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run-command API
===============
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.
Functions
---------
`start_command`::
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.
`finish_command`::
Wait for the completion of a sub-process that was started with
start_command().
`run_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_dir`, `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`, or
`RUN_COMMAND_STDOUT_TO_STDERR` that correspond to the members
.no_stdin, .git_cmd, .stdout_to_stderr of `struct child_process`.
The argument dir corresponds the member .dir. The argument env
corresponds to the member .env.
`start_async`::
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.
`finish_async`::
Wait for the completion of an asynchronous function that was
started with start_async().
Data structures
---------------
* `struct child_process`
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:
. Specify 0 to request no special redirection. No new file descriptor
is allocated. The child process simply inherits the channel from the
parent.
. 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
stdout/stderr.
The caller of start_command() must close the so returned FDs
after it has completed reading from/writing to it!
. 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!
. 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 the
parent's stderr (i.e. *not* to what .err or
.no_stderr specify).
To modify the environment of the sub-process, specify an array of
string pointers (NULL terminated) in .env:
. If the string is of the form "VAR=value", i.e. it contains '='
the variable is added to the child process's environment.
. 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.
* `struct async`
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);
. 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.
. data is the value that the caller has specified in the .data member
of struct async.
. 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:
. 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.
. It must not change the program's state that the caller of the
facility also uses.
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