execve.2

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 2 代码 · 共 261 行

.\" SCCSID: @(#)execve.2	6.6	3/15/90
.TH execve 2
.SH Name
execve \- execute a file
.SH Syntax
.ft B
execve(name, argv, envp)
.br
char *name, *argv[], *envp[];
.fi
.SH Description
.NXR "execve system call"
.NXA "execve system call" "environ global variable"
.NXR "file" "executing"
The
.PN execve
system call transforms the calling process into a new process.
The new process is constructed from an ordinary file
called the \fInew process file\fP.
.NXR "new process file" "defined"
This file is either an executable object file,
.NXR "executable object file" "defined"
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 can be specified
by the header to be initialized with zero data.
For further information, see 
.MS a.out 5 .
.PP
An interpreter file begins with a 
line of the form ``#! \fIinterpreter\fP''.
.NXR "interpreter file" "defined"
When an interpreter file is executed
the system 
executes 
the specified interpreter, giving
it the name of the originally 
executed 
file as an argument,
shifting over the rest of the original arguments.
.PP
There can be no return from
a successful 
.PN execve
because the calling
core image is lost.
This is the mechanism whereby different process images become active.
.PP
The argument \fIargv\fP is an 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, the last component of \fIname\fP.
.PP
The argument \fIenvp\fP is also an array of character pointers
to null-terminated strings.  These strings pass information to the
new process, but they are not directly arguments to the command.
For further information, see 
.MS environ 7 .
.PP
Descriptors open in the calling process remain open in
the new process, except for those for which the 
.I close-on-exec
flag is set.  For further information, see 
.MS close 2 .
Descriptors which remain open are unaffected by
.PN execve .
.PP
Ignored signals remain ignored across an
.PN execve ,
but signals that are caught are reset to their default values.
The signal stack is reset to be undefined.  For further information,
see 
.MS sigvec 2 .
.PP
Each process has
.I real
user and group IDs and
.I effective
user and group IDs.  The
.I real
ID identifies the person using the system; the
.I effective
ID determines his access privileges.
The
.PN execve
system call
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
.I real
user ID is not affected.
.PP
The new process also inherits the following attributes from
the calling process:
.PP
.in +5n
.nf
.ta +2i
Process ID	See getpid(2)
Parent process ID	See getpid(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)
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)
.in \-5n
.fi
.PP
When the executed program begins, it is called as follows:
.PP
.DT
.nf
	main(argc, argv, envp)
	int argc;
	char **argv, **envp;
.fi
.PP
The
.I argc
argument is the number of elements in \fIargv\fP
(the ``arg count'')
and
.I argv
is the array of character pointers
to the arguments themselves.
.PP
The
.I envp
argument is a pointer to an array of strings that constitute
the
.I environment
of the process.
A pointer to this array is also stored
in the global environ variable.
Each string consists of a name,
an equal sign ( = ), and a null-terminated value.
The array of pointers is terminated by a null pointer.
The shell 
.MS sh 1
passes an environment entry for each global shell variable
defined when the program is called.
See 
.MS environ 7
for some conventionally used names.
.PP
If
.PN execve
returns to the calling process, an error has occurred; the
return value is \-1 and the global variable
errno
contains an error code.
.SH Environment
.SS POSIX, System Five 
When your program is compiled using the POSIX or System V environment,
the
effective user ID and effective group ID of the new process image
are saved (as the saved-set-uid and saved-set-gid)
for later use by the
.PN setuid ,
.PN setgid ,
and
.PN kill
functions.
.SH Restrictions
.NXR "execve system call" "restricted"
If a program's effective user ID is not the superuser,
but it is executed when
the real user ID is root, then the program has the powers
of the superuser. 
.SH Diagnostics
.NXR "execve system call" "diagnostics"
The
.PN execve
system call fails and returns to the calling process under the following
conditions:
.TP 15
[ENOENT]
The new process file does not exist.
.TP 15
[ENOTDIR]
A component of the path prefix is not a directory.
.TP 15
[EACCES]
Search permission is denied for a 
component of the path prefix.
.TP 15
[EACCES]
The new process file is not an ordinary file.
.TP 15
[EACCES]
The new process file mode denies execute permission.
.TP 15
[ENOEXEC]
The new process file has the appropriate access
permission, but it has an invalid magic number in its header.
.TP 15
[ETXTBSY]
The new process file is a pure procedure (shared text)
file that is currently open for writing or reading by some process.
.TP 15
[ENOMEM]
The new process requires more virtual memory than
is allowed by the imposed maximum.  For further information,
see 
.MS getrlimit 2 .
.TP 15
[E2BIG]
The number of bytes in the new process's argument list
is larger than the system-imposed limit of {ARG_MAX} bytes.
.TP 15
[EFAULT]
The new process file is not as long as indicated by
the size values in its header.
.TP 15
[EFAULT]
The
\fIpath\fP\|, \fIargv\fP\|, or \fIenvp\fP points
to an illegal address.
.TP 15
[EIO]
An I/O error occurred while reading from the file system.
.TP 15
[ENAMETOOLONG]
A component of a pathname exceeded 255 characters, or an
entire path name exceeded 1023 characters.
.TP 15
[ELOOP]
Too many symbolic links were encountered in translating the
pathname.
.TP 15
[EROFS]
If binaries cannot be executed from the file system.
.TP 15
[EROFS]
If
.PN setuid
and
.PN setgid
programs cannot be executed from the file system.
.TP 15
[ESTALE]
The file handle given in the argument is invalid.  The file
referred to by that file handle no longer exists or has 
been revoked.
.TP
[ETIMEDOUT]
A connect request or remote file operation failed
because the connected party
did not properly respond after a period
of time that is dependent on the communications protocol.
.SH See Also
exit(2), fork(2), execl(3), environ(7)