1.\" $FreeBSD: src/lib/libc/sys/ptrace.2,v 1.12.2.12 2001/12/14 18:34:01 ru Exp $ 2.\" $DragonFly: src/lib/libc/sys/ptrace.2,v 1.4 2006/05/26 19:39:37 swildner Exp $ 3.\" $NetBSD: ptrace.2,v 1.2 1995/02/27 12:35:37 cgd Exp $ 4.\" 5.\" This file is in the public domain. 6.Dd January 20, 1996 7.Dt PTRACE 2 8.Os 9.Sh NAME 10.Nm ptrace 11.Nd process tracing and debugging 12.Sh LIBRARY 13.Lb libc 14.Sh SYNOPSIS 15.In sys/types.h 16.In sys/ptrace.h 17.Ft int 18.Fn ptrace "int request" "pid_t pid" "caddr_t addr" "int data" 19.Sh DESCRIPTION 20.Fn ptrace 21provides tracing and debugging facilities. It allows one process (the 22.Em tracing 23process) to control another (the 24.Em traced 25process). Most of the time, the traced process runs normally, but when 26it receives a signal 27(see 28.Xr sigaction 2 ) , 29it stops. The tracing process is expected to notice this via 30.Xr wait 2 31or the delivery of a 32.Dv SIGCHLD 33signal, examine the state of the stopped process, and cause it to 34terminate or continue as appropriate. 35.Fn ptrace 36is the mechanism by which all this happens. 37.Pp 38The 39.Fa request 40argument specifies what operation is being performed; the meaning of 41the rest of the arguments depends on the operation, but except for one 42special case noted below, all 43.Fn ptrace 44calls are made by the tracing process, and the 45.Fa pid 46argument specifies the process ID of the traced process. 47.Fa request 48can be: 49.Bl -tag -width 12n 50.It Dv PT_TRACE_ME 51This request is the only one used by the traced process; it declares 52that the process expects to be traced by its parent. All the other 53arguments are ignored. (If the parent process does not expect to trace 54the child, it will probably be rather confused by the results; once the 55traced process stops, it cannot be made to continue except via 56.Fn ptrace . ) 57When a process has used this request and calls 58.Xr execve 2 59or any of the routines built on it 60(such as 61.Xr execv 3 ) , 62it will stop before executing the first instruction of the new image. 63Also, any setuid or setgid bits on the executable being executed will 64be ignored. 65.It Dv PT_READ_I , Dv PT_READ_D 66These requests read a single 67.Vt int 68of data from the traced process' address space. Traditionally, 69.Fn ptrace 70has allowed for machines with distinct address spaces for instruction 71and data, which is why there are two requests: conceptually, 72.Dv PT_READ_I 73reads from the instruction space and 74.Dv PT_READ_D 75reads from the data space. In the current 76.Dx 77implementation, these 78two requests are completely identical. The 79.Fa addr 80argument specifies the address (in the traced process' virtual address 81space) at which the read is to be done. This address does not have to 82meet any alignment constraints. The value read is returned as the 83return value from 84.Eo \& 85.Fn ptrace 86.Ec . 87.It Dv PT_WRITE_I , Dv PT_WRITE_D 88These requests parallel 89.Dv PT_READ_I 90and 91.Dv PT_READ_D , 92except that they write rather than read. The 93.Fa data 94argument supplies the value to be written. 95.It Dv PT_CONTINUE 96The traced process continues execution. 97.Fa addr 98is an address specifying the place where execution is to be resumed (a 99new value for the program counter), or 100.Po Vt caddr_t Pc Ns 1 101to indicate that execution is to pick up where it left off. 102.Fa data 103provides a signal number to be delivered to the traced process as it 104resumes execution, or 0 if no signal is to be sent. 105.It Dv PT_STEP 106The traced process is single stepped one instruction. 107.Fa addr 108should be passed 109.Po Vt caddr_t Pc Ns 1 . 110.Fa data 111is not used. 112.It Dv PT_KILL 113The traced process terminates, as if 114.Dv PT_CONTINUE 115had been used with 116.Dv SIGKILL 117given as the signal to be delivered. 