libc/gen/exec.3

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.Dd March 22, 2020
.Dt EXEC 3
.Os
.Sh NAME
.Nm execl ,
.Nm execlp ,
.Nm execle ,
.Nm exect ,
.Nm execv ,
.Nm execvp ,
.Nm execvP
.Nd execute a file
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Vt extern char **environ ;
.Ft int
.Fn execl "const char *path" "const char *arg" ... NULL
.Ft int
.Fn execlp "const char *file" "const char *arg" ... NULL
.Ft int
.Fn execle "const char *path" "const char *arg" ... NULL "char *const envp[]"
.Fc
.Ft int
.Fn exect "const char *path" "char *const argv[]" "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 execvP "const char *file" "const char *search_path" "char *const argv[]"
.Sh DESCRIPTION
The
.Nm exec
family of functions replaces 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 2
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
.Em arg0 ,
.Em arg1 ,
\&...,
.Em argn .
Together they describe a list of one or more pointers to null-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
.Dv NULL
pointer.
.Pp
The
.Fn exect ,
.Fn execv ,
.Fn execvp ,
and
.Fn execvP
functions provide an array of pointers to null-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
.Sy must
be terminated by a
.Dv NULL
pointer.
.Pp
The
.Fn execle
and
.Fn exect
functions also specify the environment of the executed process by following
the
.Dv NULL
pointer that terminates the list of arguments in the argument list
or the pointer to the argv array with an additional argument.
This additional argument is an array of pointers to null-terminated strings
and
.Em must
be terminated by a
.Dv NULL
pointer.
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 execvP
will duplicate the actions of the shell in searching for an executable file
if the specified file name does not contain a slash
.Dq Li /
character.
For
.Fn execlp
and
.Fn execvp ,
search path is the path specified in the environment by
.Dq Ev PATH
variable.
If this variable is not specified,
the default path is set according to the
.Dv _PATH_DEFPATH
definition in
.In paths.h ,
which is set to
.Dq Ev /sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin .
For
.Fn execvP ,
the search path is specified as an argument to the function.
In addition, certain errors are treated specially.
.Pp
If an error is ambiguous (for simplicity, we shall consider all
errors except
.Er ENOEXEC
as being ambiguous here, although only the critical error
.Er EACCES
is really ambiguous),
then these functions will act as if they stat the file to determine
whether the file exists and has suitable execute permissions.
If it does, they will return immediately with the global variable
.Va errno
restored to the value set by
.Fn execve .
Otherwise, the search will be continued.
If the search completes without performing a successful
.Fn execve
or terminating due to an error,
these functions will return with the global variable
.Va errno
set to
.Er EACCES
or
.Er ENOENT
according to whether at least one file with suitable execute permissions
was found.
.Pp
If the header of a file is not recognized (the attempted
.Fn 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.)
.Pp
The function
.Fn exect
executes a file with the program tracing facilities enabled (see
.Xr ptrace 2 ) .
.Sh RETURN VALUES
If any of the
.Fn exec
functions returns, an error will have 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
The shell.
.El
.Sh COMPATIBILITY
Historically, the default path for the
.Fn execlp
and
.Fn execvp
functions was
.Dq Pa :/bin:/usr/bin .
This was changed to remove the current directory to enhance system
security.
.Pp
The behavior of
.Fn execlp
and
.Fn execvp
when errors occur while attempting to execute the file is not quite historic
practice, and 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 ETXTBSY ,
upon which they retried after sleeping for several seconds, and
.Er ENOMEM
and
.Er E2BIG ,
upon which they returned.
They now return for
.Er ETXTBSY ,
and determine existence and executability more carefully.
In particular,
.Er EACCES
for inaccessible directories in the path prefix is no longer
confused with
.Er EACCES
for files with unsuitable execute permissions.
In
.Bx 4.4 ,
they returned upon all errors except
.Er EACCES ,
.Er ENOENT ,
.Er ENOEXEC
and
.Er ETXTBSY .
This was inferior to the traditional error handling,
since it breaks the ignoring of errors for path prefixes
and only improves the handling of the unusual ambiguous error
.Er EFAULT
and the unusual error
.Er EIO .
The behaviour was changed to match the behaviour of
.Xr sh 1 .
.Sh ERRORS
The
.Fn execl ,
.Fn execle ,
.Fn execlp ,
.Fn execvp
and
.Fn execvP
functions
may fail and set
.Va errno
for any of the errors specified for the library functions
.Xr execve 2
and
.Xr malloc 3 .
.Pp
The
.Fn exect
and
.Fn execv
functions
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
The
.Fn execl ,
.Fn execv ,
.Fn execle ,
.Fn execlp
and
.Fn execvp
functions
conform to
.St -p1003.1-88 .
.Sh HISTORY
The
.Fn exec
function appeared in
.At v1 .
The
.Fn execl
and
.Fn execv
functions appeared in
.At v2 .
The
.Fn execlp ,
.Fn execle ,
.Fn execve ,
and
.Fn execvp
functions appeared in
.At v7 .
The
.Fn execvP
function first appeared in
.Fx 5.2 .
.Sh BUGS
The type of the
.Fa argv
and
.Fa envp
parameters to
.Fn execle ,
.Fn exect ,
.Fn execv ,
.Fn execvp ,
and
.Fn execvP
is a historical accident and no sane implementation should modify the provided
strings.
The bogus parameter types trigger false positives from
.Li const
correctness analyzers.
On
.Fx ,
the
.Fn __DECONST
macro may be used to work around this limitation.
.Pp
Due to a fluke of the C standard, on platforms other than
.Fx
the definition of
.Dv NULL
may be the untyped number zero, rather than a
.Ad (void *)0
expression.
To distinguish the concepts, they are referred to as a
.Dq null pointer constant
and a
.Dq null pointer ,
respectively.
On exotic computer architectures that
.Fx
does not support, the null pointer constant and null pointer may have a
different representation.
In general, where this document and others reference a
.Dv NULL
value, they actually imply a null pointer.
E.g., for portability to non-FreeBSD operating systems on exotic computer
architectures, one may use
.Li (char *)NULL
in place of
.Dv NULL
when invoking
.Fn execl ,
.Fn execle ,
and
.Fn execlp .