->fd)

typedef struct {
    int reqnum;
} FAMRequest;

enum FAMCodes { FAMChanged=1, FAMDeleted=2, FAMStartExecuting=3,
    FAMStopExecuting=4, FAMCreated=5, FAMMoved=6, FAMAcknowledge=7,
    FAMExists=8, FAMEndExist=9 };

typedef struct {
    FAMConnection* fc;
    FAMRequest fr;
    char hostname[MAXHOSTNAMELEN];
    char filename[NAME_MAX];
    void *userdata;
    FAMCodes code;
} FAMEvent;

extern int FAMErrno;

extern char *FamErrlist[];

DESCRIPTION
FAM, the File Alteration Monitor, is a subsystem that applications can use to be notified when specific files or directories are changed. It is intended as a replacement for mechanisms such as poll and select.

FAM comes in two parts: famd, the daemon that listens for requests and provides notifications, and libfam a library that client applications can use to communicate with FAM.

libfam's routines are found in libfam.a, which is loaded if the option -lfam is used with cc or ld.

An application calls routines described here to establish a list of files for famdto monitor. famdgenerates events on a socket to communicate with the application. The famd process is started when the first connection from any application to it is opened. It exits after all connections to it have been closed.

USAGE
Here are the steps required to use FAM in an application:

1.  Create a connection to famd by calling FAMOpen. This routine will pass back a FAMConnection structure used in all FAM procedures.

2.  Tell famd which files and directories to monitor by calling FAMMonitorFile and FAMMonitorDirectory to express interest in files and directories, respectively.

3.  Select on the famd socket file descriptor and call FAMPending when the famd socket is active, and FAMNextEvent when FAMPending indicates that an event is available. Alternatively, call FAMPending (or FAMNextEvent) periodically to check the socket connection to famd to see if any new information has arrived. If there are no events pending, FAMNextEvent blocks until an event occurs.

4.  When the application is through monitoring a file or directory, it should call FAMCancelMonitor. If the application wants to temporarily suspend monitoring of a file or directory, it may call FAMSuspendMonitor. When the application is ready to start monitoring again, it calls FAMResumeMonitor.

5.  Before the application exits, it should call FAMClose to free resources associated with files still being monitored and to close the connection to famd.

DATA STRUCTURES
The FAMConnection Structure

The FAMConnection data structure is created when opening a connection to famd. Subsequently it is passed into all FAM procedures. This structure has all the information in it to communicate to fam.

Use the macro FAMCONNECTION_GETFD to access the file descriptor inside the FAMConnection, rather than accessing it directly.

The FAMRequest Structure

When famd is called on to monitor a file, it passes back a FAMRequest structure. This structure uniquely identifies the request so that it may be cancelled, using FAMCancelMonitor or suspended, using FAMSuspendMonitor.

The FAMEvent Structure

Changes to files and directories are encoded in the FAMEvent structure. The code field of this structure contains one of the following enumeration constants:

FAMChanged
    Some value which can be obtained with fstat changed for a file or directory being monitored.

FAMDeleted
    A file or directory being monitored was deleted or its name was changed. This event is also generated when monitoring starts on a nonexistent file or directory.

FAMStartExecuting
    An executable file or shared library being monitored started executing. If multiple processes execute the same file, this event only occurs when the first process starts.

FAMStopExecuting
    An executable file being monitored which was running finished. If multiple processes from an executable are running, this event is only generated when the last one finishes.

FAMCreated
    A file was created in a directory being monitored. Note: this event is only generated for files created directly in a directory being monitored; subdirectories are not automatically monitored.

FAMMoved
    FAMMoved events never occur. The name remains defined so that programs that reference it will still compile.

FAMAcknowledge
    After a FAMCancelMonitor, famd generates a FAMAcknowledge event. Also, if an invalid pathname is specified, famd generates a FAMAcknowledge event.

FAMExists
    When the application requests a file be monitored, famd generates a FAMExists event for that file. When the application requests a directory be monitored, famd generates a FAMExists event for that directory and every file directly contained in that directory.

FAMEndExist
    When the application requests a file directory be monitored, a series of FAMExists events is generated as described above. After the last FAMExists message, famd generates a FAMEndExist message.

