The run-command API offers a versatile tool to run sub-processes with redirected input and output as well as with a modified environment and an alternate current directory.

A similar API offers the capability to run a function asynchronously, which is primarily used to capture the output that the function produces in the caller in order to process it.



Start a sub-process. Takes a pointer to a struct child_process that specifies the details and returns pipe FDs (if requested). See below for details.


Wait for the completion of a sub-process that was started with start_command().


A convenience function that encapsulates a sequence of start_command() followed by finish_command(). Takes a pointer to a struct child_process that specifies the details.

run_command_v_opt, run_command_v_opt_cd_env

Convenience functions that encapsulate a sequence of start_command() followed by finish_command(). The argument argv specifies the program and its arguments. The argument opt is zero or more of the flags RUN_COMMAND_NO_STDIN, RUN_GIT_CMD, RUN_COMMAND_STDOUT_TO_STDERR, or RUN_SILENT_EXEC_FAILURE that correspond to the members .no_stdin, .git_cmd, .stdout_to_stderr, .silent_exec_failure of struct child_process. The argument dir corresponds the member .dir. The argument env corresponds to the member .env.

The functions above do the following:

  1. If a system call failed, errno is set and -1 is returned. A diagnostic is printed.

  2. If the program was not found, then -1 is returned and errno is set to ENOENT; a diagnostic is printed only if .silent_exec_failure is 0.

  3. Otherwise, the program is run. If it terminates regularly, its exit code is returned. No diagnistic is printed, even if the exit code is non-zero.

  4. If the program terminated due to a signal, then the return value is the signal number - 128, ie. it is negative and so indicates an unusual condition; a diagnostic is printed. This return value can be passed to exit(2), which will report the same code to the parent process that a POSIX shell’s $? would report for a program that died from the signal.


    Run a function asynchronously. Takes a pointer to a struct async that specifies the details and returns a pipe FD from which the caller reads. See below for details.


    Wait for the completion of an asynchronous function that was started with start_async().


    Run a hook. The first argument is a pathname to an index file, or NULL if the hook uses the default index file or no index is needed. The second argument is the name of the hook. The further arguments correspond to the hook arguments. The last argument has to be NULL to terminate the arguments list. If the hook does not exist or is not executable, the return value will be zero. If it is executable, the hook will be executed and the exit status of the hook is returned. On execution, .stdout_to_stderr and .no_stdin will be set. (See below.)

Data structures

This describes the arguments, redirections, and environment of a command to run in a sub-process.

The caller:

  1. allocates and clears (memset(&chld, 0, sizeof(chld));) a struct child_process variable;

  2. initializes the members;

  3. calls start_command();

  4. processes the data;

  5. closes file descriptors (if necessary; see below);

  6. calls finish_command().

The .argv member is set up as an array of string pointers (NULL terminated), of which .argv[0] is the program name to run (usually without a path). If the command to run is a git command, set argv[0] to the command name without the git- prefix and set .git_cmd = 1.

The members .in, .out, .err are used to redirect stdin, stdout, stderr as follows:

  1. Specify 0 to request no special redirection. No new file descriptor is allocated. The child process simply inherits the channel from the parent.

  2. Specify -1 to have a pipe allocated; start_command() replaces -1 by the pipe FD in the following way:

    .in: Returns the writable pipe end into which the caller writes;
            the readable end of the pipe becomes the child's stdin.
    .out, .err: Returns the readable pipe end from which the caller
            reads; the writable end of the pipe end becomes child's
    The caller of start_command() must close the so returned FDs
    after it has completed reading from/writing to it!
  3. Specify a file descriptor > 0 to be used by the child:

    .in: The FD must be readable; it becomes child's stdin.
    .out: The FD must be writable; it becomes child's stdout.
    .err > 0 is not supported.
    The specified FD is closed by start_command(), even if it fails to
    run the sub-process!
  4. Special forms of redirection are available by setting these members to 1:

    .no_stdin, .no_stdout, .no_stderr: The respective channel is
            redirected to /dev/null.
    .stdout_to_stderr: stdout of the child is redirected to its
            stderr. This happens after stderr is itself redirected.
            So stdout will follow stderr to wherever it is

To modify the environment of the sub-process, specify an array of string pointers (NULL terminated) in .env:

  1. If the string is of the form "VAR=value", i.e. it contains = the variable is added to the child process’s environment.

  2. If the string does not contain =, it names an environment variable that will be removed from the child process’s environment.

To specify a new initial working directory for the sub-process, specify it in the .dir member.

If the program cannot be found, the functions return -1 and set errno to ENOENT. Normally, an error message is printed, but if .silent_exec_failure is set to 1, no message is printed for this special error condition.

This describes a function to run asynchronously, whose purpose is to produce output that the caller reads.

The caller:

  1. allocates and clears (memset(&asy, 0, sizeof(asy));) a struct async variable;

  2. initializes .proc and .data;

  3. calls start_async();

  4. processes the data by reading from the fd in .out;

  5. closes .out;

  6. calls finish_async().

The function pointer in .proc has the following signature:

int proc(int fd, void *data);
  1. fd specifies a writable file descriptor to which the function must write the data that it produces. The function must close this descriptor before it returns.

  2. data is the value that the caller has specified in the .data member of struct async.

  3. The return value of the function is 0 on success and non-zero on failure. If the function indicates failure, finish_async() will report failure as well.

There are serious restrictions on what the asynchronous function can do because this facility is implemented by a pipe to a forked process on UNIX, but by a thread in the same address space on Windows:

  1. It cannot change the program’s state (global variables, environment, etc.) in a way that the caller notices; in other words, .out is the only communication channel to the caller.

  2. It must not change the program’s state that the caller of the facility also uses.