GLib-2.0 ▶ GLib ▶ spawn_async_with_pipes_and_fdsSince 2.68

Executes a child program asynchronously (your program will not block waiting for the child to exit).

The child program is specified by the only argument that must be provided, argv. argv should be a null-terminated array of strings, to be passed as the argument vector for the child. The first string in argv is of course the name of the program to execute. By default, the name of the program must be a full path. If flags contains the GLib.SpawnFlags.SEARCH_PATH flag, the PATH environment variable is used to search for the executable. If flags contains the GLib.SpawnFlags.SEARCH_PATH_FROM_ENVP flag, the PATH variable from envp is used to search for the executable. If both the GLib.SpawnFlags.SEARCH_PATH and GLib.SpawnFlags.SEARCH_PATH_FROM_ENVP flags are set, the PATH variable from envp takes precedence over the environment variable.

If the program name is not a full path and GLib.SpawnFlags.SEARCH_PATH flag is not used, then the program will be run from the current directory (or working_directory, if specified); this might be unexpected or even dangerous in some cases when the current directory is world-writable.

On Windows, note that all the string or string vector arguments to this function and the other g_spawn*() functions are in UTF-8, the GLib file name encoding. Unicode characters that are not part of the system codepage passed in these arguments will be correctly available in the spawned program only if it uses wide character API to retrieve its command line. For C programs built with Microsoft's tools it is enough to make the program have a wmain() instead of main(). wmain() has a wide character argument vector as parameter.

At least currently, mingw doesn't support wmain(), so if you use mingw to develop the spawned program, it should call g_win32_get_command_line() to get arguments in UTF-8.

On Windows the low-level child process creation API CreateProcess() doesn't use argument vectors, but a command line. The C runtime library's spawn*() family of functions (which g_spawn_async_with_pipes() eventually calls) paste the argument vector elements together into a command line, and the C runtime startup code does a corresponding reconstruction of an argument vector from the command line, to be passed to main(). Complications arise when you have argument vector elements that contain spaces or double quotes. The spawn*() functions don't do any quoting or escaping, but on the other hand the startup code does do unquoting and unescaping in order to enable receiving arguments with embedded spaces or double quotes. To work around this asymmetry, g_spawn_async_with_pipes() will do quoting and escaping on argument vector elements that need it before calling the C runtime spawn() function.

The returned child_pid on Windows is a handle to the child process, not its identifier. Process handles and process identifiers are different concepts on Windows.

envp is a null-terminated array of strings, where each string has the form KEY=VALUE. This will become the child's environment. If envp is null, the child inherits its parent's environment.

flags should be the bitwise OR of any flags you want to affect the function's behaviour. The GLib.SpawnFlags.DO_NOT_REAP_CHILD means that the child will not automatically be reaped; you must use a child watch (g_child_watch_add()) to be notified about the death of the child process, otherwise it will stay around as a zombie process until this process exits. Eventually you must call g_spawn_close_pid() on the child_pid, in order to free resources which may be associated with the child process. (On Unix, using a child watch is equivalent to calling waitpid() or handling the SIGCHLD signal manually. On Windows, calling g_spawn_close_pid() is equivalent to calling CloseHandle() on the process handle returned in child_pid). See g_child_watch_add().

Open UNIX file descriptors marked as FD_CLOEXEC will be automatically closed in the child process. GLib.SpawnFlags.LEAVE_DESCRIPTORS_OPEN means that other open file descriptors will be inherited by the child; otherwise all descriptors except stdin/stdout/stderr will be closed before calling exec() in the child. GLib.SpawnFlags.SEARCH_PATH means that argv[0] need not be an absolute path, it will be looked for in the PATH environment variable. GLib.SpawnFlags.SEARCH_PATH_FROM_ENVP means need not be an absolute path, it will be looked for in the PATH variable from envp. If both GLib.SpawnFlags.SEARCH_PATH and GLib.SpawnFlags.SEARCH_PATH_FROM_ENVP are used, the value from envp takes precedence over the environment.

GLib.SpawnFlags.CHILD_INHERITS_STDIN means that the child will inherit the parent's standard input (by default, the child's standard input is attached to /dev/null). GLib.SpawnFlags.STDIN_FROM_DEV_NULL explicitly imposes the default behavior. Both flags cannot be enabled at the same time and, in both cases, the stdin_pipe_out argument is ignored.

GLib.SpawnFlags.STDOUT_TO_DEV_NULL means that the child's standard output will be discarded (by default, it goes to the same location as the parent's standard output). GLib.SpawnFlags.CHILD_INHERITS_STDOUT explicitly imposes the default behavior. Both flags cannot be enabled at the same time and, in both cases, the stdout_pipe_out argument is ignored.

