PTRACE(2)

NAME
ptrace - process tracing and debugging

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/ptrace.h>
int
ptrace(int request, pid_t pid, caddr_t addr, int data);

DESCRIPTION
The ptrace() system call provides tracing and debugging facilities. It allows one process (the tracing process) to control another (the traced process). The tracing process must first attach to the traced process, and then issue a series of ptrace() system calls to control the execution of the process, as well as access process memory and register state. For the duration of the tracing session, the traced process will be “re-parented”, with its parent process ID (and resulting behavior) changed to the tracing process. It is permissible for a tracing process to attach to more than one other process at a time. When the tracing process has completed its work, it must detach the traced process; if a tracing process exits without first detaching all processes it has attached, those processes will be killed.

Most of the time, the traced process runs normally, but when it receives a signal (see sigaction(2)), it stops. The tracing process is expected to notice this via wait(2) or the delivery of a SIGCHLD signal, examine the state of the stopped process, and cause it to terminate or continue as appropriate. The signal may be a normal process signal, generated as a result of traced process behavior, or use of the kill(2) system call; alternatively, it may be generated by the tracing facility as a result of attaching, system calls, or stepping by the tracing process. The tracing process may choose to intercept the signal, using it to observe process behavior (such as SIGTRAP), or forward the signal to the process if appropriate. The ptrace() system call is the mechanism by which all this happens.

The request argument specifies what operation is being performed; the meaning of the rest of the arguments depends on the operation, but except for one special case noted below, all ptrace() calls are made by the tracing process, and the pid argument specifies the process ID of the traced process or a corresponding thread ID. The request argument can be:

PT_TRACE_ME
    This request is the only one used by the traced process; it declares that the process expects to be traced by its parent. All the other arguments are ignored. (If the parent process does not expect to trace the child, it will probably be rather confused by the results; once the traced process stops, it cannot be made to continue except via ptrace().) When a process has used this request and calls execve(2) or any of the routines built on it (such as execv(3)), it will stop before executing the first instruction of the new image. Also, any setuid or setgid bits on the executable being executed will be ignored. If the child was created by vfork(2) system call or rfork(2) call with the RFMEM flag specified, the debugging events are reported to the parent only after the execve(2) is executed.

PT_READ_I, PT_READ_D
    These requests read a single int of data from the traced process's address space. Traditionally, ptrace() has allowed for machines with distinct address spaces for instruction and data, which is why there are two requests: conceptually, PT_READ_I reads from the instruction space and PT_READ_D reads from the data space. In the current FreeBSD implementation, these two requests are completely identical. The addr argument specifies the address (in the traced process's virtual address space) at which the read is to be done. This address does not have to meet any alignment constraints. The value read is returned as the return value from ptrace().

PT_WRITE_I, PT_WRITE_D
    These requests parallel PT_READ_I and PT_READ_D, except that they write rather than read. The data argument supplies the value to be written.

PT_IO
    This request allows reading and writing arbitrary amounts of data in the traced process's address space. The addr argument specifies a pointer to a struct ptrace_io_desc, which is defined as follows:

        struct ptrace_io_desc {
        	int	piod_op;	/* I/O operation */
        	void	*piod_offs;	/* child offset */
        	void	*piod_addr;	/* parent offset */
        	size_t	piod_len;	/* request length */
        };
        /*
         * Operations in piod_op.
         */
        #define PIOD_READ_D	1	/* Read from D space */
        #define PIOD_WRITE_D	2	/* Write to D space */
        #define PIOD_READ_I	3	/* Read from I space */
        #define PIOD_WRITE_I	4	/* Write to I space */

    The data argument is ignored. The actual number of bytes read or written is stored in piod_len upon return.

PT_CONTINUE
    The traced process continues execution. The addr argument is an address specifying the place where execution is to be resumed (a new value for the program counter), or (caddr_t)1 to indicate that execution is to pick up where it left off. The data argument provides a signal number to be delivered to the traced process as it resumes execution, or 0 if no signal is to be sent.

PT_STEP
    The traced process is single stepped one instruction. The addr argument should be passed (caddr_t)1. The data argument provides a signal number to be delivered to the traced process as it resumes execution, or 0 if no signal is to be sent.

PT_KILL
    The traced process terminates, as if PT_CONTINUE had been used with SIGKILL given as the signal to be delivered.

