NAME

Pgrab —

grab and control a process

LIBRARY

Process Control Library (libproc, -lproc)

SYNOPSIS

#include <libproc.h>

struct ps_prochandle *
Pgrab(pid_t pid, int flags, int *perr);

DESCRIPTION

The Pgrab() function attempts to grab the process identified by pid and returns a handle to it that allows the process to be controlled, interrogated, and manipulated. This interface only works with processes that already exist. Use Pgrab_core(3PROC) for core files and Pcreate(3PROC) to create processes.

A grabbed process undergoes the following changes unless flags is set to the contrary:

The process is stopped
All other tracing flags are cleared
The grab is exclusive. If any existing handles to this process exist or anyone else is using the underlying facilities of the /proc file system to control this process, it will fail.
Unless the process is already stopped, the PR_RLC flag is set indicating the process should run-on-last-close. Allowing the process to resume running if its controlling process dies.

Grabbing a process is a destructive action. Stopping a process stops execution of all its threads. The impact of stopping a process depends on the purpose of that process. For example, if one stops a process that's primarily doing computation, then its computation is delayed the entire time that it is stopped. However, if instead this is an active TCP server, then the accept backlog may fill causing connection errors and potentially connection time out errors.

Special care must be taken to ensure that a stopped process continues, even if the controlling process terminates. If the controlling process disables the PR_RLC flag or the process was already stopped, then the process remains stopped after the controlling process terminates. Exercise caution when changing this behavior.

Many of these default behaviors can be controlled by passing values to the flags argument. Values for flags are constructed by a bitwise-inclusive-OR of flags from the following list:

PGRAB_RETAIN: Indicates that any existing tracing flags on pid should be retained. If this flag is not specified, they will be cleared as part of creating the libproc handle for this process.
Normally extant tracing flags are cleared when a process is grabbed.
PGRAB_FORCE: Indicates that the process should not be grabbed exclusively. Care should be taken with this option. If other consumers are manipulating the process, then this may result in surprising behavior as the process is being manipulated from multiple points of control at the same time.
Normally an attempt will be made to grab the process exclusively and fail if it is already in use.
PGRAB_RDONLY: Indicates that the process should be grabbed in a read-only fashion. This implies that both the PGRAB_RETAIN and PGRAB_NOSTOP flags should be set. If a process is opened read-only, then a caller can only read information about a process and cannot manipulate it, change its current state, or inject systems calls into it.
Normally when a process is grabbed, it does so for both reading and writing.
PGRAB_NOSTOP: Do not stop a process as it is grabbed. Note, any extant tracing flags on the process will still be cleared unless the PGRAB_RETAIN flag has been set.
Normally a process is stopped as a result of grabbing the process.

The perr argument must be a non-NULL pointer which will store a more detailed error in the event that the Pgrab() function fails. A human-readable form of the error can be obtained with Pgrab_error(3PROC).

Once a caller is done with the library handle it should call Prelease(3PROC) to release the grabbed process. Failure to properly release the handle may leave a process stopped and interfere with the ability of other software to obtain a handle.

Permissions

Unprivileged users may grab and control their own processes only if both the user and group IDs of the target process match those of the calling process. In addition, the caller must have a super set of the target's privileges. Processes with the PRIV_PROC_OWNER privilege may manipulate any process on the system, as long as it has an equal privilege set. For more details on the security and programming considerations, please see the section PROGRAMMING NOTES in proc(5).

RETURN VALUES

Upon successful completion, the Pgrab() function returns a control handle to the process. Otherwise, NULL is returned with perr containing the error code.

ERRORS

The Pgrab() function will fail if:

G_BUSY: The process pid is already being traced and the PGRAB_FORCE flag was not passed in flags.
G_LP64: The calling process is a 32-bit process and process pid is 64-bit.
G_NOFD: Too many files are open. This is logically equivalent to receiving EMFILE.
G_NOPROC: The process referred to by pid does not exist.
G_PERM: The calling process has insufficient permissions or privileges to open the specified process. See Permissions for more information.
G_SYS: The process referred to by pid is a system process and cannot be grabbed.
G_SELF: The process referred to by pid is the process ID of the caller and the PGRAB_RDONLY was not passed. A process may only grab itself if it's read-only.
G_STRANGE: An unanticipated system error occurred while trying to grab the process file and create the handle. The value of errno indicates the system failure.
G_ZOMB: The process referred to by pid is a zombie and cannot be grabbed.

INTERFACE STABILITY

Uncommitted

MT-LEVEL

MT-Safe