glibmm: Spawning Processes

glibmm 2.88.1

Classes | Typedefs | Functions
Spawning Processes

Process launching with fork()/exec(). More...

## Classes
class | Glib::SpawnError
| Exception class for errors occuring when spawning processes. More...

## Typedefs
using | Glib::SlotSpawnChildSetup = sigc::slot<void()>
| For instance, void on_child_setup();.

## Functions
void | Glib::spawn_async_with_pipes (const std::string & working_directory, const std::vector< std::string > & argv, const std::vector< std::string > & envp, SpawnFlags flags=SpawnFlags::DEFAULT, const SlotSpawnChildSetup & child_setup={}, Pid * child_pid=nullptr, int * standard_input=nullptr, int * standard_output=nullptr, int * standard_error=nullptr)
| Executes a child program asynchronously (your program will not block waiting for the child to exit).
void | Glib::spawn_async_with_pipes (const std::string & working_directory, const std::vector< std::string > & argv, SpawnFlags flags=SpawnFlags::DEFAULT, const SlotSpawnChildSetup & child_setup={}, Pid * child_pid=nullptr, int * standard_input=nullptr, int * standard_output=nullptr, int * standard_error=nullptr)
| Like the main spawn_async_with_pipes() method, but inheriting the parent's environment.
void | Glib::spawn_async (const std::string & working_directory, const std::vector< std::string > & argv, const std::vector< std::string > & envp, SpawnFlags flags=SpawnFlags::DEFAULT, const SlotSpawnChildSetup & child_setup={}, Pid * child_pid=nullptr)
| See spawn_async_with_pipes() for a full description.
void | Glib::spawn_async (const std::string & working_directory, const std::vector< std::string > & argv, SpawnFlags flags=SpawnFlags::DEFAULT, const SlotSpawnChildSetup & child_setup={}, Pid * child_pid=nullptr)
| Like the main spawn_async() method, but inheriting the parent's environment.
void | Glib::spawn_sync (const std::string & working_directory, const std::vector< std::string > & argv, const std::vector< std::string > & envp, SpawnFlags flags=SpawnFlags::DEFAULT, const SlotSpawnChildSetup & child_setup={}, std::string * standard_output=nullptr, std::string * standard_error=nullptr, int * wait_status=nullptr)
| Executes a child synchronously (waits for the child to exit before returning).
void | Glib::spawn_sync (const std::string & working_directory, const std::vector< std::string > & argv, SpawnFlags flags=SpawnFlags::DEFAULT, const SlotSpawnChildSetup & child_setup={}, std::string * standard_output=nullptr, std::string * standard_error=nullptr, int * wait_status=nullptr)
| Like the main spawn_sync() method, but inheriting the parent's environment.
void | Glib::spawn_command_line_async (const std::string & command_line)
| A simple version of spawn_async() that parses a command line with shell_parse_argv() and passes it to spawn_async().
void | Glib::spawn_command_line_sync (const std::string & command_line, std::string * standard_output=nullptr, std::string * standard_error=nullptr, int * wait_status=nullptr)
| A simple version of spawn_sync() with little-used parameters removed, taking a command line instead of an argument vector.
void | Glib::spawn_close_pid (Pid pid)
| On some platforms, notably WIN32, the Pid type represents a resource which must be closed to prevent resource leaking.

## Detailed Description

Process launching with fork()/exec().

## Typedef Documentation

## ◆ SlotSpawnChildSetup

using Glib::SlotSpawnChildSetup = sigc::slot<void()>

For instance,
void on_child_setup();.

## Function Documentation

## ◆ spawn_async() [1/2]

void Glib::spawn_async | ( | const std::string & | working_directory,
|  | const std::vector< std::string > & | argv,
|  | const std::vector< std::string > & | envp,
|  | SpawnFlags | flags = SpawnFlags::DEFAULT,
|  | const SlotSpawnChildSetup & | child_setup = {},
|  | Pid * | child_pid = nullptr )

See spawn_async_with_pipes() for a full description.

This function simply calls the spawn_async_with_pipes() without any pipes.