If a FAM event applies to a file or directory being monitored, the FAMEvent's filename field contains the full pathname that was passed to famd. If an event applies to an entry in a monitored directory, the filename field contains the relative path only. For example, if the directory /tmp/xyzzy were monitored, and the file /tmp/xyzzy/plugh were deleted, a FAMDeleted event would be generated containing "plugh" in filename. If the directory itself were deleted, filename would contain "/tmp/xyzzy".

PROCEDURES

FAMOpen, FAMClose

The application opens a connection to famd by calling FAMOpen. FAMOpen initializes the FAMConnection structure passed in to it and returns 0 if successful, otherwise -1. The variable char* appName should be set to the name of your application. The FAMConnection structure is passed to all subsequent FAM procedure calls.

FAMClose frees resources associated with files still being monitored and closes a famd connection. It returns 0 if successful and -1 otherwise.

FAMMonitorDirectory, FAMMonitorFile

FAMMonitorDirectory and FAMMonitorFile tell famd to start monitoring a directory or file, respectively. The parameters to this function are a FAMConnection (initialized by FAMOpen), a FAMRequest structure, a filename and a user data pointer. The FAMRequest structure is modified to subsequently identify this request. When the file or directory changes, a FAM event structure will be generated. The application can retrieve this structure by calling FAMNextEvent (see description under FAMNextEvent).

FAMMonitorDirectory monitors changes that happens to the contents of the directory (as well as the directory file itself); FAMMonitorFile monitors only what happens to a particular file. Both routines return 0 if successful and -1 otherwise.

The filename argument must be a full pathname.

FAMSuspendMonitor, FAMResumeMonitor

FAMSuspendMonitor temporarily suspends monitoring of files or directories. This is useful when an application is not displaying information about files, when it is iconified, for example. FAMResumeMonitor signals famd to start monitoring the file or directory again. Changes which occur while monitoring is suspended are enqueued and delivered when monitoring is resumed.

Both of these routines take a FAMConnection and a FAMRequest structure. The FAMRequest Structure is returned from the FAMMonitorFile or FAMMonitorDirectory routines and return 0 if successful and -1 otherwise.

Because famd runs as an asynchronous process, FAMNextEvent may return a few events regarding a given request after that request has been suspended.

FAMCancelMonitor

When an application is finished monitoring a file or directory, it should call FAMCancelMonitor. This routine will signal famd not to monitor this directory anymore. The FAMRequest structure is returned from the FAMMonitorFile or FAMMonitorDirectory routines. FAMCancelMonitor returns 0 if successful and -1 otherwise.

FAMPending, FAMNextEvent

FAMPending returns 1 if an event is waiting and 0 if no event is waiting. It also returns 1 if an error has been encountered. This routine returns immediately to the caller.

FAMNextEvent will get the next FAM event. If there are no FAM events waiting, then the calling application blocks until a FAM event is received. If blocking is not desirable, call FAMPending before FAMNextEvent, and only call FAMNextEvent when FAMPending says an event is available.

There are two ways to for applications to receive FAM events:

1. The Select approach - The application selects on the file
    descriptor returned from FAMOpen, in the FAMConnection structure.
    When this file descriptor becomes active, the application calls
    FAMPending to determine whether a complete event is ready, and
    FAMNextEvent to retrieve the pending FAM event.

2. The Polling approach - The application calls FAMPending
    periodically (usually when the system is waiting for input).
    When FAMPending returns 1, the application calls FAMNextEvent to
    retrieve the pending FAM event.

FAMNextEvent reads any information that is on the famd socket, and returns it to the application in the form of a FAMEvent.

FAMNextEvent returns 1 if successful and -1 otherwise.

SEE ALSO
famd(8), fstat(1), poll(2), select(2)

BUGS
The FAMMoved event is not currently supported.

FAMNextEvent may not initialize the FAMEvent's filename field for FAMEndExist and FAMAcknowledge events. Use the request number to determine the file or directory to which those events refer.

FAMErrno and FamErrlist are not set when errors occur.

When a shell script is run, notification is generated for the shell executing the script.

Each process is limited to 1000 active requests at a time.

When using the Linux DNotify kernel monitor, a file handle will be opened for each file famd is asked to monitor, meaning the file system the file resides on can not be unmounted.