execve(2)
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 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 initialized with zero data. See a.out(5).
An interpreter file begins with a line of the form ``#! interpreter''.
(Minix-vmd only.) 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 shifting 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 character pointers
to null-terminated strings. These strings pass information to the new
process that is not directly an argument 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 consists 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 variable 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 permission.
[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 system as released is 4096 bytes for 16-bit Minix,
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).