proc(4) UNIX System V proc(4)
NAME
/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 O_EXCL
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 O_EXCL flag, fail until the exclusively-opened file
descriptor is closed. (Exception: a super-user open that does not
specify O_EXCL 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 permissions. 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
operations that affect process control.
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Process information and control operations involve the use of sets of
flags. The set types sigset_t, fltset_t, and sysset_t 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 pr_flags; /* Process flags */
short pr_why; /* Reason for process stop (if stopped) */
short pr_what; /* More detailed reason */
struct siginfo pr_info; /* Info associated with signal or fault */
short pr_cursig; /* Current signal */
sigset_t pr_sigpend; /* Set of other pending signals */
sigset_t pr_sighold; /* Set of held signals */
struct sigaltstack pr_altstack; /* Alternate signal stack info */
struct sigaction pr_action; /* Signal action for current signal */
pid_t pr_pid; /* Process id */
pid_t pr_ppid; /* Parent process id */
pid_t pr_pgrp; /* Process group id */
pid_t pr_sid; /* Session id */
timestruc_t pr_utime; /* Process user cpu time */
timestruc_t pr_stime; /* Process system cpu time */
timestruc_t pr_cutime; /* Sum of children's user times */
timestruc_t pr_cstime; /* Sum of children's system times */
char pr_clname[8]; /* Scheduling class name */
long pr_filler[20];/* Filler area for future expansion */
long pr_instr; /* Current instruction */
gregset_t pr_reg; /* General registers */
} prstatus_t;
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pr_flags is a bit-mask holding these flags:
PR_STOPPED process is stopped
PR_ISTOP process is stopped on an event of interest (see
PIOCSTOP)
PR_DSTOP process has a stop directive in effect (see PIOCSTOP)
PR_ASLEEP process is in an interruptible sleep within a system
call
PR_FORK process has its inherit-on-fork flag set (see
PIOCSFORK)
PR_RLC process has its run-on-last-close flag set (see
PIOCSRLC)
PR_PTRACE process is being traced via ptrace
PR_PCINVAL process program counter refers to an invalid address
PR_ISSYS process is a system process (see PIOCSTOP)
pr_why and pr_what together describe, for a stopped process, the reason
that the process is stopped. Possible values of pr_why are:
PR_REQUESTED indicates that the process stopped because PIOCSTOP
was applied; pr_what is unused in this case.
PR_SIGNALLED indicates that the process stopped on receipt of a
signal (see PIOCSTRACE); pr_what holds the signal number that
caused the stop (for a newly-stopped process, the same value is in
pr_cursig).
PR_FAULTED indicates that the process stopped on incurring a
hardware fault (see PIOCSFAULT); pr_what holds the fault number
that caused the stop.
PR_SYSENTRY and PR_SYSEXIT indicate a stop on entry to or exit from
a system call (see PIOCSENTRY and PIOCSEXIT); pr_what holds the
system call number.
PR_JOBCONTROL indicates that the process stopped due to the default
action of a job control stop signal (see sigaction); pr_what holds
the stopping signal number.
pr_info, when the process is in a PR_SIGNALLED or PR_FAULTED stop,
contains additional information pertinent to the particular signal or
fault (see sys/siginfo.h).
pr_cursig names the current signal-that is, the next signal to be
delivered to the process. pr_sigpend identifies any other pending
signals. pr_sighold identifies those signals whose delivery is being
delayed if sent to the process.
pr_altstack contains the alternate signal stack information for the
process (see sigaltstack). pr_action contains the signal action
information pertaining to the current signal (see sigaction); it is
undefined if pr_cursig is zero.
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pr_pid, pr_ppid, pr_pgrp, and pr_sid are, respectively, the process ID,
the ID of the process's parent, the process's process group ID, and the
process's session ID.
pr_utime, pr_stime, pr_cutime, and pr_cstime are, respectively, the user
CPU and system CPU time consumed by the process, and the cumulative user
CPU and system CPU time consumed by the process's children, in seconds
and nanoseconds.
pr_clname contains the name of the process's scheduling class.
The pr_filler area is reserved for future use.
pr_instr 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, PR_PCINVAL is set and pr_instr is undefined.
pr_reg is an array holding the contents of the general registers. On the
3B2 the predefined constants R_R0, R_R1, ... R_R8, R_FP, R_AP, R_PS,
R_SP, and R_PC can be used as indices to refer to the corresponding
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
complete 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
prstatus_t to be filled with status information for the stopped process.
An ``event of interest'' is either a PR_REQUESTED stop or a stop that has
been specified in the process's tracing flags (set by PIOCSTRACE,
PIOCSFAULT, PIOCSENTRY, and PIOCSEXIT). A PR_JOBCONTROL stop is
specifically not an event of interest. (A process may stop twice due to
a stop signal, first showing PR_SIGNALLED if the signal is traced and
again showing PR_JOBCONTROL if the process is set running without
clearing the signal.) If the process is controlled by ptrace, it comes
to a PR_SIGNALLED stop on receipt of any signal; this is an event of
interest only if the signal is in the traced signal set. If PIOCSTOP is
applied to a process that is stopped, but not on an event of interest,
the stop directive takes effect when the process is restarted by the
competing mechanism; at that time the process enters a PR_REQUESTED stop
before executing any user-level code.
ioctls are interruptible by signals so that, for example, an alarm 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 directive
remains in effect even though the ioctl returns an error.
