proc(4) proc(4)
NAME
proc - /proc process file system
DESCRIPTION
/proc is a file system that provides access to the image of each
active process in the system. The name of each entry in the /proc
directory is a decimal number corresponding to the process ID. The
owner of each "file" is determined by the process's user-ID.
Standard system call interfaces are used to access /proc files: open,
close, read, write, and ioctl. An open for reading and writing enables
process control; a read-only open allows inspection but not control.
As with ordinary files, more than one process can open the same /proc
file at the same time. Exclusive open is provided to allow controlling
processes to avoid collisions: an open for writing that specifies
OEXCL fails if the file is already open for writing; if such an
exclusive open succeeds, subsequent attempts to open the file for
writing, with or without the OEXCL flag, fail until the exclusively-
opened file descriptor is closed. (Exception: a superuser open that
does not specify OEXCL succeeds even if the file is exclusively
opened.) There can be any number of read-only opens, even when an
exclusive write open is in effect on the file.
Data may be transferred from or to any locations in the traced
process's address space by applying lseek to position the file at the
virtual address of interest followed by read or write. The PIOCMAP
operation can be applied to determine the accessible areas (mappings)
of the address space. A contiguous area of the address space may
appear as multiple mappings due to varying read/write/execute permis-
sions. I/O transfers may span contiguous mappings. An I/O request
extending into an unmapped area is truncated at the boundary.
Information and control operations are provided through ioctl. These
have the form:
#include <sys/types.h>
#include <sys/signal.h>
#include <sys/fault.h>
#include <sys/syscall.h>
#include <sys/procfs.h>
void *p;
retval = ioctl(fildes, code, p);
The argument p is a generic pointer whose type depends on the specific
ioctl code. Where not specifically mentioned below, its value should
be zero. <sys/procfs.h> contains definitions of ioctl codes and data
structures used by the operations. Certain operations can be performed
only if the process file is open for writing; these include all opera-
tions that affect process control.
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Process information and control operations involve the use of sets of
flags. The set types sigsett, fltsett, and syssett correspond,
respectively, to signal, fault, and system call enumerations defined
in <sys/signal.h>, <sys/fault.h>, and <sys/syscall.h>.
Each set type is large enough to hold flags for its own enumeration.
Although they are of different sizes, they have a common structure and
can be manipulated by these macros:
prfillset(&set); /* turn on all flags in set */
premptyset(&set); /* turn off all flags in set */
praddset(&set, flag); /* turn on the specified flag */
prdelset(&set, flag); /* turn off the specified flag */
r = prismember(&set, flag); /* != 0 iff flag is turned on */
One of prfillset or premptyset must be used to initialize set before
it is used in any other operation. flag must be a member of the
enumeration corresponding to set.
The allowable ioctl codes follow. Those requiring write access are
marked with an asterisk (*). Except where noted, an ioctl to a process
that has terminated elicits the error ENOENT.
PIOCSTATUS
This returns status information for the process; p is a pointer to a
prstatus structure:
typedef struct prstatus
{
long prflags; /* Process flags */
short prwhy; /* Reason for process stop (if stopped) */
short prwhat; /* More detailed reason */
struct siginfo prinfo; /* Info associated with signal or fault */
short prcursig; /* Current signal */
sigsett prsigpend; /* Set of other pending signals */
sigsett prsighold; /* Set of held signals */
struct sigaltstack praltstack; /* Alternate signal stack info */
struct sigaction praction; /* Signal action for current signal */
pidt prpid; /* Process id */
pidt prppid; /* Parent process id */
pidt prpgrp; /* Process group id */
pidt prsid; /* Session id */
timestruct prutime; /* Process user cpu time */
timestruct prstime; /* Process system cpu time */
timestruct prcutime; /* Sum of children's user times */
timestruct prcstime; /* Sum of children's system times */
char prclname[8]; /* Scheduling class name */
long prfiller[20]; /* Filler area for future expansion */
long prinstr; /* Current instruction */
gregsett prreg; /* General registers */
} prstatust;
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prflags is a bit-mask holding these flags:
PRSTOPPED process is stopped
PRISTOP process is stopped on an event of interest (see
PIOCSTOP)
PRDSTOP process has a stop directive in effect (see PIOCSTOP)
PRASLEEP process is in an interruptible sleep within a system
call
PRFORK process has its inherit-on-fork flag set (see PIOCSFORK)
PRRLC process has its run-on-last-close flag set (see
PIOCSRLC)
PRPTRACE process is being traced via ptrace
PRPCINVAL process program counter refers to an invalid address
PRISSYS process is a system process (see PIOCSTOP)
prwhy and prwhat together describe, for a stopped process, the rea-
son that the process is stopped. Possible values of prwhy are:
- PRREQUESTED indicates that the process stopped because PIOCSTOP
was applied; prwhat is unused in this case.
