.\" Copyright (c) 2013-2026 Devin Teske <dteske@FreeBSD.org>
.\" Copyright (c) 2021-2026 Faraz Vahedi <kfv@FreeBSD.org>
.\"
.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.Dd September 16, 2026
.Dt BSDCONF_PUT 3
.Os
.Sh NAME
.Nm bsdconf_put ,
.Nm bsdconf_set_option
.Nd resilient configuration file writing
.Sh LIBRARY
.Lb libbsdconf
.Sh SYNOPSIS
.In bsdconf.h
.Ft int
.Fo bsdconf_put
.Fa "struct bsdconf_option options[]"
.Fa "const char *path"
.Fa "uint16_t processing_options"
.Fa "uint16_t put_options"
.Fc
.Ft int
.Fo bsdconf_set_option
.Fa "struct bsdconf_option options[]"
.Fa "const char *directive"
.Fa "union bsdconf_value *value"
.Fc
.Sh DESCRIPTION
.Fn bsdconf_put
rewrites the configuration file at
.Fa path ,
applying the per-directive action of each option in the array:
.Bl -tag -width BSDCONF_ACTION_SET_VALUE
.It Dv BSDCONF_ACTION_SET_VALUE
Set the directive to
.Va value.str ,
editing it in place if present or appending it to the file if absent.
.It Dv BSDCONF_ACTION_CHECK
Report
.Pq via Va result
whether the current value differs from
.Va value.str ,
without modifying the file.
.It Dv BSDCONF_ACTION_REMOVE
Delete the directive from the file.
.El
.Pp
Unlike
.Fn bsdconf_parse ,
directives are matched exactly
.Pq a pattern is not a writable target .
Comments,
blank lines,
statement ordering,
and the formatting of untouched statements are preserved.
For each processed option,
.Va result
is set to a bitmask of
.Dv BSDCONF_DIRECTIVE_FOUND ,
.Dv BSDCONF_VALUE_CHANGED ,
.Dv BSDCONF_DIRECTIVE_ADDED ,
and
.Dv BSDCONF_DIRECTIVE_REMOVED ,
and
.Va line
is set to the line of the first match
.Pq if found .
A non-zero
.Va match_line
on input restricts the put to the statement on that physical line
.Pq zero matches any statement, as before ;
this allows a single
.Ql name+=value
among several to be rewritten or removed without appending a new line
or touching the others
.Pq make(1) list-strike edits in Xr sysconf 8 .
.Pp
The
.Fa path
argument to
.Fn bsdconf_put
must name a regular file
.Pq only a regular file can be atomically replaced ;
anything else is rejected with
.Er EINVAL .
When every option is
.Dv BSDCONF_ACTION_CHECK ,
or every
.Dv BSDCONF_ACTION_SET_VALUE
and
.Dv BSDCONF_ACTION_REMOVE
would leave the file unchanged,
the original is left untouched:
no temporary is created,
.Va mtime
is not bumped,
and hard links are not severed.
The original is read in full, subject to the
.Ev BSDCONF_MAX_BYTES
cap documented in
.Xr bsdconf 3 .
Otherwise the file is replaced atomically:
output is streamed to a temporary file created with
.Xr mkstemp 3
in the target's own directory,
flushed to stable storage with
.Xr fsync 2 ,
given the mode
.Pq and, if permitted, the ownership
of the original,
and then moved over the original with
.Xr rename 2 .
An unexpected system failure or power loss mid-transaction leaves the
original untouched
.Pq see also Sx SECURITY CONSIDERATIONS .
.Pp
.Fn bsdconf_put
operates on exactly one file.
For a format backed by more than one file
.Pq see Xr bsdconf_format 3 ,
the caller decides which file to hand it,
and the deterministic sourcing order makes that decision mechanical:
because the last file to list a directive dictates its effective value,
a directive should be rewritten in the last file of the format's ordered
list that currently lists it
.Pq found by parsing the files in order with Fn bsdconf_parse ,
or appended to the format's default file when no file lists it.
A removal,
by contrast,
must be applied to every file listing the directive,
lest deleting the authoritative definition merely unmask an earlier one.
This is the policy implemented by
.Xr sysconf 8 .
.Pp
The
.Fa put_options
argument is a mask of the following bit fields:
.Bl -tag -width BSDCONF_PUT_NO_DUPLICATES
.It Dv BSDCONF_PUT_NO_DUPLICATES
Fail with
.Er EEXIST
if a directive to be written appears more than once in the file.
.It Dv BSDCONF_PUT_ALLOW_EMPTY
Permit setting an empty value.