GLib.SpawnFlags.STDERR_TO_DEV_NULL means that the child's standard error will be discarded (by default, it goes to the same location as the parent's standard error). GLib.SpawnFlags.CHILD_INHERITS_STDERR explicitly imposes the default behavior. Both flags cannot be enabled at the same time and, in both cases, the stderr_pipe_out argument is ignored.

It is valid to pass the same FD in multiple parameters (e.g. you can pass a single FD for both stdout_fd and stderr_fd, and include it in source_fds too).

source_fds and target_fds allow zero or more FDs from this process to be remapped to different FDs in the spawned process. If n_fds is greater than zero, source_fds and target_fds must both be non-null and the same length. Each FD in source_fds is remapped to the FD number at the same index in target_fds. The source and target FD may be equal to simply propagate an FD to the spawned process. FD remappings are processed after standard FDs, so any target FDs which equal stdin_fd, stdout_fd or stderr_fd will overwrite them in the spawned process.

source_fds is supported on Windows since 2.72.

GLib.SpawnFlags.FILE_AND_ARGV_ZERO means that the first element of argv is the file to execute, while the remaining elements are the actual argument vector to pass to the file. Normally g_spawn_async_with_pipes() uses argv[0] as the file to execute, and passes all of argv to the child.

child_setup and user_data are a function and user data. On POSIX platforms, the function is called in the child after GLib has performed all the setup it plans to perform (including creating pipes, closing file descriptors, etc.) but before calling exec(). That is, child_setup is called just before calling exec() in the child. Obviously actions taken in this function will only affect the child, not the parent.

On Windows, there is no separate fork() and exec() functionality. Child processes are created and run with a single API call, CreateProcess(). There is no sensible thing child_setup could be used for on Windows so it is ignored and not called.

If non-null, child_pid will on Unix be filled with the child's process ID. You can use the process ID to send signals to the child, or to use g_child_watch_add() (or waitpid()) if you specified the GLib.SpawnFlags.DO_NOT_REAP_CHILD flag. On Windows, child_pid will be filled with a handle to the child process only if you specified the GLib.SpawnFlags.DO_NOT_REAP_CHILD flag. You can then access the child process using the Win32 API, for example wait for its termination with the WaitFor*() functions, or examine its exit code with GetExitCodeProcess(). You should close the handle with CloseHandle() or g_spawn_close_pid() when you no longer need it.

If non-null, the stdin_pipe_out, stdout_pipe_out, stderr_pipe_out locations will be filled with file descriptors for writing to the child's standard input or reading from its standard output or standard error. The caller of g_spawn_async_with_pipes() must close these file descriptors when they are no longer in use. If these parameters are null, the corresponding pipe won't be created.

If stdin_pipe_out is null, the child's standard input is attached to /dev/null unless GLib.SpawnFlags.CHILD_INHERITS_STDIN is set.

If stderr_pipe_out is NULL, the child's standard error goes to the same location as the parent's standard error unless GLib.SpawnFlags.STDERR_TO_DEV_NULL is set.

If stdout_pipe_out is NULL, the child's standard output goes to the same location as the parent's standard output unless GLib.SpawnFlags.STDOUT_TO_DEV_NULL is set.

error can be null to ignore errors, or non-null to report errors. If an error is set, the function returns false. Errors are reported even if they occur in the child (for example if the executable in ``argv[0] is not found). Typically the message field of returned errors should be displayed to users. Possible errors are those from the G_SPAWN_ERROR domain.

If an error occurs, child_pid, stdin_pipe_out, stdout_pipe_out, and stderr_pipe_out will not be filled with valid values.

If child_pid is not null and an error does not occur then the returned process reference must be closed using g_spawn_close_pid().

On modern UNIX platforms, GLib can use an efficient process launching codepath driven internally by posix_spawn(). This has the advantage of avoiding the fork-time performance costs of cloning the parent process address space, and avoiding associated memory overcommit checks that are not relevant in the context of immediately executing a distinct process. This optimized codepath will be used provided that the following conditions are met:

1. GLib.SpawnFlags.DO_NOT_REAP_CHILD is set
2. GLib.SpawnFlags.LEAVE_DESCRIPTORS_OPEN is set
3. GLib.SpawnFlags.SEARCH_PATH_FROM_ENVP is not set
4. working_directory is null
5. child_setup is null
6. The program is of a recognised binary format, or has a shebang. Otherwise, GLib will have to execute the program through the shell, which is not done using the optimized codepath.

If you are writing a GTK application, and the program you are spawning is a graphical application too, then to ensure that the spawned program opens its windows on the right screen, you may want to use GdkAppLaunchContext, GAppLaunchContext, or set the DISPLAY environment variable.