- PRSIGNALLED indicates that the process stopped on receipt of a
signal (see PIOCSTRACE); prwhat holds the signal number that
caused the stop (for a newly-stopped process, the same value is in
prcursig).
- PRFAULTED indicates that the process stopped on incurring a
hardware fault (see PIOCSFAULT); prwhat holds the fault number
that caused the stop.
- PRSYSENTRY and PRSYSEXIT indicate a stop on entry to or exit from
a system call (see PIOCSENTRY and PIOCSEXIT); prwhat holds the
system call number.
- PRJOBCONTROL indicates that the process stopped due to the default
action of a job control stop signal [see sigaction(2)]; prwhat
holds the stopping signal number.
prinfo, when the process is in a PRSIGNALLED or PRFAULTED stop,
contains additional information pertinent to the particular signal or
fault (see <sys/siginfo.h>).
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prcursig names the current signal - that is, the next signal to be
delivered to the process. prsigpend identifies any other pending sig-
nals. prsighold identifies those signals whose delivery is being
delayed if sent to the process.
praltstack contains the alternate signal stack information for the
process [see sigaltstack(2)]. praction contains the signal action
information pertaining to the current signal [see sigaction(2)]; it is
undefined if prcursig is zero.
prpid, prppid, prpgrp, and prsid are, respectively, the process
id, the id of the process's parent, the process's process group id,
and the process's session id.
prutime, prstime, prcutime, and prcstime are, respectively, the
user CPU and system CPU time consumed by the process, and the cumula-
tive user CPU and system CPU time consumed by the process's children,
in seconds and nanoseconds.
prclname contains the name of the process's scheduling class.
The prfiller area is reserved for future use.
prinstr contains the machine instruction to which the program counter
refers. The amount of data retrieved from the process is machine-
dependent; on the 3B2, it is a single byte. In general, the size is
that of the machine's smallest instruction. If the program counter
refers to an invalid address, PRPCINVAL is set and prinstr is unde-
fined.
prreg is an array holding the contents of the general registers. On
the 3B2 the predefined constants RR0, RR1, ... RR8, RFP, RAP,
RPS, RSP, and RPC can be used as indices to refer to the corre-
sponding registers.
PIOCSTOP*, PIOCWSTOP
PIOCSTOP directs the process to stop and waits until it has stopped;
PIOCWSTOP simply waits for the process to stop. These operations com-
plete when the process stops on an event of interest, immediately if
already so stopped. If p is non-zero it points to an instance of
prstatust to be filled with status information for the stopped pro-
cess.
An "event of interest" is either a PRREQUESTED stop or a stop that
has been specified in the process's tracing flags (set by PIOCSTRACE,
PIOCSFAULT, PIOCSENTRY, and PIOCSEXIT). A PRJOBCONTROL stop is
specifically not an event of interest. (A process may stop twice due
to a stop signal, first showing PRSIGNALLED if the signal is traced
and again showing PRJOBCONTROL if the process is set running without
clearing the signal.) If the process is controlled by ptrace, it comes
to a PRSIGNALLED stop on receipt of any signal; this is an event of
interest only if the signal is in the traced signal set. If PIOCSTOP
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is applied to a process that is stopped, but not on an event of
interest, the stop directive takes effect when the process is restar-
ted by the competing mechanism; at that time the process enters a
PRREQUESTED stop before executing any user-level code.
ioctls are interruptible by signals so that, for example, an alarm(2)
can be set to avoid waiting forever for a process that may never stop
on an event of interest. If PIOCSTOP is interrupted, the stop direc-
tive remains in effect even though the ioctl returns an error.
A system process (indicated by the PRISSYS flag) never executes at
user level, has no user-level address space visible through /proc, and
cannot be stopped. Applying PIOCSTOP or PIOCWSTOP to a system process
elicits the error EBUSY.