Parameters

  working_directory | Child's current working directory, or an empty string to inherit parent's.
  argv | Child's argument vector.
  envp | Child's environment.
  flags | Flags from SpawnFlags.
  child_setup | Slot to run in the child just before exec(), or an empty slot.
  child_pid | Return location for child process ID, or nullptr

Exceptions

  SpawnError | 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.

## ◆ spawn_async() [2/2]

void Glib::spawn_async | ( | const std::string & | working_directory,
|  | const std::vector< std::string > & | argv,
|  | SpawnFlags | flags = SpawnFlags::DEFAULT,
|  | const SlotSpawnChildSetup & | child_setup = {},
|  | Pid * | child_pid = nullptr )

Like the main spawn_async() method, but inheriting the parent's environment.

Parameters

  working_directory | Child's current working directory, or an empty string to inherit parent's.
  argv | Child's argument vector.
  flags | Flags from SpawnFlags.
  child_setup | Slot to run in the child just before exec(), or an empty slot.
  child_pid | Return location for child process ID, or nullptr

Exceptions

  SpawnError | 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.

## ◆ spawn_async_with_pipes() [1/2]

void Glib::spawn_async_with_pipes | ( | const std::string & | working_directory,
|  | const std::vector< std::string > & | argv,
|  | const std::vector< std::string > & | envp,
|  | SpawnFlags | flags = SpawnFlags::DEFAULT,
|  | const SlotSpawnChildSetup & | child_setup = {},
|  | Pid * | child_pid = nullptr,
|  | int * | standard_input = nullptr,
|  | int * | standard_output = nullptr,
|  | int * | standard_error = nullptr )

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. 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; the PATH shell variable will only be searched if you pass the SpawnFlags::SEARCH_PATH flag.

On Windows, note that all the string or string vector arguments to this function and the other 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 will have to call the undocumented function __wgetmainargs() to get the wide character argument vector and environment. See gspawn-win32-helper.c in the GLib sources or init.c in the mingw runtime sources for a prototype for that function. Alternatively, you can retrieve the Win32 system level wide character command line passed to the spawned program using the GetCommandLineW() function.

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 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 of 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, spawn_async_with_pipes() will do quoting and escaping on argument vector elements that need it before calling the C runtime spawn() function.

envp is a lists of strings, where each string has the form KEY=VALUE. This will become the child's environment.

flags should be the bitwise OR of any flags you want to affect the function's behaviour. The SpawnFlags::DO_NOT_REAP_CHILD flags means that the child will not automatically be reaped; you must use a ChildWatch source to be notified about the death of the child process. Eventually you must call spawn_close_pid() on the child_pid, in order to free resources which may be associated with the child process. (On Unix, using a ChildWatch source is equivalent to calling waitpid() or handling the SIGCHLD signal manually. On Windows, calling spawn_close_pid() is equivalent to calling CloseHandle() on the process handle returned in child_pid).

SpawnFlags::LEAVE_DESCRIPTORS_OPEN means that the parent's 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. SpawnFlags::SEARCH_PATH means that argv[0] need not be an absolute path, it will be looked for in the user's PATH. SpawnFlags::STDOUT_TO_DEV_NULL means that the child's standard output will be discarded, instead of going to the same location as the parent's standard output. If you use this flag, standard_output must be nullptr. SpawnFlags::STDERR_TO_DEV_NULL means that the child's standard error will be discarded, instead of going to the same location as the parent's standard error. If you use this flag, standard_error must be nullptr. 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). If you use this flag, standard_input must be nullptr. 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 spawn_async_with_pipes() uses argv[0] as the file to execute, and passes all of argv to the child.

child_setup is a callback slot. 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(). child_setup is called in the parent process just before creating the child process. You should carefully consider what you do in child_setup if you intend your software to be portable to Windows.

If non-nullptr, 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 child_watch_add() (or waitpid()) if you specified the 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 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 spawn_close_pid() when you no longer need it.

If non-nullptr, the standard_input, standard_output, standard_error 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 spawn_async_with_pipes() must close these file descriptors when they are no longer in use. If these parameters are nullptr, the corresponding pipe won't be created.

