neomantra · December 22, 2017 09:38 · Dec 11, 2013 · Dec 11, 2013
diff --git a/spawn.lua b/spawn.lua
@@ -28,7 +28,9 @@ int execvp(const char *file, char *const argv[]);
 
 local bor = bit.bor
 
-local char_arr_t = ffi.typeof('const char * [?]')
+local ffi_cast = ffi.cast
+local k_char_p_arr_t = ffi.typeof('const char * [?]')
+local char_p_k_p_t   = ffi.typeof('char * const *')
 
 local octal = function(n) return tonumber(n, 8) end 
 local O_WRONLY = octal('0001')
@@ -107,12 +109,12 @@ local function spawn(cmd_line, stdout_redirect, stderr_redirect)
         redirect(stdout_redirect, FD_STDOUT)
         redirect(stderr_redirect, FD_STDERR)
 
-        local argv = char_arr_t(#args + 1)  -- automatically NULL terminated
+        local argv = k_char_p_arr_t(#args + 1)  -- automatically NULL terminated
         for i = 1, #args do
             argv[i-1] = args[i]   -- args is 1-based Lua table, argv is 0-based C array
         end
 
-        local res = C.execvp(args[1], ffi.cast('char *const *', argv))
+        local res = C.execvp(args[1], ffi_cast(char_p_k_p_t, argv))
         if res == -1 then error("execvp failed with " .. ffi.errno()) end
         -- HERE SHOULD BE UNREACHABLE!!
     end

diff --git a/spawn.lua b/spawn.lua
@@ -0,0 +1,122 @@
+-- Spawn a command in the background, optionally redirecting stderr and stdout
+--
+-- requiring this file returns a function(cmd_line, stdout_redirect, stderr_redirect)
+--
+-- `cmd_line` is the command with possible arguments
+-- optional `stdout_redirect` is io.stdout, io.stderr, or a filename.  default/nil is io.stdout
+-- optional `stderr_redirect` is io.stdout, io.stderr, or a filename.  default/nil is io.stderr
+--
+-- Example:
+--     luajit -e 'require("spawn")("cat /etc/network/interfaces", "foo1", io.stdout)'
+--
+
+local ffi = require 'ffi'
+local C = ffi.C
+
+ffi.cdef([[
+
+typedef int32_t pid_t;
+pid_t fork(void);
+
+int open(const char *pathname, int flags, int mode);
+int close(int fd);
+int dup2(int oldfd, int newfd);
+
+int execvp(const char *file, char *const argv[]);
+
+]])
+
+local bor = bit.bor
+
+local char_arr_t = ffi.typeof('const char * [?]')
+
+local octal = function(n) return tonumber(n, 8) end 
+local O_WRONLY = octal('0001')
+local O_CREAT  = octal('0100')
+local S_IRUSR  = octal('00400') -- user has read permission
+local S_IWUSR  = octal('00200') -- user has write permission
+
+local FD_STDOUT = 1
+local FD_STDERR = 2
+
+
+-- split a string by spaces, except that single-quoted items are kept as a single token
+local function tokenize_args( s )
+    local t = {}
+    local i, prev = 1, 1
+    local in_q = nil
+
+    local function capture_token()
+        local w = s:sub(prev, i-1)
+        if #w ~= 0 then t[#t+1] = w end
+        prev = i + 1
+    end
+
+    while i <= #s do
+        local c = s:sub(i, i)
+        if in_q then -- close quote?
+            if c == in_q then
+                capture_token()
+                in_q = nil
+            end
+        elseif c == ' ' then
+            capture_token()
+        elseif c == '\'' then
+            in_q = '\''
+            capture_token()
+        end
+
+        i = i + 1
+    end
+
+    -- final cleanup
+    capture_token()
+    return t
+end
+
+
+-- dest should be either 0 or 1 (FD_STDOUT or FD_STDERR)
+local function redirect(io_or_filename, dest_fd)
+    if io_or_filename == nil then return end
+
+    -- first check for regular 
+    if (io_or_filename == io.stdout or io_or_filename == FD_STDOUT) and dest_fd ~= FD_STDOUT then
+        C.dup2(FD_STDERR, FD_STDOUT)
+    elseif (io_or_filename == io.stderr or io_or_filename == FD_STDERR) and dest_fd ~= FD_STDERR then
+        C.dup2(FD_STDOUT, FD_STDERR)
+
+    -- otherwise handle file-based redirection
+    else
+        local fd = C.open(io_or_filename, bor(O_WRONLY, O_CREAT), bor(S_IRUSR, S_IWUSR))
+        if fd < 0 then error("couldn't open file '" .. fname .. "': " .. ffi.errno()) end
+        C.dup2(fd, dest_fd)
+        C.close(fd)
+    end
+end
+
+
+local function spawn(cmd_line, stdout_redirect, stderr_redirect)
+    local args = tokenize_args(cmd_line)
+    if not args or #args == 0 then error("couldn't tokenize cmd_line") end
+
+    local pid = C.fork()
+    if pid < 0 then
+        error("fork failed " .. ffi.errno())
+    elseif pid == 0 then -- child process
+
+        redirect(stdout_redirect, FD_STDOUT)
+        redirect(stderr_redirect, FD_STDERR)
+
+        local argv = char_arr_t(#args + 1)  -- automatically NULL terminated
+        for i = 1, #args do
+            argv[i-1] = args[i]   -- args is 1-based Lua table, argv is 0-based C array
+        end
+
+        local res = C.execvp(args[1], ffi.cast('char *const *', argv))
+        if res == -1 then error("execvp failed with " .. ffi.errno()) end
+        -- HERE SHOULD BE UNREACHABLE!!
+    end
+end
+
+
+return spawn