tail head cat sleep
QR code linking to this page

Manual Pages  — FILEMON


filemon – the filemon device



#include <dev/filemon/filemon.h>


The filemon device allows a process to collect file operations data of its children. The device /dev/filemon responds to two ioctl(2) calls.

filemon is not intended to be a security auditing tool. Many system calls are not tracked and binaries of foreign ABI will not be fully audited. It is intended for auditing of processes for the purpose of determining its dependencies in an efficient and easily parsable format. An example of this is make(1) which uses this module with .MAKE.MODE=meta to handle incremental builds more smartly.

System calls are denoted using the following single letters:

A openat(2). The next log entry may be lacking an absolute path or be inaccurate.
C chdir(2)
D unlink(2)
E exec(2)
F fork(2), vfork(2)
L link(2), linkat(2), symlink(2), symlinkat(2)
M rename(2)
R open(2) or openat(2) for read
W open(2) or openat(2) for write
X _exit(2)

Note that ‘R’ following ‘W’ records can represent a single open(2) for R/W, or two separate open(2) calls, one for ‘R’ and one for ‘W’. Note that only successful system calls are captured.


User mode programs communicate with the filemon driver through a number of ioctls which are described below. Each takes a single argument.
FILEMON_SET_FD Write the internal tracing buffer to the supplied open file descriptor.
  Child process ID to trace. This should normally be done under the control of a parent in the child after fork(2) but before anything else. See the example below.


The ioctl() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.


The ioctl() system call with FILEMON_SET_FD will fail if:
  The filemon handle is already associated with a file descriptor.

The ioctl() system call with FILEMON_SET_PID will fail if:
  No process having the specified process ID exists.
  The process ID specified is already being traced and was not the current process.

The close() system call on the filemon file descriptor may fail with the errors from write(2) if any error is encountered while writing the log. It may also fail if:
  An invalid address was used for a traced system call argument, resulting in no log entry for the system call.
  An argument for a traced system call was too long, resulting in no log entry for the system call.




#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/ioctl.h>
#include <dev/filemon/filemon.h>
#include <fcntl.h>
#include <err.h>
#include <unistd.h>

static void open_filemon(void) {         pid_t child;         int fm_fd, fm_log;

        if ((fm_fd = open("/dev/filemon", O_RDWR | O_CLOEXEC)) == -1)                 err(1, "open(\"/dev/filemon\", O_RDWR)");         if ((fm_log = open("filemon.out",          O_CREAT | O_WRONLY | O_TRUNC | O_CLOEXEC, DEFFILEMODE)) == -1)                 err(1, "open(filemon.out)");

        if (ioctl(fm_fd, FILEMON_SET_FD, &fm_log) == -1)                 err(1, "Cannot set filemon log file descriptor");

        if ((child = fork()) == 0) {                 child = getpid();                 if (ioctl(fm_fd, FILEMON_SET_PID, &child) == -1)                         err(1, "Cannot set filemon PID");                 /* Do something here. */         } else {                 wait(&child);                 close(fm_fd);         } }

Creates a file named filemon.out and configures the filemon device to write the filemon buffer contents to it.


dtrace(1), ktrace(1), script(1), truss(1), ioctl(2)


A filemon device appeared in FreeBSD 9.1 .


Unloading the module may panic the system, thus requires using kldunload -f.

FILEMON (4) March 22, 2016

tail head cat sleep
QR code linking to this page

Please direct any comments about this manual page service to Ben Bullock. Privacy policy.