If standard_input is nullptr, the child's standard input is attached to /dev/null unless SpawnFlags::CHILD_INHERITS_STDIN is set.

If standard_error is nullptr, the child's standard error goes to the same location as the parent's standard error unless SpawnFlags::STDERR_TO_DEV_NULL is set.

If standard_output is nullptr, the child's standard output goes to the same location as the parent's standard output unless SpawnFlags::STDOUT_TO_DEV_NULL is set.

If child_pid is not nullptr and an error does not occur then the returned pid must be closed using spawn_close_pid().

Parameters

  working_directory | Child's current working directory, or an empty string to inherit the parent's, in the GLib file name encoding.
  argv | Child's argument vector.
  envp | Child's environment.
  flags | Flags from SpawnFlags
  child_setup | Slot to run in the child just before exec(), or an empty slot.
  child_pid | Return location for child process ID, or nullptr.
  standard_input | Return location for file descriptor to write to child's stdin, or nullptr.
  standard_output | Return location for file descriptor to read child's stdout, or nullptr.
  standard_error | Return location for file descriptor to read child's stderr, or nullptr.

Exceptions

  SpawnError | 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. If an error occurs, child_pid, standard_input, standard_output, and standard_error will not be filled with valid values.

## ◆ spawn_async_with_pipes() [2/2]

void Glib::spawn_async_with_pipes | ( | const std::string & | working_directory,
|  | const std::vector< std::string > & | argv,
|  | SpawnFlags | flags = SpawnFlags::DEFAULT,
|  | const SlotSpawnChildSetup & | child_setup = {},
|  | Pid * | child_pid = nullptr,
|  | int * | standard_input = nullptr,
|  | int * | standard_output = nullptr,
|  | int * | standard_error = nullptr )

Like the main spawn_async_with_pipes() method, but inheriting the parent's environment.

Parameters

  working_directory | Child's current working directory, or an empty string to inherit the parent's, in the GLib file name encoding.
  argv | Child's argument vector.
  flags | Flags from SpawnFlags
  child_setup | Slot to run in the child just before exec(), or an empty slot.
  child_pid | Return location for child process ID, or nullptr.
  standard_input | Return location for file descriptor to write to child's stdin, or nullptr.
  standard_output | Return location for file descriptor to read child's stdout, or nullptr.
  standard_error | Return location for file descriptor to read child's stderr, or nullptr.

Exceptions

  SpawnError | 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. If an error occurs, child_pid, standard_input, standard_output, and standard_error will not be filled with valid values.

## ◆ spawn_close_pid()

void Glib::spawn_close_pid | ( | Pid | pid | ) |

On some platforms, notably WIN32, the Pid type represents a resource which must be closed to prevent resource leaking.

spawn_close_pid() is provided for this purpose. It should be used on all platforms, even though it doesn't do anything under UNIX.

Parameters

  pid | The process identifier to close.

## ◆ spawn_command_line_async()

void Glib::spawn_command_line_async | ( | const std::string & | command_line | ) |

A simple version of spawn_async() that parses a command line with shell_parse_argv() and passes it to spawn_async().

It runs a command line in the background. Unlike spawn_async(), the SpawnFlags::SEARCH_PATH flag is enabled, other flags are not. Note that SpawnFlags::SEARCH_PATH can have security implications, so consider using spawn_async() directly if appropriate.

The same concerns on Windows apply as for spawn_command_line_sync().

Parameters

  command_line | A command line.

Exceptions

  SpawnError | 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.
  ShellError | If the command line could not be parsed.

## ◆ spawn_command_line_sync()

void Glib::spawn_command_line_sync | ( | const std::string & | command_line,
|  | std::string * | standard_output = nullptr,
|  | std::string * | standard_error = nullptr,
|  | int * | wait_status = nullptr )

A simple version of spawn_sync() with little-used parameters removed, taking a command line instead of an argument vector.

See spawn_sync() for full details. command_line will be parsed by shell_parse_argv(). Unlike spawn_sync(), the SpawnFlags::SEARCH_PATH flag is enabled. Note that SpawnFlags::SEARCH_PATH can have security implications, so consider using spawn_sync() directly if appropriate.