PIOCRUN*
The traced process is made runnable again after a stop. If p is non-
zero it points to a prrun structure describing additional actions to
be performed:
typedef struct prrun
{
long prflags; /* Flags */
sigsett prtrace; /* Set of signals to be traced */
sigsett prsighold; /* Set of signals to be held */
fltsett prfault; /* Set of faults to be traced */
caddrt prvaddr; /* Virtual address at which to resume */
long prfiller[8]; /* Filler area for future expansion */
} prrunt;
prflags is a bit-mask describing optional actions; the remainder of
the entries are meaningful only if the appropriate bits are set in
prflags. prfiller is reserved for future use; this area must be
filled with zeros by the user's program. Flag definitions:
- PRCSIG clears the current signal, if any (see PIOCSSIG).
- PRCFAULT clears the current fault, if any (see PIOCCFAULT).
- PRSTRACE sets the traced signal set to prtrace (see PIOCSTRACE).
- PRSHOLD sets the held signal set to prsighold (see PIOCSHOLD).
- PRSFAULT sets the traced fault set to prfault (see PIOCSFAULT).
- PRSVADDR sets the address at which execution resumes to prvaddr.
- PRSTEP directs the process to single-step - i.e. to run and to exe-
cute a single machine instruction. On completion of the instruc-
tion, a hardware trace trap occurs. If FLTTRACE is being traced,
the process stops, otherwise it is sent SIGTRAP; if SIGTRAP is
being traced and not held, the process stops. This operation
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requires hardware support and may not be implemented on all proces-
sors.
- PRSABORT is meaningful only if the process is in a PRSYSENTRY stop
or is marked PRASLEEP; it instructs the process to abort execution
of the system call (see PIOCSENTRY, PIOCSEXIT).
- PRSTOP directs the process to stop again as soon as possible after
resuming execution (see PIOCSTOP). In particular if the process is
stopped on PRSIGNALLED or PRFAULTED, the next stop will show
PRREQUESTED, no other stop will have intervened, and the process
will not have executed any user-level code.
PIOCRUN fails (EBUSY) if applied to a process that is not stopped on
an event of interest. Once PIOCRUN has been applied, the process is no
longer stopped on an event of interest even if, due to a competing
mechanism, it remains stopped.
PIOCSTRACE*
This defines a set of signals to be traced: the receipt of one of
these signals causes the traced process to stop. The set of signals is
defined via an instance of sigsett addressed by p. Receipt of SIGKILL
cannot be traced.
If a signal that is included in the held signal set is sent to the
traced process, the signal is not received and does not cause a pro-
cess stop until it is removed from the held signal set, either by the
process itself or by setting the held signal set with PIOCSHOLD or the
PRSHOLD option of PIOCRUN.
PIOCGTRACE
The current traced signal set is returned in an instance of sigsett
addressed by p.
PIOCSSIG*
The current signal and its associated signal information are set
according to the contents of the siginfo structure addressed by p (see
<sys/siginfo.h>). If the specified signal number is zero or if p is
zero, the current signal is cleared. The semantics of this operation
are different from those of kill or PIOCKILL in that the signal is
delivered to the process immediately after execution is resumed (even
if it is being held) and an additional PRSIGNALLED stop does not
intervene even if the signal is traced. Setting the current signal to
SIGKILL terminates the process immediately, even if it is stopped.
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PIOCKILL*
A signal is sent to the process with semantics identical to those of
kill; p points to an int naming the signal. Sending SIGKILL terminates
the process immediately.
PIOCUNKILL*
A signal is deleted, i.e. it is removed from the set of pending sig-
nals; the current signal (if any) is unaffected. p points to an int
naming the signal. It is an error to attempt to delete SIGKILL.
PIOCGHOLD, PIOCSHOLD*
PIOCGHOLD returns the set of held signals (signals whose delivery will
be delayed if sent to the process) in an instance of sigsett
addressed by p. PIOCSHOLD correspondingly sets the held signal set but
does not allow SIGKILL or SIGSTOP to be held.
