*** UNIX MANUAL PAGE BROWSER ***

ptrace(2) System Calls Manual ptrace(2) NAME ptrace - Traces the execution of a child process SYNOPSIS #include <sys/signal.h> #include <sys/ptrace.h> int ptrace ( long request, long int process, ulong_t *address, ulong_t data ); PARAMETERS Determines the action to be taken by the ptrace function. Specifies the process ID. Determined by the value of the request parameter. De- termined by the value of the request parameter. DESCRIPTION The ptrace function permits a parent process to control execution of a child process. It is primarily used by utility programs to enable breakpoint debugging. The child process behaves normally until it receives a signal. When a signal is delivered, the child process is stopped, and a SIGCHLD signal is sent to its parent. The parent process can wait for the child process to stop using the wait function. When the child process is stopped, its parent process can use the ptrace function to examine and modify the image memory of the child process, to either terminate the child process or permit it to con- tinue. As a security measure, the ptrace function inhibits the set-user ID fa- cility when any subsequent exec function is issued by the child process. When a traced process calls one of the exec functions, it stops before executing the first instruction of the new image as if it had received the SIGTRAP signal. The request parameter is set to one of the values in the following list. Only the PT_TRACE_ME request can be issued by child processes; the remaining requests can only be used by the parent process. For each request, the process parameter is the process ID of the child process. The child process must be in a stopped state before the fol- lowing requests are made. This request sets the child process trace flag. It must be issued by the child process that is to be traced by its parent process. When the trace flag is set, the child process is left in a stopped state on receipt of a signal, and the action speci- fied by the sigaction function is ignored. The process, address, and data parameters are ignored, and the return value is not defined for this request. Do not issue this request when the parent process does not expect to trace the child process. These requests return the ad- dress space data of the child process at the location pointed to by the address parameter. The PT_READ_I and PT_READ_D requests produce equal results. The data parameter is ignored. These requests fail when the value of the address parameter is not in the address space of the child process or, on some machines, when the address parameter is not prop- erly aligned. These errors return a value of -1, and the parent process errno is set to [EIO]. This request returns the variable of the system's per-process data area for the child, specified by the ad- dress parameter. This area contains the register values and other in- formation about the process. On some machines, the address parameter is subject to alignment constraints. The data parameter is ignored. This request fails when the value of the address parameter is outside of the system's per-process data area for the child. On failure, a value of -1 is returned and the parent process errno is set to [EIO]. These requests write the value of the data parameter into the address space variable of the child process at the location pointed to by the address parameter. On some machines, where necessary, the PT_WRITE_I request synchronizes any hardware caches, if present. In all other re- spects, the PT_WRITE_I and PT_WRITE_D requests can be used with equal results. On some machines, these requests return the previous contents of the address space variable of the child process, while on other ma- chines no useful value is returned with the exception of System V be- havior. System V behavior for libsys5 returns the value written, and for libc behavior it returns zero (0) for success. These requests fail when the address parameter points to a loca- tion in a pure procedure space and a copy cannot be made. These requests also fail when the value of the address parameter is out of range and on some machines, when the address parameter is not properly aligned. On failure a value of -1 is returned and the parent process errno is set to [EIO]. This request writes the value of the data parameter into the variable of the sys- tem's per-process data area for the child, specified by the ad- dress parameter. This area contains the register values and other information about the process. On some machines, the ad- dress parameter is subject to alignment constraints. Not all locations within the system's per-process data area for the child may be written. This request fails when the value of the address parameter is outside of the system's per-process data area for the child. On failure, a value of -1 is returned and the parent process errno is set to indicate the error. This re- quest permits the child process to resume execution. When the data parameter is zero (0), the signal that caused the child process to stop is canceled before the child process resumes ex- ecution. When the data parameter has a valid signal value, the child process resumes execution as though that signal had been re- ceived. When the address parameter is equal to 1, execution continues from where it stopped. When the address parameter is not 1, it is assumed to be the address at which the process should resume execution. System V behavior for libsys5 returns the value written, and for libc behavior it returns zero (0) for success. This request fails when the data parameter is not zero (0) or a valid signal value. On failure, a value of -1 is returned to the parent process and the parent process errno is set to [EIO]. This re- quest terminates a child process as if the child process called the exit function. This request permits execution to continue in the same manner as PT_CONTINUE. However, as soon as possible after the execution of at least one instruction, execution stops again as if the child process had received the SIGTRAP signal. ERRORS If the ptrace function fails, errno is set to one of the following val- ues: Specifies that the request parameter does not have one of the listed values, or is not valid for the machine type on which the process is executing. Specifies that the given signal number is in- valid. Specifies that the addressed used is either out of bounds or improperly aligned. Specifies that the process parameter identifies a child process that does not exist or that has not executed this func- tion with the request parameter PT_TRACE_ME. Specifies that the speci- fied process cannot be traced. Specifies that an invalid location was used for the system's per-process data area or that the process parame- ter is out of range. Specifies that the location within the system's per-process data area could not be modified. RELATED INFORMATION Functions: exec(2), sigaction(2), wait(2). delim off ptrace(2)