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.TH EXECVEAT 2 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
execveat \- execute program relative to a directory file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int execveat(int " dirfd ", const char *" pathname ,
.BI " const char *const " argv "[], const char *const " envp [],
.BI " int " flags );
.fi
.PP
.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
.\" commit 51f39a1f0cea1cacf8c787f652f26dfee9611874
The
.BR execveat ()
system call executes the program referred to by the combination of
.I dirfd
and
.IR pathname .
It operates in exactly the same way as
.BR execve (2),
except for the differences described in this manual page.
.PP
If the pathname given in
.I pathname
is relative, then it is interpreted relative to the directory
referred to by the file descriptor
.I dirfd
(rather than relative to the current working directory of
the calling process, as is done by
.BR execve (2)
for a relative pathname).
.PP
If
.I pathname
is relative and
.I dirfd
is the special value
.BR AT_FDCWD ,
then
.I pathname
is interpreted relative to the current working
directory of the calling process (like
.BR execve (2)).
.PP
If
.I pathname
is absolute, then
.I dirfd
is ignored.
.PP
If
.I pathname
is an empty string and the
.BR AT_EMPTY_PATH
flag is specified, then the file descriptor
.I dirfd
specifies the file to be executed (i.e.,
.IR dirfd
refers to an executable file, rather than a directory).
.PP
The
.I flags
argument is a bit mask that can include zero or more of the following flags:
.TP
.BR AT_EMPTY_PATH
If
.I pathname
is an empty string, operate on the file referred to by
.IR dirfd
(which may have been obtained using the
.BR open (2)
.B O_PATH
flag).
.TP
.B AT_SYMLINK_NOFOLLOW
If the file identified by
.I dirfd
and a non-NULL
.I pathname
is a symbolic link, then the call fails with the error
.BR ELOOP .
.SH RETURN VALUE
On success,
.BR execveat ()
does not return.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
The same errors that occur for
.BR execve (2)
can also occur for
.BR execveat ().
The following additional errors can occur for
.BR execveat ():
.TP
.B EBADF
.I dirfd
is not a valid file descriptor.
.TP
.B EINVAL
Invalid flag specified in
.IR flags .
.TP
.B ELOOP
.I flags
includes
.BR AT_SYMLINK_NOFOLLOW
and the file identified by
.I dirfd
and a non-NULL
.I pathname
is a symbolic link.
.TP
.B ENOENT
The program identified by
.I dirfd
and
.I pathname
requires the use of an interpreter program
(such as a script starting with "#!"), but the file descriptor
.I dirfd
was opened with the
.B O_CLOEXEC
flag, with the result that
the program file is inaccessible to the launched interpreter.
See BUGS.
.TP
.B ENOTDIR
.I pathname
is relative and
.I dirfd
is a file descriptor referring to a file other than a directory.
.SH VERSIONS
.BR execveat ()
was added to Linux in kernel 3.19.
.SH CONFORMING TO
The
.BR execveat ()
system call is Linux-specific.
.SH NOTES
In addition to the reasons explained in
.BR openat (2),
the
.BR execveat ()
system call is also needed to allow
.BR fexecve (3)
to be implemented on systems that do not have the
.I /proc
filesystem mounted.
.PP
When asked to execute a script file, the
.IR argv[0]
that is passed to the script interpreter is a string of the form
.IR /dev/fd/N
or
.IR /dev/fd/N/P ,
where
.I N
is the number of the file descriptor passed via the
.IR dirfd
argument.
A string of the first form occurs when
.BR AT_EMPTY_PATH
is employed.
A string of the second form occurs when the script is specified via both
.IR dirfd
and
.IR pathname ;
in this case,
.IR P
is the value given in
.IR pathname .
.PP
For the same reasons described in
.BR fexecve (3),
the natural idiom when using
.BR execveat ()
is to set the close-on-exec flag on
.IR dirfd .
(But see BUGS.)
.PP
Glibc does not provide a wrapper for this system call; call it using
.BR syscall (2).
.SH BUGS
The
.B ENOENT
error described above means that it is not possible to set the
close-on-exec flag on the file descriptor given to a call of the form:
.PP
execveat(fd, "", argv, envp, AT_EMPTY_PATH);
.PP
However, the inability to set the close-on-exec flag means that a file
descriptor referring to the script leaks through to the script itself.
As well as wasting a file descriptor,
this leakage can lead to file-descriptor exhaustion in scenarios
where scripts recursively employ
.BR execveat ().
.\" For an example, see Michael Kerrisk's 2015-01-10 reply in this LKML
.\" thread (http://thread.gmane.org/gmane.linux.kernel/1836105/focus=20229):
.\"
.\" Subject: [PATCHv10 man-pages 5/5] execveat.2: initial man page.\" for execveat(2
.\" Date: Mon, 24 Nov 2014 11:53:59 +0000
.SH SEE ALSO
.BR execve (2),
.BR openat (2),
.BR fexecve (3)