PIOCMAXSIG, PIOCACTION
These operations provide information about the signal actions associ-
ated with the traced process [see sigaction(2)]. PIOCMAXSIG returns,
in the int addressed by p, the maximum signal number understood by the
system. This can be used to allocate storage for use with the
PIOCACTION operation, which returns the traced process's signal
actions in an array of sigaction structures addressed by p. Signal
numbers are displaced by 1 from array indices, so that the action for
signal number n appears in position n-1 of the array.
PIOCSFAULT*
This defines a set of hardware faults to be traced: on incurring one
of these faults the traced process stops. The set is defined via an
instance of fltsett addressed by p. Fault names are defined in
<sys/fault.h> and include the following. Some of these may not occur
on all processors; there may be processor-specific faults in addition
to these.
FLTILL illegal instruction
FLTPRIV privileged instruction
FLTBPT breakpoint trap
FLTTRACE trace trap
FLTACCESS memory access fault
FLTBOUNDS memory bounds violation
FLTIOVF integer overflow
FLTIZDIV integer zero divide
FLTFPE floating-point exception
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FLTSTACK unrecoverable stack fault
FLTPAGE recoverable page fault
When not traced, a fault normally results in the posting of a signal
to the process that incurred the fault. If the process stops on a
fault, the signal is posted to the process when execution is resumed
unless the fault is cleared by PIOCCFAULT or by the PRCFAULT option of
PIOCRUN. FLTPAGE is an exception; no signal is posted. There may be
additional processor-specific faults like this. prinfo in the
prstatus structure identifies the signal to be sent and contains
machine-specific information about the fault.
PIOCGFAULT
The current traced fault set is returned in an instance of fltsett
addressed by p.
PIOCCFAULT*
The current fault (if any) is cleared; the associated signal is not
sent to the process.
PIOCSENTRY*, PIOCSEXIT*
These operations instruct the process to stop on entry to or exit from
specified system calls. The set of system calls to be traced is
defined via an instance of syssett addressed by p.
When entry to a system call is being traced, the traced process stops
after having begun the call to the system but before the system call
arguments have been fetched from the process. When exit from a system
call is being traced, the traced process stops on completion of the
system call just prior to checking for signals and returning to user
level. At this point all return values have been stored into the
traced process's saved registers.
If the traced process is stopped on entry to a system call
(PRSYSENTRY) or when sleeping in an interruptible system call
(PRASLEEP is set), it may be instructed to go directly to system call
exit by specifying the PRSABORT flag in a PIOCRUN request. Unless exit
from the system call is being traced the process returns to user level
showing error EINTR.
PIOCGENTRY, PIOCGEXIT
These return the current traced system call entry or exit set in an
instance of syssett addressed by p.
PIOCSFORK*, PIOCRFORK*
PIOCSFORK sets the inherit-on-fork flag in the traced process: the
process's tracing flags are inherited by the child of a fork.
PIOCRFORK turns this flag off: child processes start with all tracing
flags cleared.
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PIOCSRLC*, PIOCRRLC*
PIOCSRLC sets the run-on-last-close flag in the traced process: when
the last writable /proc file descriptor referring to the traced pro-
cess is closed, all of the process's tracing flags are cleared, any
outstanding stop directive is canceled, and if the process is stopped,
it is set running as though PIOCRUN had been applied to it. PIOCRRLC
turns this flag off: the process's tracing flags are retained and the
process is not set running when the process file is closed.
PIOCGREG, PIOCSREG*
These operations respectively get and set the saved process registers
into or out of an array addressed by p; the array has type gregsett.
Register contents are accessible using a set of predefined indices
(see PIOCSTATUS). Only certain bits of the processor-status word (PSW)
can be modified by PIOCSREG; on the 3B2 these include the condition-
code bits and the trace-enable bit. Other privileged registers cannot
be modified at all. PIOCSREG fails (EBUSY) if applied to a process
that is not stopped on an event of interest.
PIOCGFPREG, PIOCSFPREG*
These operations respectively get and set the saved process floating-
point registers into or out of a structure addressed by p; the struc-
ture has type fpregsett. An error (EINVAL) is returned if there is no
floating-point hardware on the machine. PIOCSFPREG fails (EBUSY) if
applied to a process that is not stopped on an event of interest.
PIOCNICE*
The traced process's nice(1) priority is incremented by the amount
contained in the int addressed by p. Only the superuser may better a
process's priority in this way, but any user may make the priority
worse.
