execl.3

操作系统设计与实现源码

.\" Copyright (c) 1983 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)execl.3	6.2 (Berkeley) 4/25/86
.\"
.TH EXECL 3 "April 25, 1986"
.UC 5
.SH NAME
execl, execv, execle, execlp, execvp, exec, environ \- execute a file
.SH SYNOPSIS
.ft B
#include <unistd.h>

.in +.5i
.ti -.5i
int execl(const char *\fIname\fP, const char *\fIarg0\fP, ..., (char *) NULL)
.ti -.5i
int execv(const char *\fIname\fP, char *const \fIargv\fP[])
.ti -.5i
int execle(const char *\fIname\fP, const char *\fIarg0\fP, ..., (char *) NULL, char *const \fIenvp\fP[])
.ti -.5i
int execlp(const char *\fIname\fP, const char *\fIarg0\fP, ..., (char *) NULL)
.ti -.5i
int execvp(const char *\fIname\fP, char *const \fIargv\fP[])
.in -.5i

extern char *const *environ;
.fi
.SH DESCRIPTION
These routines provide various interfaces to the
.B execve 
system call.  Refer to 
.BR  execve (2)
for a description of their properties; only
brief descriptions are provided here.
.PP
.B Exec
in all its forms
overlays the calling process with the named file, then
transfers to the
entry point of the core image of the file.
There can be no return from a successful exec; the calling
core image is lost.
.PP
The
.I name
argument
is a pointer to the name of the file
to be executed.
The pointers
.IR arg [ 0 ],
.IR arg [ 1 "] ..."
address null-terminated strings.
Conventionally
.IR arg [ 0 ]
is the name of the
file.
.PP
Two interfaces are available.
.B execl
is useful when a known file with known arguments is
being called;
the arguments to
.B execl
are the character strings
constituting the file and the arguments;
the first argument is conventionally
the same as the file name (or its last component).
A null pointer argument must end the argument list.
(Note that the
.B execl*
functions are variable argument functions.  This means that the type
of the arguments beyond
.I arg0
is not checked.  So the null pointer requires an explicit cast to type
.B "(char *)"
if not of that type already.)
.PP
The
.B execv
version is useful when the number of arguments is unknown
in advance;
the arguments to
.B execv
are the name of the file to be
executed and a vector of strings containing
the arguments.
The last argument string must be followed
by a null pointer.
.PP
When a C program is executed,
it is called as follows:
.PP
.RS
.ft B
.nf
int main(int \fIargc\fP, char *const \fIargv\fP[], char *const \fIenvp\fP[]);

exit(main(\fIargc\fP, \fIargv\fP, \fIenvp\fP));
.fi
.ft R
.RE
.PP
where
.I argc
is the argument count
and
.I argv 
is an array of character pointers
to the arguments themselves.
As indicated,
.I argc
is conventionally at least one
and the first member of the array points to a
string containing the name of the file.
.PP
.I Argv
is directly usable in another
.B execv
because
.IR argv [ argc ]
is 0.
.PP
.I Envp
is a pointer to an array of strings that constitute
the
.I environment
of the process.
Each string consists of a name, an \*(lq=\*(rq, and a null-terminated value.
The array of pointers is terminated by a null pointer.
The shell
.BR sh (1)
passes an environment entry for each global shell variable
defined when the program is called.
See
.BR environ (7)
for some conventionally
used names.
The C run-time start-off routine places a copy of
.I envp
in the global cell
.BR environ ,
which is used
by
.B execv
and
.B execl
to pass the environment to any subprograms executed by the
current program.
.PP
.B Execlp
and
.B execvp
are called with the same arguments as
.B execl
and
.BR execv ,
but duplicate the shell's actions in searching for an executable
file in a list of directories.
The directory list is obtained from the environment variable
.BR PATH .
Under standard Minix, if a file is found that is executable, but does
not have the proper executable header then it is assumed to be
a shell script.
.B Execlp
and
.B execvp
execute
.B /bin/sh
to interpret the script.
Under Minix-vmd this does not happen, a script must begin with
.B #!
and the full path name of the interpreter if it is to be an
executable script.
.SH "SEE ALSO"
.BR execve (2),
.BR fork (2),
.BR environ (7),
.BR sh (1).
.SH DIAGNOSTICS
If the file cannot be found,
if it is not executable,
if it does not start with a valid magic number (see
.BR a.out (5)),
if maximum memory is exceeded,
or if the arguments require too much space,
a return
constitutes the diagnostic;
the return value is \-1 and
.B errno
is set as for
.BR execve .
Even for the super-user,
at least one of the execute-permission bits must be set for
a file to be executed.