# Functions for opening and closing file descriptors
#
# Copyright (C) 2013  Patrick McDermott
#
# This file is part of the ProteanOS Development Kit.
#
# The ProteanOS Development Kit is free software: you can redistribute
# it and/or modify it under the terms of the GNU General Public License
# as published by the Free Software Foundation, either version 3 of the
# License, or (at your option) any later version.
#
# The ProteanOS Development Kit is distributed in the hope that it
# will be useful, but WITHOUT ANY WARRANTY; without even the implied
# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
# See the GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with the ProteanOS Development Kit.  If not, see
# <http://www.gnu.org/licenses/>.

# FD 3 is available as a temporary FD.
_FD_MIN=4
# Shells and the file descriptors they reserve for the user:
#   * Debian Almquist Shell
#     - The shell doesn't understand any file descriptor greater than 9.
#     - File descriptors starting at 10 are used for I/O redirection.
#   * BusyBox ash
#     - File descriptors starting at 10 are used for job control and I/O
#       redirection.
#   * GNU Bash:
#     - GNU Readline uses file descriptor 255 in interactive shells.
#     - File descriptors starting at 10 are used for I/O redirection.
#   * MirOS Korn Shell
#     - File descriptors starting at either 10 or 24 are used for the shell's
#       tty_fd and shl_dbg_fd.
#   * Solaris ksh
#     - The shell doesn't understand any file descriptor greater than 9.
_FD_MAX=9

FD=

fdalloc()
{
	local i=
	local fd=

	# Find first available file descriptor.
	i=${_FD_MIN}
	while [ ${i} -le ${_FD_MAX} ]; do
		if [ "x$(eval "printf '%s' \"\${_fd_${i}+set}\"")" != 'xset' ]
		then
			fd=${i}
			break
		fi
		i=$((${i} + 1))
	done
	if [ "x${fd:+set}" != 'xset' ]; then
		error "$(get_msg 'emfile')"
		return 1
	fi

	FD="${fd}"
	return 0
}

fopen()
{
	local path="${1}"
	local mode="${2}"
	shift 2

	case "${mode}" in
		'r')  mode='<' ;;
		'w')  mode='>' ;;
		'a')  mode='>>';;
		'rw') mode='<>';;
		*) return 125;;
	esac

	fdalloc || return 1

	if eval "exec ${FD}${mode}\"\${path}\""; then
		eval "_fd_${FD}=\"\${mode}\${path}\""
		return 0
	else
		error "$(get_msg 'cant_fopen')"
		FD=
		return 1
	fi
}

fclose()
{
	local fd="${1}"
	shift 1

	# Make sure the file descriptor is open.
	if [ "x$(eval "printf '%s' \"\${_fd_${fd}+set}\"")" != 'xset' ]; then
		error "$(get_msg 'ebadf')"
		return 1
	fi

	eval "exec ${fd}>&-"
	unset "_fd_${fd}"

	return 0
}