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 18, 2015 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_IO 96This request allows reading and writing arbitrary amounts of data in 97the traced process's address space. 98The 99.Fa addr 100argument specifies a pointer to a 101.Vt "struct ptrace_io_desc" , 102which is defined as follows: 103.Bd -literal 104struct ptrace_io_desc { 105 int piod_op; /* I/O operation */ 106 void *piod_offs; /* child offset */ 107 void *piod_addr; /* parent offset */ 108 size_t piod_len; /* request length */ 109}; 110 111/* 112 * Operations in piod_op. 113 */ 114#define PIOD_READ_D 1 /* Read from D space */ 115#define PIOD_WRITE_D 2 /* Write to D space */ 116#define PIOD_READ_I 3 /* Read from I space */ 117#define PIOD_WRITE_I 4 /* Write to I space */ 118.Ed 119.Pp 120The 121.Fa data 122argument is ignored. 123The actual number of bytes read or written is stored in 124.Va piod_len 125upon return. 126.It Dv PT_CONTINUE 127The traced process continues execution. 128.Fa addr 129is an address specifying the place where execution is to be resumed (a 130new value for the program counter), or 131.Po Vt caddr_t Pc Ns 1 132to indicate that execution is to pick up where it left off. 133.Fa data 134provides a signal number to be delivered to the traced process as it 135resumes execution, or 0 if no signal is to be sent. 136.It Dv PT_STEP 137The traced process is single stepped one instruction. 138.Fa addr 139should be passed 140.Po Vt caddr_t Pc Ns 1 . 141.Fa data 142is not used. 143.It Dv PT_KILL 144The traced process terminates, as if 145.Dv PT_CONTINUE 146had been used with 147.Dv SIGKILL 148given as the signal to be delivered. 149.It Dv PT_ATTACH 150This request allows a process to gain control of an otherwise unrelated 151process and begin tracing it. It does not need any cooperation from 152the to-be-traced process. In this case, 153.Fa pid 154specifies the process ID of the to-be-traced process, and the other two 155arguments are ignored. This request requires that the target process 156must have the same real UID as the tracing process, and that it must 157not be executing a setuid or setgid executable. (If the tracing 158process is running as root, these restrictions do not apply.) The 159tracing process will see the newly-traced process stop and may then 160control it as if it had been traced all along. 161.It Dv PT_DETACH 162This request is like PT_CONTINUE, except that it does not allow 163specifying an alternate place to continue execution, and after it 164succeeds, the traced process is no longer traced and continues 165execution normally. 166.El 167.Pp 168Additionally, machine-specific requests can exist. On the i386, these 169are: 170.Bl -tag -width 12n 171.It Dv PT_GETREGS 172This request reads the traced process' machine registers into the 173.Do 174.Vt "struct reg" 175.Dc 176(defined in 177.In machine/reg.h ) 178pointed to by 179.Fa addr . 180.It Dv PT_SETREGS 181This request is the converse of 182.Dv PT_GETREGS ; 183it loads the traced process' machine registers from the 184.Do 185.Vt "struct reg" 186.Dc 187(defined in 188.In machine/reg.h ) 189pointed to by 190.Fa addr . 191.It Dv PT_GETFPREGS 192This request reads the traced process' floating-point registers into 193the 194.Do 195.Vt "struct fpreg" 196.Dc 197(defined in 198.In machine/reg.h ) 199pointed to by 200.Fa addr . 201.It Dv PT_SETFPREGS 202This request is the converse of 203.Dv PT_GETFPREGS ; 204it loads the traced process' floating-point registers from the 205.Do 206.Vt "struct fpreg" 207.Dc 208(defined in 209.In machine/reg.h ) 210pointed to by 211.Fa addr . 212.It Dv PT_GETDBREGS 213This request reads the traced process' debug registers into 214the 215.Do 216.Vt "struct dbreg" 217.Dc 218(defined in 219.In machine/reg.h ) 220pointed to by 221.Fa addr . 222.It Dv PT_SETDBREGS 223This request is the converse of 224.Dv PT_GETDBREGS ; 225it loads the traced process' debug registers from the 226.Do 227.Vt "struct dbreg" 228.Dc 229(defined in 230.In machine/reg.h ) 231pointed to by 232.Fa addr . 233.El 234.Sh RETURN VALUES 235Some requests can cause 236.Fn ptrace 237to return 238.Li -1 239as a non-error value; to disambiguate, 240.Va errno 241can be set to 0 before the call and checked afterwards. 242.Sh ERRORS 243The 244.Fn ptrace 245function may fail if: 246.Bl -tag -width Er 247.It Bq Er ESRCH 248.Bl -bullet -compact 249.It 250No process having the specified process ID exists. 251.El 252.It Bq Er EINVAL 253.Bl -bullet -compact 254.It 255A process attempted to use 256.Dv PT_ATTACH 257on itself. 258.It 259The 260.Fa request 261was not one of the legal requests. 262.It 263The signal number (in 264.Fa data ) 265to 266.Dv PT_CONTINUE 267was neither 0 nor a legal signal number. 268.It 269The 270.Fa pid 271represents a system process. 272.It 273.Dv PT_GETREGS , 274.Dv PT_SETREGS , 275.Dv PT_GETFPREGS , 276.Dv PT_SETFPREGS , 277.Dv PT_GETDBREGS , 278or 279.Dv PT_SETDBREGS 280was attempted on a process with no valid register set. (This is 281normally true only of system processes.) 282.El 283.It Bq Er EBUSY 284.Bl -bullet -compact 285.It 286.Dv PT_ATTACH 287was attempted on a process that was already being traced. 288.It 289A request attempted to manipulate a process that was being traced by 290some process other than the one making the request. 291.It 292A request (other than 293.Dv PT_ATTACH ) 294specified a process that wasn't stopped. 295.El 296.It Bq Er EPERM 297.Bl -bullet -compact 298.It 299A request (other than 300.Dv PT_ATTACH ) 301attempted to manipulate a process that wasn't being traced at all. 302.It 303An attempt was made to use 304.Dv PT_ATTACH 305on a process in violation of the requirements listed under 306.Dv PT_ATTACH 307above. 308.El 309.El 310.Sh SEE ALSO 311.Xr execve 2 , 312.Xr sigaction 2 , 313.Xr wait 2 , 314.Xr execv 3 , 315.Xr i386_clr_watch 3 , 316.Xr i386_set_watch 3 317.Sh HISTORY 318A 319.Fn ptrace 320function call appeared in 321.At v7 . 322