PT_ATTACH
    This request allows a process to gain control of an otherwise unrelated process and begin tracing it. It does not need any cooperation from the to-be-traced process. In this case, pid specifies the process ID of the to-be-traced process, and the other two arguments are ignored. This request requires that the target process must have the same real UID as the tracing process, and that it must not be executing a setuid or setgid executable. (If the tracing process is running as root, these restrictions do not apply.) The tracing process will see the newly-traced process stop and may then control it as if it had been traced all along.

PT_DETACH
    This request is like PT_CONTINUE, except that it does not allow specifying an alternate place to continue execution, and after it succeeds, the traced process is no longer traced and continues execution normally.

PT_GETREGS
    This request reads the traced process's machine registers into the “ struct reg ” (defined in <machine/reg.h>) pointed to by addr.

PT_SETREGS
    This request is the converse of PT_GETREGS; it loads the traced process's machine registers from the “ struct reg ” (defined in <machine/reg.h>) pointed to by addr.

PT_GETFPREGS
    This request reads the traced process's floating-point registers into the “ struct fpreg ” (defined in <machine/reg.h>) pointed to by addr.

PT_SETFPREGS
    This request is the converse of PT_GETFPREGS; it loads the traced process's floating-point registers from the “ struct fpreg ” (defined in <machine/reg.h>) pointed to by addr.

PT_GETDBREGS
    This request reads the traced process's debug registers into the “ struct dbreg ” (defined in <machine/reg.h>) pointed to by addr.

PT_SETDBREGS
    This request is the converse of PT_GETDBREGS; it loads the traced process's debug registers from the “ struct dbreg ” (defined in <machine/reg.h>) pointed to by addr.

PT_LWPINFO
    This request can be used to obtain information about the kernel thread, also known as light-weight process, that caused the traced process to stop. The addr argument specifies a pointer to a struct ptrace_lwpinfo, which is defined as follows:

        struct ptrace_lwpinfo {
        	lwpid_t pl_lwpid;
        	int	pl_event;
        	int	pl_flags;
        	sigset_t pl_sigmask;
        	sigset_t pl_siglist;
        	siginfo_t pl_siginfo;
        	char	pl_tdname[MAXCOMLEN + 1];
        	int	pl_child_pid;
        };

    The data argument is to be set to the size of the structure known to the caller. This allows the structure to grow without affecting older programs.

    The fields in the struct ptrace_lwpinfo have the following meaning:
    pl_lwpid
        LWP id of the thread
    pl_event
        Event that caused the stop. Currently defined events are
        PL_EVENT_NONE
            No reason given
        PL_EVENT_SIGNAL
            Thread stopped due to the pending signal
    pl_flags
        Flags that specify additional details about observed stop. Currently defined flags are:
        PL_FLAG_SCE
            The thread stopped due to system call entry, right after the kernel is entered. The debugger may examine syscall arguments that are stored in memory and registers according to the ABI of the current process, and modify them, if needed.
        PL_FLAG_SCX
            The thread is stopped immediately before syscall is returning to the usermode. The debugger may examine system call return values in the ABI-defined registers and/or memory.
        PL_FLAG_EXEC
            When PL_FLAG_SCX is set, this flag may be additionally specified to inform that the program being executed by debuggee process has been changed by successful execution of a system call from the execve(2) family.
        PL_FLAG_SI
            Indicates that pl_siginfo member of struct ptrace_lwpinfo contains valid information.
        PL_FLAG_FORKED
            Indicates that the process is returning from a call to fork(2) that created a new child process. The process identifier of the new process is available in the pl_child_pid member of struct ptrace_lwpinfo.
        PL_FLAG_CHILD
            The flag is set for first event reported from a new child, which is automatically attached due to PT_FOLLOW_FORK enabled.
    pl_sigmask
        The current signal mask of the LWP
    pl_siglist
        The current pending set of signals for the LWP. Note that signals that are delivered to the process would not appear on an LWP siglist until the thread is selected for delivery.
    pl_siginfo
        The siginfo that accompanies the signal pending. Only valid for PL_EVENT_SIGNAL stop when PL_FLAG_SI is set in pl_flags.
    pl_tdname
        The name of the thread.
    pl_child_pid
        The process identifier of the new child process. Only valid for a PL_EVENT_SIGNAL stop when PL_FLAG_FORKED is set in pl_flags.

