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         #include <unistd.h>
         execve(const char *path, char *const argv[], char *const envp[]);
         fexecve(int fd, char *const argv[], char *const envp[]);


         The execve() system call transforms the calling process into a new pro-
         cess.  The new process is constructed from an ordinary file, whose name
         is pointed to by path, called the new process file.  The fexecve() system
         call is equivalent to execve() except that the file to be executed is
         determined by the file descriptor fd instead of a path.  This file is
         either an executable object file, or a file of data for an interpreter.
         An executable object file consists of an identifying header, followed by
         pages of data representing the initial program (text) and initialized
         data pages.  Additional pages may be specified by the header to be ini-
         tialized with zero data; see elf(5) and a.out(5).
         An interpreter file begins with a line of the form:
               #! interpreter [arg]
         When an interpreter file is execve'd, the system actually execve's the
         specified interpreter.  If the optional arg is specified, it becomes the
         first argument to the interpreter, and the name of the originally
         execve'd file becomes the second argument; otherwise, the name of the
         originally execve'd file becomes the first argument.  The original argu-
         ments are shifted over to become the subsequent arguments.  The zeroth
         argument is set to the specified interpreter.
         The argument argv is a pointer to a null-terminated array of character
         pointers to null-terminated character strings.  These strings construct
         the argument list to be made available to the new process.  At least one
         argument must be present in the array; by custom, the first element
         should be the name of the executed program (for example, the last compo-
         nent of path).
         The argument envp is also a pointer to a null-terminated array of charac-
         ter pointers to null-terminated strings.  A pointer to this array is nor-
         mally stored in the global variable environ.  These strings pass informa-
         tion to the new process that is not directly an argument to the command
         (see environ(7)).
         File descriptors open in the calling process image remain open in the new
         process image, except for those for which the close-on-exec flag is set
         (see close(2) and fcntl(2)).  Descriptors that remain open are unaffected
         by execve().  If any of the standard descriptors (0, 1, and/or 2) are
         closed at the time execve() is called, and the process will gain privi-
         the new process image file is set, the effective group ID of the new pro-
         cess image is set to the group ID of the new process image file.  (The
         effective group ID is the first element of the group list.)  The real
         user ID, real group ID and other group IDs of the new process image
         remain the same as the calling process image.  After any set-user-ID and
         set-group-ID processing, the effective user ID is recorded as the saved
         set-user-ID, and the effective group ID is recorded as the saved set-
         group-ID.  These values may be used in changing the effective IDs later
         (see setuid(2)).
         The set-ID bits are not honored if the respective file system has the
         nosuid option enabled or if the new process file is an interpreter file.
         Syscall tracing is disabled if effective IDs are changed.
         The new process also inherits the following attributes from the calling
               process ID           see getpid(2)
               parent process ID    see getppid(2)
               process group ID     see getpgrp(2)
               access groups        see getgroups(2)
               working directory    see chdir(2)
               root directory       see chroot(2)
               control terminal     see termios(4)
               resource usages      see getrusage(2)
               interval timers      see getitimer(2)
               resource limits      see getrlimit(2)
               file mode mask       see umask(2)
               signal mask          see sigvec(2), sigsetmask(2)
         When a program is executed as a result of an execve() system call, it is
         entered as follows:
               main(argc, argv, envp)
               int argc;
               char **argv, **envp;
         where argc is the number of elements in argv (the ''arg count'') and argv
         points to the array of character pointers to the arguments themselves.
         The fexecve() ignores the file offset of fd.  Since execute permission is
         checked by fexecve(), the file descriptor fd need not have been opened
         with the O_EXEC flag.  However, if the file to be executed denies read
         permission for the process preparing to do the exec, the only way to pro-
         vide the fd to fexecve() is to use the O_EXEC flag when opening fd.  Note
         that the file to be executed can not be open for writing.


         As the execve() system call overlays the current process image with a new
         process image the successful call has no process to return to.  If
         execve() does return to the calling process an error has occurred; the
         return value will be -1 and the global variable errno is set to indicate
         [ELOOP]            Too many symbolic links were encountered in translat-
                            ing the pathname.
         [EACCES]           Search permission is denied for a component of the
                            path prefix.
         [EACCES]           The new process file is not an ordinary file.
         [EACCES]           The new process file mode denies execute permission.
         [ENOEXEC]          The new process file has the appropriate access per-
                            mission, but has an invalid magic number in its
         [ETXTBSY]          The new process file is a pure procedure (shared text)
                            file that is currently open for writing by some pro-
         [ENOMEM]           The new process requires more virtual memory than is
                            allowed by the imposed maximum (getrlimit(2)).
         [E2BIG]            The number of bytes in the new process' argument list
                            is larger than the system-imposed limit.  This limit
                            is specified by the sysctl(3) MIB variable
         [EFAULT]           The new process file is not as long as indicated by
                            the size values in its header.
         [EFAULT]           The path, argv, or envp arguments point to an illegal
         [EIO]              An I/O error occurred while reading from the file sys-
         In addition, the fexecve() will fail and return to the calling process
         [EBADF]            The fd argument is not a valid file descriptor open
                            for executing.


         If a program is setuid to a non-super-user, but is executed when the real
         uid is ''root'', then the program has some of the powers of a super-user
         as well.
         When executing an interpreted program through fexecve(), kernel supplies
         /dev/fd/n as a second argument to the interpreter, where n is the file
         descriptor passed in the fd argument to fexecve().  For this construction
         to work correctly, the fdescfs(5) filesystem shall be mounted on /dev/fd.
         The execve() system call appeared in 4.2BSD.  The fexecve() system call
         appeared in FreeBSD 8.0.

    BSD April 10, 2008 BSD


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