V8/usr/man/man2/exec.2

.TH EXEC 2 
.SH NAME
execl, execv, execle, execve, execlp, execvp, exec, exece, environ \- execute a file
.SH SYNOPSIS
.nf
.B execl(name, arg0, arg1, ..., argn, 0)
.B char *name, *arg0, *arg1, ..., *argn;
.PP
.B execv(name, argv)
.B char *name, *argv[];
.PP
.B "execle(name, arg0, arg1, ..., argn, 0, envp)"
.B "char *name, *arg0, *arg1, ..., *argn, *envp[];"
.PP
.B execve(name, argv, envp)
.B char *name, *argv[], *envp[];
.PP
.B extern char **environ;
.fi
.SH DESCRIPTION
.I 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
Files remain open across
.I exec
unless explicit arrangement has been made;
see
.IR ioctl (2).
Ignored/held signals remain ignored/held across
these calls, but
signals that are caught (see
.IR signal (2))
are reset
to their default values.
.PP
Each user has a
.I real
user ID and group ID and an
.I effective
user ID and group ID.
The
real
ID
identifies the person using the system;
the
effective
ID
determines his access privileges.
.I Exec
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.
.PP
The
.I name
argument
is a pointer to the name of the file
to be executed.
If the first two bytes of that file are the ASCII
string `#!', then the first line of the file is taken to be ASCII and
determines the name of the program to execute.
The first nonblank string following `#!' in that line is substituted for
.I name.
Any second string, separated from the first by blanks or tabs,
is inserted between
the first two arguments (arguments 0 and 1) passed to the invoked file.
.PP
The argument pointers
.I arg0, arg1, ...
or the pointers in
.I argv
address null-terminated strings.
Conventionally argument 0
is the name of the
file.
.PP
.I Execl
is useful when a known file with known arguments is
being called;
the arguments to
.I execl
are the character strings
constituting the file and the arguments.
A 0 argument must end the argument list.
.PP
.I Execv
is useful when the number of arguments is unknown
in advance;
the arguments to
.I 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 0 pointer.
.PP
When a C program is executed,
it is called as follows:
.IP
.nf
main(argc, argv, envp)
int argc;
char **argv, **envp;
.fi
.PP
where
.IR argc ""
is the argument count
and
.IR argv ""
is an array of character pointers
to the arguments themselves.
As indicated,
.IR 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
.I 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, a `=', and a null-terminated value;
or a name, a pair of parentheses (), a value bracketed by { and }, and
by a null.
The array of pointers is terminated by a null pointer.
The shell
.IR sh (1)
passes an environment entry for each global shell variable
defined when the program is called.
See
.IR environ (5)
for some conventionally
used names.
.PP
The C run-time start-off routine places a copy of
.I envp
in the global cell
.I environ,
which is used
by
.I execv
and
.I execl
to pass the environment to any subprograms executed by the
current program.
The
.I exec
routines use lower-level routines as follows
to pass an environment explicitly:
.IP
.nf
execve(file, argv, environ);
execle(file, arg0, arg1, . . . , argn, 0, environ);
.fi
.PP
.I Execlp
and
.I execvp
are called with the same arguments as
.I execl
and
.I 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.
.I Exect
is the same as
.I execve,
except it arranges for a stop to occur on the first instruction
of the new core image for the benefit of tracers, see
.IR proc (4).
.SH FILES
.ta \w'/bin/sh  'u
/bin/sh	shell, invoked if command file found
by
.I execlp
or
.I execvp
.SH "SEE ALSO"
fork(2), environ(5)
.SH DIAGNOSTICS
If the file cannot be found,
if it is not executable,
if it does not start with a valid magic number (see
.IR 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.
Even for the super-user,
at least one of the execute-permission bits must be set for
a file to be executed.
.SH BUGS
If
.I execvp
is called to execute a file that turns out to be a shell
command file,
and if it is impossible to execute the shell,
the values of
.IR argv [0]
and
.IR argv [\-1]
will be modified before return.
.br
The path search of
.I execlp
and
.I execvp
does not extend to names substituted by `#!'.