118.It Dv PT_ATTACH 119This request allows a process to gain control of an otherwise unrelated 120process and begin tracing it. It does not need any cooperation from 121the to-be-traced process. In this case, 122.Fa pid 123specifies the process ID of the to-be-traced process, and the other two 124arguments are ignored. This request requires that the target process 125must have the same real UID as the tracing process, and that it must 126not be executing a setuid or setgid executable. (If the tracing 127process is running as root, these restrictions do not apply.) The 128tracing process will see the newly-traced process stop and may then 129control it as if it had been traced all along. 130.It Dv PT_DETACH 131This request is like PT_CONTINUE, except that it does not allow 132specifying an alternate place to continue execution, and after it 133succeeds, the traced process is no longer traced and continues 134execution normally. 135.El 136.Pp 137Additionally, machine-specific requests can exist. On the i386, these 138are: 139.Bl -tag -width 12n 140.It Dv PT_GETREGS 141This request reads the traced process' machine registers into the 142.Do 143.Vt "struct reg" 144.Dc 145(defined in 146.In machine/reg.h ) 147pointed to by 148.Fa addr . 149.It Dv PT_SETREGS 150This request is the converse of 151.Dv PT_GETREGS ; 152it loads the traced process' machine registers from the 153.Do 154.Vt "struct reg" 155.Dc 156(defined in 157.In machine/reg.h ) 158pointed to by 159.Fa addr . 160.It Dv PT_GETFPREGS 161This request reads the traced process' floating-point registers into 162the 163.Do 164.Vt "struct fpreg" 165.Dc 166(defined in 167.In machine/reg.h ) 168pointed to by 169.Fa addr . 170.It Dv PT_SETFPREGS 171This request is the converse of 172.Dv PT_GETFPREGS ; 173it loads the traced process' floating-point registers from the 174.Do 175.Vt "struct fpreg" 176.Dc 177(defined in 178.In machine/reg.h ) 179pointed to by 180.Fa addr . 181.It Dv PT_GETDBREGS 182This request reads the traced process' debug registers into 183the 184.Do 185.Vt "struct dbreg" 186.Dc 187(defined in 188.In machine/reg.h ) 189pointed to by 190.Fa addr . 191.It Dv PT_SETDBREGS 192This request is the converse of 193.Dv PT_GETDBREGS ; 194it loads the traced process' debug registers from the 195.Do 196.Vt "struct dbreg" 197.Dc 198(defined in 199.In machine/reg.h ) 200pointed to by 201.Fa addr . 202.El 203.Sh RETURN VALUES 204Some requests can cause 205.Fn ptrace 206to return 207.Li -1 208as a non-error value; to disambiguate, 209.Va errno 210can be set to 0 before the call and checked afterwards. 211.Sh ERRORS 212The 213.Fn ptrace 214function may fail if: 215.Bl -tag -width Er 216.It Bq Er ESRCH 217.Bl -bullet -compact 218.It 219No process having the specified process ID exists. 220.El 221.It Bq Er EINVAL 222.Bl -bullet -compact 223.It 224A process attempted to use 225.Dv PT_ATTACH 226on itself. 227.It 228The 229.Fa request 230was not one of the legal requests. 231.It 232The signal number (in 233.Fa data ) 234to 235.Dv PT_CONTINUE 236was neither 0 nor a legal signal number. 237.It 238.Dv PT_GETREGS , 239.Dv PT_SETREGS , 240.Dv PT_GETFPREGS , 241.Dv PT_SETFPREGS , 242.Dv PT_GETDBREGS , 243or 244.Dv PT_SETDBREGS 245was attempted on a process with no valid register set. (This is 246normally true only of system processes.) 247.El 248.It Bq Er EBUSY 249.Bl -bullet -compact 250.It 251.Dv PT_ATTACH 252was attempted on a process that was already being traced. 253.It 254A request attempted to manipulate a process that was being traced by 255some process other than the one making the request. 256.It 257A request (other than 258.Dv PT_ATTACH ) 259specified a process that wasn't stopped. 260.El 261.It Bq Er EPERM 262.Bl -bullet -compact 263.It 264A request (other than 265.Dv PT_ATTACH ) 266attempted to manipulate a process that wasn't being traced at all. 267.It 268An attempt was made to use 269.Dv PT_ATTACH 270on a process in violation of the requirements listed under 271.Dv PT_ATTACH 272above. 273.El 274.El 275.Sh SEE ALSO 276.Xr execve 2 , 277.Xr sigaction 2 , 278.Xr wait 2 , 279.Xr execv 3 , 280.Xr i386_clr_watch 3 , 281.Xr i386_set_watch 3 282.Sh HISTORY 283A 284.Fn ptrace 285function call appeared in 286.At v7 . 287