PIOCPSINFO
This returns miscellaneous process information such as that reported
by ps(1). p is a pointer to a prpsinfo structure containing at least
the following fields:
typedef struct prpsinfo
{
char prstate; /* numeric process state (see prsname) */
char prsname; /* printable character representing prstate */
char przomb; /* !=0: process terminated but not waited for */
char prnice; /* nice for cpu usage */
ulong prflag; /* process flags */
uidt pruid; /* real user id */
gidt prgid; /* real group id */
pidt prpid; /* unique process id */
pidt prppid; /* process id of parent */
pidt prpgrp; /* pid of process group leader */
pidt prsid; /* session id */
caddrt praddr; /* physical address of process */
long prsize; /* size of process image in pages */
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long prrssize; /* resident set size in pages */
caddrt prwchan; /* wait addr for sleeping process */
timestruct prstart; /* process start time, sec+nsec since epoch */
timestruct prtime; /* usr+sys cpu time for this process */
long prpri; /* priority, high value is high priority */
char proldpri; /* pre-V5.41, low value is high priority */
char prcpu; /* pre-V5.41, cpu usage for scheduling */
devt prttydev; /* controlling tty device (PRNODEV if none) */
char prclname[8]; /* Scheduling class name */
char prfname[16]; /* last component of execed pathname */
char prpsargs[PRARGSZ]; /* initial characters of arg list */
long prfiller[20]; /* for future expansion */
} prpsinfot;
Some of the entries in prpsinfo, such as prstate and prflag, are
system-specific and should not be expected to retain their meanings
across different versions of the operating system. praddr is a ves-
tige of the past and has no real meaning in current systems.
PIOCPSINFO can be applied to a zombie process (one that has terminated
but whose parent has not yet performed a wait on it).
PIOCNMAP, PIOCMAP
These operations provide information about the memory mappings (vir-
tual address ranges) associated with the traced process. PIOCNMAP
returns, in the int addressed by p, the number of mappings that are
currently active. This can be used to allocate storage for use with
the PIOCMAP operation, which returns the list of currently active map-
pings. For PIOCMAP, p addresses an array of elements of type prmapt;
one array element (one structure) is returned for each mapping, plus
an additional element containing all zeros to mark the end of the
list.
typedef struct prmap
{
caddrt prvaddr; /* Virtual address base */
ulong prsize; /* Size of mapping in bytes */
offt proff; /* Offset into mapped object, if any */
long prmflags; /* Protection and attribute flags */
long prfiller[4]; /* Filler for future expansion */
} prmapt;
prvaddr is the virtual address base (the lower limit) of the mapping
within the traced process and prsize is its size in bytes. proff is
the offset within the mapped object (if any) to which the address base
is mapped.
prmflags is a bit-mask of protection and attribute flags:
MAREAD mapping is readable by the traced process
MAWRITE mapping is writable by the traced process
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MAEXEC mapping is executable by the traced process
MASHARED mapping changes are shared by the mapped object
MABREAK mapping is grown by the brk system call
MASTACK mapping is grown automatically on stack faults
PIOCOPENM
The return value retval provides a read-only file descriptor for a
mapped object associated with the traced process. If p is zero the
traced process's execed file (its a.out file) is found. This enables a
debugger to find the object file symbol table without having to know
the path name of the executable file. If p is non-zero it points to a
caddrt containing a virtual address within the traced process and the
mapped object, if any, associated with that address is found; this can
be used to get a file descriptor for a shared library that is attached
to the process. On error (invalid address or no mapped object for the
designated address), -1 is returned.
PIOCCRED
Fetch the set of credentials associated with the process. p points to
an instance of prcredt, which is filled by the operation:
typedef struct prcred
{
uidt preuid; /* Effective user id */
uidt prruid; /* Real user id */
uidt prsuid; /* Saved user id (from exec) */
uidt pregid; /* Effective group id */
uidt prrgid; /* Real group id */
uidt prsgid; /* Saved group id (from exec) */
uint prngroups; /* Number of supplementary groups */
} prcredt;
PIOCGROUPS
Fetch the set of supplementary group IDs associated with the process.
p points to an array of elements of type uidt, which will be filled
by the operation. PIOCCRED can be applied beforehand to determine the
number of groups (prngroups) that will be returned and the amount of
storage that should be allocated to hold them.