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A system process (indicated by the PR_ISSYS 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 pr_flags; /* Flags */
sigset_t pr_trace; /* Set of signals to be traced */
sigset_t pr_sighold; /* Set of signals to be held */
fltset_t pr_fault; /* Set of faults to be traced */
caddr_t pr_vaddr; /* Virtual address at which to resume */
long pr_filler[8]; /* Filler area for future expansion */
} prrun_t;
pr_flags is a bit-mask describing optional actions; the remainder of the
entries are meaningful only if the appropriate bits are set in pr_flags.
pr_filler 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 pr_trace (see PIOCSTRACE).
PRSHOLD sets the held signal set to pr_sighold (see PIOCSHOLD).
PRSFAULT sets the traced fault set to pr_fault (see PIOCSFAULT).
PRSVADDR sets the address at which execution resumes to pr_vaddr.
PRSTEP directs the process to single-step-i.e., to run and to
execute a single machine instruction. On completion of the
instruction, 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
requires hardware support and may not be implemented on all
processors.
PRSABORT is meaningful only if the process is in a PR_SYSENTRY stop
or is marked PR_ASLEEP; 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 PR_SIGNALLED or PR_FAULTED, the next stop will show
PR_REQUESTED, no other stop will have intervened, and the process
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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 sigset_t 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 process 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 sigset_t
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 PR_SIGNALLED 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.
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 signals;
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 sigset_t addressed by
p. PIOCSHOLD correspondingly sets the held signal set but does not allow
SIGKILL or SIGSTOP to be held.
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PIOCMAXSIG, PIOCACTION
These operations provide information about the signal actions associated
with the traced process (see sigaction). 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 fltset_t 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
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. pr_info 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 fltset_t
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 syscalls to be traced is defined via
an instance of sysset_t addressed by p.
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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 (PR_SYSENTRY)
or when sleeping in an interruptible system call (PR_ASLEEP 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 sysset_t 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.
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 process 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 gregset_t.
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 structure
has type fpregset_t. 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.
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PIOCNICE*
The traced process's nice priority is incremented by the amount contained
in the int addressed by p. Only the super-user 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 pr_state; /* numeric process state (see pr_sname) */
char pr_sname; /* printable character representing pr_state */
char pr_zomb; /* !=0: process terminated but not waited for */
char pr_nice; /* nice for cpu usage */
u_long pr_flag; /* process flags */
uid_t pr_uid; /* real user id */
gid_t pr_gid; /* real group id */
pid_t pr_pid; /* unique process id */
pid_t pr_ppid; /* process id of parent */
pid_t pr_pgrp; /* pid of process group leader */
pid_t pr_sid; /* session id */
caddr_t pr_addr; /* physical address of process */
long pr_size; /* size of process image in pages */
long pr_rssize; /* resident set size in pages */
caddr_t pr_wchan; /* wait addr for sleeping process */
timestruc_t pr_start; /* process start time, sec+nsec since epoch */
timestruc_t pr_time; /* usr+sys cpu time for this process */
long pr_pri; /* priority, high value is high priority */
char pr_oldpri; /* pre-SVR4, low value is high priority */
char pr_cpu; /* pre-SVR4, cpu usage for scheduling */
dev_t pr_ttydev; /* controlling tty device (PRNODEV if none) */
char pr_clname[8]; /* Scheduling class name */
char pr_fname[16]; /* last component of execed pathname */
char pr_psargs[PRARGSZ]; /* initial characters of arg list */
long pr_filler[20]; /* for future expansion */
} prpsinfo_t;
Some of the entries in prpsinfo, such as pr_state and pr_flag, are
system-specific and should not be expected to retain their meanings
across different versions of the operating system. pr_addr is a vestige
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 (virtual
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 mappings. For PIOCMAP, p
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addresses an array of elements of type prmap_t; 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 {
caddr_t pr_vaddr; /* Virtual address base */
u_long pr_size; /* Size of mapping in bytes */
off_t pr_off; /* Offset into mapped object, if any */
long pr_mflags; /* Protection and attribute flags */
long pr_filler[4]; /* Filler for future expansion */
} prmap_t;
pr_vaddr is the virtual address base (the lower limit) of the mapping
within the traced process and pr_size is its size in bytes. pr_off is
the offset within the mapped object (if any) to which the address base is
mapped.
pr_mflags is a bit-mask of protection and attribute flags:
MA_READ mapping is readable by the traced process
MA_WRITE mapping is writable by the traced process
MA_EXEC mapping is executable by the traced process
MA_SHARED mapping changes are shared by the mapped object
MA_BREAK mapping is grown by the brk system call
MA_STACK 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 caddr_t
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 prcred_t, which is filled by the operation:
typedef struct prcred {
uid_t pr_euid; /* Effective user id */
uid_t pr_ruid; /* Real user id */
uid_t pr_suid; /* Saved user id (from exec) */
uid_t pr_egid; /* Effective group id */
uid_t pr_rgid; /* Real group id */
uid_t pr_sgid; /* Saved group id (from exec) */
u_int pr_ngroups; /* Number of supplementary groups */
} prcred_t;
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PIOCGROUPS
Fetch the set of supplementary group IDs associated with the process. p
points to an array of elements of type uid_t, which will be filled by the
operation. PIOCCRED can be applied beforehand to determine the number of
groups (pr_ngroups) 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 structure
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 available
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).
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 super-user, 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 super-user. Even if held by the super-user, 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 closing 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 previous 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 operations
than strictly necessary.
FILES
/proc directory (list of active processes)
/proc/nnnnnprocess image
SEE ALSO
open(2), ptrace(2), sigaction(2), signal(2)
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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
process.
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 process 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 super-user 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 supplied
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 specified
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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