path: root/lib/libc/gen/exec.3

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.Dd $Mdocdate: March 21 2012 $
.Dt EXEC 3
.Os
.Sh NAME
.Nm execl ,
.Nm execlp ,
.Nm execle ,
.Nm execv ,
.Nm execvp ,
.Nm execvpe
.Nd execute a file
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Vt extern char **environ;
.Ft int
.Fn execl "const char *path" "const char *arg" ...
.Ft int
.Fn execlp "const char *file" "const char *arg" ...
.Ft int
.Fn execle "const char *path" "const char *arg" ... "char *const envp[]"
.Ft int
.Fn execv "const char *path" "char *const argv[]"
.Ft int
.Fn execvp "const char *file" "char *const argv[]"
.Ft int
.Fn execvpe "const char *file" "char *const argv[]" "char *const envp[]"
.Sh DESCRIPTION
The
.Nm exec
family of functions replace the current process image with a
new process image.
The functions described in this manual page are front-ends for the function
.Xr execve 2 .
(See the manual page for
.Xr execve
for detailed information about the replacement of the current process.)
.Pp
The initial argument for these functions is the pathname of a file which
is to be executed.
.Pp
The
.Fa "const char *arg"
and subsequent ellipses in the
.Fn execl ,
.Fn execlp ,
and
.Fn execle
functions can be thought of as
.Fa arg0 ,
.Fa arg1 ,
\&...,
.Fa argn .
Together they describe a list of one or more pointers to
NUL-terminated
strings that represent the argument list available to the executed program.
The first argument, by convention, should point to the file name associated
with the file being executed.
The list of arguments
.Em must
be terminated by a null pointer.
.Pp
The
.Fn execv ,
.Fn execvp
and
.Fn execvpe
functions provide an array of pointers to
NUL-terminated strings that
represent the argument list available to the new program.
The first argument, by convention, should point to the file name associated
with the file being executed.
The array of pointers
.Em must
be terminated by a null pointer itself.
.Pp
The
.Fn execle
function also specifies the environment of the executed process by following
the null
pointer that terminates the list of arguments in the parameter list
or the pointer to the
.Va argv
array with an additional parameter.
This additional parameter is an array of pointers to NUL-terminated
strings and
.Em must
be terminated by a null pointer itself.
The other functions take the environment for the new process image from the
external variable
.Va environ
in the current process.
.Pp
Some of these functions have special semantics.
.Pp
The functions
.Fn execlp ,
.Fn execvp
and
.Fn execvpe
will duplicate the actions of the shell in searching for an executable file
if the specified file name does not contain a slash
.Pq Sq \&/
character.
The search path is the path specified in the environment by
.Ev PATH
variable.
If this variable isn't specified,
.Dv _PATH_DEFPATH
from
.Aq Pa paths.h
is used instead, its value being:
.Pa /usr/bin:/bin:/usr/sbin:/sbin:/usr/X11R6/bin:/usr/local/bin
.Pp
In addition, certain errors are treated specially.
.Pp
If permission is denied for a file (the attempted
.Xr execve
returned
.Er EACCES ) ,
these functions will continue searching the rest of
the search path.
If no other file is found, however, they will return with the global variable
.Va errno
set to
.Er EACCES .
.Pp
If the header of a file isn't recognized (the attempted
.Xr execve
returned
.Er ENOEXEC ) ,
these functions will execute the shell with the path of
the file as its first argument.
(If this attempt fails, no further searching is done.)
.Sh RETURN VALUES
If any of the
.Nm exec
functions return, an error has occurred.
The return value is \-1, and the global variable
.Va errno
will be set to indicate the error.
.Sh FILES
.Bl -tag -width /bin/sh -compact
.It Pa /bin/sh
default shell program
.El
.Sh ERRORS
.Fn execl ,
.Fn execle ,
.Fn execlp ,
and
.Fn execvp
may fail and set
.Va errno
for any of the errors specified for the library functions
.Xr execve 2
and
.Xr malloc 3 .
.Pp
.Fn execv
may fail and set
.Va errno
for any of the errors specified for the library function
.Xr execve 2 .
.Sh SEE ALSO
.Xr sh 1 ,
.Xr execve 2 ,
.Xr fork 2 ,
.Xr ktrace 2 ,
.Xr ptrace 2 ,
.Xr environ 7
.Sh STANDARDS
Historically, the default path for the
.Fn execlp
and
.Fn execvp
functions was
.Pa \&.:/bin:/usr/bin .
This was changed to improve security and behaviour.
.Pp
The behavior of
.Fn execlp
and
.Fn execvp
when errors occur while attempting to execute the file is historic
practice, but has not traditionally been documented and is not specified
by the
.Tn POSIX
standard.
.Pp
Traditionally, the functions
.Fn execlp
and
.Fn execvp
ignored all errors except for the ones described above and
.Er ENOMEM
and
.Er E2BIG ,
upon which they returned.
They now return if any error other than the ones described above occurs.
.Pp
.Fn execl ,
.Fn execv ,
.Fn execle ,
.Fn execlp
and
.Fn execvp
conform to
.St -p1003.1-88 .
.Fn execvpe
is a GNU extension.