PIOCGETPR, PIOCGETU
These operations copy, respectively, the traced process's proc struc-
ture and user area into the buffer addressed by p. They are provided
for completeness but it should be unnecessary to access either of
these structures directly since relevant status information is avail-
able through other control operations. Their use is discouraged
because a program making use of them is tied to a particular version
of the operating system.
PIOCGETPR can be applied to a zombie process (see PIOCPSINFO).
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DIAGNOSTICS
Errors that can occur in addition to the errors normally associated
with file system access:
ENOENT The traced process has exited after being opened.
EIO I/O was attempted at an illegal address in the traced pro-
cess.
EBADF An I/O or ioctl operation requiring write access was
attempted on a file descriptor not open for writing.
EBUSY PIOCSTOP or PIOCWSTOP was applied to a system process; an
exclusive open was attempted on a process file already
already open for writing; an open for writing was
attempted and an exclusive open is in effect on the pro-
cess file; PIOCRUN, PIOCSREG or PIOCSFPREG was applied to
a process not stopped on an event of interest; an attempt
was made to mount /proc when it is already mounted.
EPERM Someone other than the superuser attempted to better a
process's priority by issuing PIOCNICE.
ENOSYS An attempt was made to perform an unsupported operation
(such as create, remove, link, or unlink) on an entry in
/proc.
EFAULT An I/O or ioctl request referred to an invalid address in
the controlling process.
EINVAL In general this means that some invalid argument was sup-
plied to a system call. The list of conditions eliciting
this error includes: the ioctl code is undefined; an ioctl
operation was issued on a file descriptor referring to the
/proc directory; an out-of-range signal number was speci-
fied with PIOCSSIG, PIOCKILL, or PIOCUNKILL; SIGKILL was
specified with PIOCUNKILL; an illegal virtual address was
specified in a PIOCOPENM request; PIOCGFPREG or PIOCSFPREG
was issued on a machine without floating-point hardware.
EINTR A signal was received by the controlling process while
waiting for the traced process to stop via PIOCSTOP or
PIOCWSTOP.
EAGAIN The traced process has performed an exec of a
setuid/setgid object file or of an object file that it
cannot read; all further operations on the process file
descriptor (except close) elicit this error.
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NOTES
Each operation (ioctl or I/O) is guaranteed to be atomic with respect
to the traced process, except when applied to a system process.
For security reasons, except for the superuser, an open of a /proc
file fails unless both the user-ID and group-ID of the caller match
those of the traced process and the process's object file is readable
by the caller. Files corresponding to setuid and setgid processes can
be opened only by the superuser. Even if held by the superuser, an
open process file descriptor becomes invalid if the traced process
performs an exec of a setuid/setgid object file or an object file that
it cannot read. Any operation performed on an invalid file descriptor,
except close, fails with EAGAIN. In this situation, if any tracing
flags are set and the process file is open for writing, the process
will have been directed to stop and its run-on-last-close flag will
have been set (see PIOCSRLC). This enables a controlling process (if
it has permission) to reopen the process file to get a new valid file
descriptor, close the invalid file descriptor, and proceed. Just clos-
ing the invalid file descriptor causes the traced process to resume
execution with no tracing flags set. Any process not currently open
for writing via /proc but that has left-over tracing flags from a pre-
vious open and that execs a setuid/setgid or unreadable object file
will not be stopped but will have all its tracing flags cleared.
For reasons of symmetry and efficiency there are more control opera-
tions than strictly necessary.
To wait for one or more of a set of processes to stop, /proc file
descriptors can be used in a poll(2) system call. When requested and
returned, the polling event POLLPRI indicates that the process stopped
on an event of interest. Although they cannot be requested, the pol-
ling events POLLHUP, POLLERR and POLLNVAL may be returned. POLLHUP
indicates that the process has terminated. POLLERR indicates that the
file descriptor has become invalid. POLLNVAL is returned immediately
if POLLPRI is requested on a file descriptor referring to a system
process (see PIOCSTOP).
FILES
/proc
directory (list of active processes)
/proc/nnnnn
process image
SEE ALSO
open(2), poll(2), ptrace(2), sigaction(2), sigaltstack(2), signal(2),
sigset(2), siginfo(5).
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