MirOS Manual: exect(2), execve(2)

EXECVE(2)                  BSD Programmer's Manual                   EXECVE(2)

NAME

     execve, exect - execute a file

SYNOPSIS

     #include <unistd.h>

     int
     execve(const char *path, char *const argv[], char *const envp[]);

     int
     exect(const char *path, char *const argv[], char *const envp[]);

DESCRIPTION

     execve() transforms the calling process into a new process. The new pro-
     cess is constructed from an ordinary file, whose name is pointed to by
     path, called the new process file. This file is either an executable ob-
     ject 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. Addi-
     tional pages may be specified by the header to be initialized with zero
     data;  see a.out(5) and elf(5).

     An interpreter file begins with a line of the form:

           #! interpreter [arg]

     When an interpreter file is execve(Ap, d), the system execve(Ap, s) runs
     the specified interpreter. If the optional arg is specified, it becomes
     the first argument to the interpreter, and the name of the originally
     execve(Ap, d) file becomes the second argument; otherwise, the name of
     the originally execve(Ap, d) file becomes the first argument. The origi-
     nal arguments are shifted over to become the subsequent arguments. The
     zeroth argument, normally the name of the execve(Ap, d) file, is left un-
     changed.

     The argument argv is a pointer to a null-terminated array of character
     pointers to NUL-terminated character strings. These strings construct the
     argument list to be made available to the new process. At least one argu-
     ment must be present in the array; by custom, the first element should be
     the name of the executed program (for example, the last component of
     path).

     The argument envp is also a pointer to a null-terminated array of charac-
     ter pointers to NUL-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(). In the case of a new setuid or setgid executable being exe-
     cuted, if file descriptors 0, 1, or 2 (representing stdin, stdout, and
     stderr) are currently unallocated, these descriptors will be opened to
     point to some system file like /dev/null. The intent is to ensure these
     descriptors are not unallocated, since many libraries make assumptions
     about the use of these 3 file descriptors.

     Signals set to be ignored in the calling process are set to be ignored in
     the new process. Signals which are set to be caught in the calling pro-
     cess image are set to default action in the new process image. Blocked
     signals remain blocked regardless of changes to the signal action. The
     signal stack is reset to be undefined (see sigaction(2) for more informa-
     tion).
     If the set-user-ID mode bit of the new process image file is set (see
     chmod(2)), the effective user ID of the new process image is set to the
     owner ID of the new process image file. If the set-group-ID mode bit of
     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 ef-
     fective 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-user-ID and set-group-ID bits have no effect if the new process
     image file is located on a file system mounted with the nosuid flag. The
     process will be started without the new permissions.

     The new process also inherits the following attributes from the calling
     process:

           process ID           see getpid(2)
           parent process ID    see getppid(2)
           process group ID     see getpgrp(2)
           session ID           see getsid(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) (unless process image file is
                                setuid or setgid, in which case all timers are
                                disabled)
           resource limits      see getrlimit(2)
           file mode mask       see umask(2)
           signal mask          see sigaction(2), sigsetmask(3)

     When a program is executed as a result of an execve() call, it is entered
     as follows:

           main(int argc, char **argv, char **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 exect() function is equivalent to execve() with the additional pro-
     perty that it executes the file with the program tracing facilities en-
     abled (see ptrace(2)).

RETURN VALUES

     As the execve() function overlays the current process image with a new
     process image the successful call has no process to return to. If ex-
     ecve() does return to the calling process an error has occurred; the re-
     turn value will be -1 and the global variable errno is set to indicate
     the error.

ERRORS

     execve() will fail and return to the calling process if:

     [ENOTDIR]     A component of the path prefix is not a directory.

     [ENAMETOOLONG]
                   A component of a pathname exceeded {NAME_MAX} characters,
                   or an entire path name exceeded {PATH_MAX} characters.

     [ENOENT]      The new process file does not exist.

     [ELOOP]       Too many symbolic links were encountered in translating 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.

     [EACCES]      The new process file is on a filesystem mounted with execu-
                   tion disabled (MNT_NOEXEC in <sys/mount.h>).

     [ENOEXEC]     The new process file has the appropriate access permission,
                   but has an invalid magic number in its header.

     [ETXTBSY]     The new process file is a pure procedure (shared text) file
                   that is currently open for writing or reading by some pro-
                   cess.

     [ENOMEM]      The new process requires more virtual memory than is al-
                   lowed by the imposed maximum (getrlimit(2)).

     [E2BIG]       The number of bytes in the new process's argument list is
                   larger than the system-imposed limit. The limit in the sys-
                   tem as released is 262144 bytes (NCARGS in <sys/param.h>).

     [EFAULT]      The new process file is not as long as indicated by the
                   size values in its header.

     [EFAULT]      path, argv, or envp point to an illegal address.

     [EIO]         An I/O error occurred while reading from the file system.

     [ENFILE]      During startup of an interpreter, the system file table was
                   found to be full.

SEE ALSO

     _exit(2), fork(2), execl(3), exit(3), a.out(5), elf(5), environ(7)

STANDARDS

     The exect() function should not be used in portable applications.

HISTORY

     The execve() function call first appeared in Version 7 AT&T UNIX and
     3BSD.

CAVEATS

     If a program is setuid to a non-superuser, but is executed when the real
     uid is "root", then the program has some of the powers of a superuser as
     well.

MirOS BSD #10-current          January 24, 1994                              2

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