.It Dv BSDCONF_PUT_BACKUP
Save a copy of the original file with a
.Ql .bak
suffix before replacing it.
.It Dv BSDCONF_PUT_UNQUOTED
Emit
.Va value.str
as literal file text:
no quotes are added and no characters are backslash-escaped
.Pq the caller supplies the bytes that should appear on disk .
Embedded whitespace is fine
.Pq the value runs to the end of the line .
A value that could not round-trip under the parser
.Pq an embedded newline, unescaped comment marker, or trailing unescaped backslash
is rejected with
.Er EINVAL
rather than silently corrupting the file.
The reader still runs
.Fn bsdconf_strunexpand
on input, so escape sequences present in the file become the logical value
on read; this flag does not re-encode them on write.
.It Dv BSDCONF_PUT_QUOTE_ALWAYS
Enclose every value in double-quotes,
escaping embedded quotes and backslashes.
.El
.Pp
.Fn bsdconf_set_option
traverses the options-array and stages
.Fa value
into the option whose directive matches
.Fa directive
via
.Xr strcmp 3 .
.Sh RETURN VALUES
.Fn bsdconf_put
returns zero on success;
otherwise -1 is returned and the global variable
.Va errno
is set to indicate the error.
.Fn bsdconf_set_option
returns 1 if a matching directive was staged,
otherwise 0.
.Sh SEE ALSO
.Xr bsdconf 3 ,
.Xr bsdconf_format 3 ,
.Xr sysconf 8
.Sh HISTORY
The
.Fn bsdconf_put
and
.Fn bsdconf_set_option
functions first appeared in
.Fx 16.0
as part of
.Xr bsdconf 3 .
.Sh AUTHORS
.An Devin Teske Aq Mt dteske@FreeBSD.org
.An Faraz Vahedi Aq Mt kfv@FreeBSD.org
.Sh LIMITATIONS
.Fn bsdconf_put
matches directives by exact name and, by default, treats each name as
single-valued: the first matching statement is rewritten, or a new line
is appended when none matches.
That suits the built-in formats when the last assignment wins.
.Pp
Make(1)-style cumulative
.Ql +=
chains are supported only in part.
With
.Va match_line
unset, a
.Ql +=
.Dv BSDCONF_ACTION_SET_VALUE
appends a new statement rather than collapsing earlier ones; with
.Va match_line
set, one physical line can be rewritten or removed
.Pq as Xr sysconf 8 does for make/src list strikes .
Word-level
.Ql -=
policy and effective-value accumulation remain the caller's
responsibility;
the library does not implement
.Ql -=
itself.
.Pp
Formats whose directives legitimately repeat without make(1) operators
.Pq for example Apache-style cumulative keywords; see Xr bsdconf 3
likewise parse with caller-supplied callbacks, but
.Fn bsdconf_put
offers no first-class insert, remove, or reorder of occurrence
.Em N
among equals—only exact-name match or
.Va match_line
selection.
.Sh SECURITY CONSIDERATIONS
The write transaction is designed to be safe against both interruption and
interference.
The temporary file is created by
.Xr mkstemp 3
.Pq Dv O_EXCL ; never predictable, never followed
in the target's own directory,
so the data never crosses a filesystem boundary and never transits a
world-writable directory such as
.Pa /tmp .
The mode and ownership propagated onto it are taken by
.Xr fstat 2
from the descriptor actually read,
not by re-looking up the path,
and are applied with
.Xr fchmod 2
.Pq the original mode masked to 0666, so setuid, setgid, and execute bits are not copied
and
.Xr fchown 2
on the open descriptor,
so a concurrently swapped file cannot influence them.
A backup requested with
.Dv BSDCONF_PUT_BACKUP
is created mode 0600 then
.Xr fchmod 2 Ns 'd
to the original mode masked to 0777
.Pq setuid and setgid bits are not restored
and refuses to follow a symbolic link planted at the
.Ql .bak
name
.Pq Dv O_NOFOLLOW .
.Pp
Symbolic links in the
.Fa path
argument to
.Fn bsdconf_put
are resolved
.Pq with Xr realpath 3
before work begins:
writing through a symbolic link rewrites the file it points at and
preserves the link itself,
rather than replacing the link with a regular file.
The resolution is not re-verified at
.Xr open 2
time;
as with any path-based interface,
an actor with write access to a directory along the path can redirect it,
so the containing directories must be trustworthy
.Pq as those of system configuration files are .
Because replacement is by
.Xr rename 2 ,
a target with multiple hard links is severed from its other names,
which afterwards continue to reference the old content;
this is inherent to atomic replacement.