PT_GETNUMLWPS
    This request returns the number of kernel threads associated with the traced process.

PT_GETLWPLIST
    This request can be used to get the current thread list. A pointer to an array of type lwpid_t should be passed in addr, with the array size specified by data. The return value from ptrace() is the count of array entries filled in.

PT_SETSTEP
    This request will turn on single stepping of the specified process.

PT_CLEARSTEP
    This request will turn off single stepping of the specified process.

PT_SUSPEND
    This request will suspend the specified thread.

PT_RESUME
    This request will resume the specified thread.

PT_TO_SCE
    This request will trace the specified process on each system call entry.

PT_TO_SCX
    This request will trace the specified process on each system call exit.

PT_SYSCALL
    This request will trace the specified process on each system call entry and exit.

PT_FOLLOW_FORK
    This request controls tracing for new child processes of a traced process. If data is non-zero, then new child processes will enable tracing and stop before executing their first instruction. If data is zero, then new child processes will execute without tracing enabled. By default, tracing is not enabled for new child processes. Child processes do not inherit this property. The traced process will set the PL_FLAG_FORKED flag upon exit from a system call that creates a new process.

PT_VM_TIMESTAMP
    This request returns the generation number or timestamp of the memory map of the traced process as the return value from ptrace(). This provides a low-cost way for the tracing process to determine if the VM map changed since the last time this request was made.

PT_VM_ENTRY
    This request is used to iterate over the entries of the VM map of the traced process. The addr argument specifies a pointer to a struct ptrace_vm_entry, which is defined as follows:

        struct ptrace_vm_entry {
        	int		pve_entry;
        	int		pve_timestamp;
        	u_long		pve_start;
        	u_long		pve_end;
        	u_long		pve_offset;
        	u_int		pve_prot;
        	u_int		pve_pathlen;
        	long		pve_fileid;
        	uint32_t	pve_fsid;
        	char		*pve_path;
        };

    The first entry is returned by setting pve_entry to zero. Subsequent entries are returned by leaving pve_entry unmodified from the value returned by previous requests. The pve_timestamp field can be used to detect changes to the VM map while iterating over the entries. The tracing process can then take appropriate action, such as restarting. By setting pve_pathlen to a non-zero value on entry, the pathname of the backing object is returned in the buffer pointed to by pve_path, provided the entry is backed by a vnode. The pve_pathlen field is updated with the actual length of the pathname (including the terminating null character). The pve_offset field is the offset within the backing object at which the range starts. The range is located in the VM space at pve_start and extends up to pve_end (inclusive).

    The data argument is ignored.

Additionally, machine-specific requests can exist.

RETURN VALUES
Some requests can cause ptrace() to return -1 as a non-error value; to disambiguate, errno can be set to 0 before the call and checked afterwards.

ERRORS
The ptrace() system call may fail if:

[ESRCH]
    •   No process having the specified process ID exists.

[EINVAL]
    •   A process attempted to use PT_ATTACH on itself.
    •   The request argument was not one of the legal requests.
    •   The signal number (in data) to PT_CONTINUE was neither 0 nor a legal signal number.
    •   PT_GETREGS, PT_SETREGS, PT_GETFPREGS, PT_SETFPREGS, PT_GETDBREGS, or PT_SETDBREGS was attempted on a process with no valid register set. (This is normally true only of system processes.)
    •   PT_VM_ENTRY was given an invalid value for pve_entry. This can also be caused by changes to the VM map of the process.

[EBUSY]
    •   PT_ATTACH was attempted on a process that was already being traced.
    •   A request attempted to manipulate a process that was being traced by some process other than the one making the request.
    •   A request (other than PT_ATTACH) specified a process that was not stopped.

[EPERM]
    •   A request (other than PT_ATTACH) attempted to manipulate a process that was not being traced at all.
    •   An attempt was made to use PT_ATTACH on a process in violation of the requirements listed under PT_ATTACH above.

[ENOENT]
    •   PT_VM_ENTRY previously returned the last entry of the memory map. No more entries exist.

[ENAMETOOLONG]
    •   PT_VM_ENTRY cannot return the pathname of the backing object because the buffer is not big enough. pve_pathlen holds the minimum buffer size required on return.

SEE ALSO
execve(2), sigaction(2), wait(2), execv(3), i386_clr_watch(3), i386_set_watch(3)

HISTORY
The ptrace() function appeared in AT&T UNIX.