If wait_status is non-nullptr, the wait status of the child is stored there as it would be returned by waitpid(); standard UNIX macros such as WIFEXITED() and WEXITSTATUS() must be used to evaluate the wait status.

On Windows, please note the implications of shell_parse_argv() parsing command_line. Parsing is done according to Unix shell rules, not Windows command interpreter rules. Space is a separator, and backslashes are special. Thus you cannot simply pass a command_line containing canonical Windows paths, like "c:\\program files\\app\\app.exe", as the backslashes will be eaten, and the space will act as a separator. You need to enclose such paths with single quotes, like "'c:\\program files\\app\\app.exe' 'e:\\folder\\argument.txt'".

Parameters

  command_line | A command line.
  standard_output | Return location for child output.
  standard_error | Return location for child errors.
  wait_status | Return location for child wait status, as returned by waitpid().

Exceptions

  SpawnError | 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.
  ShellError | If the command line could not be parsed.

## ◆ spawn_sync() [1/2]

void Glib::spawn_sync | ( | const std::string & | working_directory,
|  | const std::vector< std::string > & | argv,
|  | const std::vector< std::string > & | envp,
|  | SpawnFlags | flags = SpawnFlags::DEFAULT,
|  | const SlotSpawnChildSetup & | child_setup = {},
|  | std::string * | standard_output = nullptr,
|  | std::string * | standard_error = nullptr,
|  | int * | wait_status = nullptr )

Executes a child synchronously (waits for the child to exit before returning).

All output from the child is stored in standard_output and standard_error, if those parameters are non-nullptr. Note that you must set the SpawnFlags::STDOUT_TO_DEV_NULL and SpawnFlags::STDERR_TO_DEV_NULL flags when passing nullptr for standard_output and standard_error. If wait_status is non-nullptr, the wait status of the child is stored there as it would be returned by waitpid(); standard UNIX macros such as WIFEXITED() and WEXITSTATUS() must be used to evaluate the wait status. Note that this function calls waitpid() even if wait_status is nullptr, and does not accept the SpawnFlags::DO_NOT_REAP_CHILD flag. If an error occurs, no data is returned in standard_output, standard_error, or wait_status.

This function calls spawn_async_with_pipes() internally; see that function for full details on the other parameters and details on how these functions work on Windows.

Parameters

  working_directory | Child's current working directory, or an empty string to inherit the parent's, in the GLib file name encoding.
  argv | Child's argument vector.
  envp | Child's environment.
  flags | Flags from SpawnFlags
  child_setup | Slot to run in the child just before exec(), or an empty slot.
  standard_output | Return location for file descriptor to read child's stdout, or nullptr.
  standard_error | Return location for file descriptor to read child's stderr, or nullptr.
  wait_status | Return location for child wait status, as returned by waitpid(), or nullptr

Exceptions

  SpawnError | 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. If an error occurs, child_pid, standard_input, standard_output, and standard_error will not be filled with valid values.

## ◆ spawn_sync() [2/2]

void Glib::spawn_sync | ( | const std::string & | working_directory,
|  | const std::vector< std::string > & | argv,
|  | SpawnFlags | flags = SpawnFlags::DEFAULT,
|  | const SlotSpawnChildSetup & | child_setup = {},
|  | std::string * | standard_output = nullptr,
|  | std::string * | standard_error = nullptr,
|  | int * | wait_status = nullptr )

Like the main spawn_sync() method, but inheriting the parent's environment.

Parameters

  working_directory | Child's current working directory, or an empty string to inherit the parent's, in the GLib file name encoding.
  argv | Child's argument vector.
  flags | Flags from SpawnFlags
  child_setup | Slot to run in the child just before exec(), or an empty slot.
  standard_output | Return location for file descriptor to read child's stdout, or nullptr.
  standard_error | Return location for file descriptor to read child's stderr, or nullptr.
  wait_status | Return location for child wait status, as returned by waitpid(), or nullptr

Exceptions

  SpawnError | 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. If an error occurs, child_pid, standard_input, standard_output, and standard_error will not be filled with valid values.