NAME
execve - execute a file
SYNOPSIS
#include <unistd.h>
int execve(const char *name, char *const argv[], char *const envp[])
DESCRIPTION
Execve transforms the calling process into a new process.
The new process is constructed from an ordinary file called
the new process file. This file is either an executable
object file, or a file of data for an interpreter. An exe-
cutable object file consists of an identifying header, fol-
lowed by pages of data representing the initial program
(text) and initialized data pages. Additional pages may be
specified by the header to be initialized with zero data.
See a.out(5).
An interpreter file begins with a line of the form ``#!
interpreter''. When an interpreter file is execve'd, the
system execve's the specified interpreter, giving it the
name of the originally exec'd file as an argument and shift-
ing over the rest of the original arguments.
There can be no return from a successful execve because the
calling core image is lost. This is the mechanism whereby
different process images become active.
The argument argv is a null-terminated array of character
pointers to null-terminated character strings. These
strings constitute the argument list to be made available to
the new process. By convention, at least one argument must
be present in this array, and the first element of this
array should be the name of the executed program (i.e., the
last component of name).
The argument envp is also a null-terminated array of charac-
ter pointers to null-terminated strings. These strings pass
information to the new process that is not directly an argu-
ment to the command (see environ(7)).
Descriptors open in the calling process remain open in the
new process, except for those for which the close-on-exec
flag is set (see close(2)). Descriptors that remain open
are unaffected by execve.
Ignored signals remain ignored across an execve, but signals
that are caught are reset to their default values. Blocked
signals remain blocked regardless of changes to the signal
action. The signal stack is reset to be undefined (see
sigaction(2) for more information).
Each process has real user and group IDs and an effective
user and group IDs. The real ID identifies the person using
the system; the effective ID determines his access
privileges. Execve changes the effective user and group ID
to the owner of the executed file if the file has the "set-
user-ID" or "set-group-ID" modes. The real user ID is not
affected.
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)
access groups see getgroups(2)
working directory see chdir(2)
root directory see chroot(2)
control terminal see tty(4)
alarm timer see alarm(2)
file mode mask see umask(2)
signal mask see sigaction(2), sigprocmask(2)
When the executed program begins, it is called as follows:
int main(int argc, char *const argv[], char *const envp[]);
exit(main(argc, argv, envp));
where argc is the number of elements in argv (the ``arg
count'') and argv is the array of character pointers to the
arguments themselves.
Envp is a pointer to an array of strings that constitute the
environment of the process. A pointer to this array is also
stored in the global variable ``environ''. Each string con-
sists of a name, an "=", and a null-terminated value. The
array of pointers is terminated by a null pointer. The
shell sh(1) passes an environment entry for each global
shell variable defined when the program is called. See
environ(7) for some conventionally used names.
RETURN VALUE
If execve returns to the calling process an error has
occurred; the return value will be -1 and the global vari-
able errno will contain an error code.
ERRORS
Execve will fail and return to the calling process if one or
more of the following are true:
[ENOTDIR] A component of the path prefix is not a
directory.
[ENAMETOOLONG] The path name exceeds PATH_MAX characters.
[ENOENT] The new process file does not exist.
[ELOOP] Too many symbolic links were encountered in
translating the pathname. (Minix-vmd)
[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 per-
mission.
[ENOEXEC] The new process file has the appropriate
access permission, but has an invalid magic
number in its header.
[ENOMEM] The new process requires more (virtual)
memory than is currently available.
[E2BIG] The number of bytes in the new process's
argument list is larger than the system-
imposed limit ARG_MAX. The limit in the sys-
tem as released is 4096 bytes for 16-bit
MINIX 3, 16384 bytes for 32-bit Minix, and
unlimited for Minix-vmd.
[EFAULT] Path, argv, or envp point to an illegal
address.
[EIO] An I/O error occurred while reading from the
file system.
CAVEATS
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.
SEE ALSO
exit(2), fork(2), execl(3), environ(7).