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#include <unistd.h>
int execl(const char *path, const char *arg0, ..., const char *argn, char * /*NULL*/);
int execv(const char *path, char *const argv[]);
int execle (const char *path,char *const arg0[], ... , const char *argn, char * /*NULL*/, char *const envp[]);
int execve (const char *path, char *const argv[], char *const envp[]);
int execlp (const char *file, const char *arg0, ..., const char *argn, char * /*NULL*/);
int execvp (const char *file, char *const argv[]);
An interpreter file begins with a line of the form
#! pathname [arg]
where pathname is the path of the interpreter, and arg is an optional argument. When an interpreter file is exec’d, the system execs the specified interpreter. The pathname specified in the interpreter file is passed as arg0 to the interpreter. If arg was specified in the interpreter file, it is passed as arg1 to the interpreter. The remaining arguments to the interpreter are arg0 through argn of the originally exec’d file.
When a C program is executed, it is called as follows:
int main (int argc, char *argv[], char *envp[]);
where argc is the argument count, argv is an array of character pointers to the arguments themselves, and envp is an array of character pointers to the environment strings. As indicated, argc is at least one, and the first member of the array points to a string containing the name of the file.
path points to a path name that identifies the new process file.
file points to the new process file. If file does not contain a slash character, the path prefix for this file is obtained by a search of the directories passed in the PATH environment variable (see environ(5) ). The environment is supplied typically by the shell. If the new process file is not an executable object file, execlp() and execvp() use the contents of that file as standard input to the shell.
The arguments arg0, ..., argn point to null-terminated character strings. These strings constitute the argument list available to the new process image. Conventionally at least arg0 should be present. It will become the name of the process, as displayed by the ps command. arg0 points to a string that is the same as path (or the last component of path). The list of argument strings is terminated by a (char *)0 argument.
argv is an array of character pointers to null-terminated strings. These strings constitute the argument list available to the new process image. By convention, argv must have at least one member, and it should point to a string that is the same as path (or its last component). argv is terminated by a null pointer.
envp is an array of character pointers to null-terminated strings. These strings constitute the environment for the new process image. envp is terminated by a null pointer. For execl(), execv(), execvp(), and execlp(), the C run-time start-off routine places a pointer to the environment of the calling process in the global object extern char **environ, and it is used to pass the environment of the calling process to the new process.
File descriptors open in the calling process remain open in the new process, except for those whose close-on-exec flag is set; (see fcntl(2) ). For those file descriptors that remain open, the file pointer is unchanged.
Signals that are being caught by the calling process are set to the default disposition in the new process image (see signal(3C) ). Otherwise, the new process image inherits the signal dispositions of the calling process.
If the set-user-ID mode bit of the new process file is set (see chmod(2) ), exec() sets the effective user ID of the new process to the owner ID of the new process file. Similarly, if the set-group-ID mode bit of the new process file is set, the effective group ID of the new process is set to the group ID of the new process file. The real user ID and real group ID of the new process remain the same as those of the calling process.
If the effective user-ID is root or super-user, the set-user-ID and set-group-ID bits will be honored when the process is being controlled by ptrace.
The shared memory segments attached to the calling process will not be attached to the new process (see shmop(2) ). Memory mappings in the calling process are unmapped before the new process begins execution (see mmap(2) ).
Profiling is disabled for the new process; see profil(2) .
Timers created by timer_create(3R) are deleted before the new process begins execution.
Any outstanding asynchronous I/O operations may be cancelled.
The new process also inherits the following attributes from the calling process:
nice value (see nice(2) )
scheduler class and priority (see priocntl(2) )
process ID
parent process ID
process group ID
supplementary group IDs
semadj values (see semop(2) )
session ID (see exit(2) and signal(3C) )
trace flag (see ptrace(2) request 0)
time left until an alarm (see alarm(2) )
current working directory
root directory
file mode creation mask (see umask(2) )
resource limits (see getrlimit(2) )
utime, stime, cutime, and cstime (see times(2) )
file-locks (see fcntl(2) and lockf(3C) )
controlling terminal
process signal mask (see sigprocmask(2) )
pending signals (see sigpending(2) )
Upon successful completion, exec() marks for update the st_atime field of the file, unless the file is on a read-only file system. Should the exec() succeed, the process image file is considered to have been open() -ed. The corresponding close() is considered to occur at a time after this open, but before process termination or successful completion of a subsequent call to exec().
exec() will fail and return to the calling process if one or more of the following are true:
If a program is setuid to a user ID other than the super-user, and the program is executed when the real user ID is super-user, then the program has some of the powers of a super-user as well.