Process

Table of Contents

• Quick Sheet
• Features
• Status
• Description
• Videos
• Blog
  • Process
  • ProcessChain
  • ProcessEnvironment
• Roadmap

🟩 Create child processes and chain them (also usable with Async library)

Process allows launching, chaining input and output, setting working directory and environment variables of child processes.

Quick Sheet

// 1. Execute child process (launch and wait for it to fully execute)
Process().exec({"cmd-exe", "-h"});
//--------------------------------------------------------------------------
// 2. Execute child process, redirecting stdout to a string
SmallString<256> output; // could be also just String
Process().exec({"where-exe", "winver"}, output);
//--------------------------------------------------------------------------
// 3. Launch a child process and explicitly wait for it to finish execution
Process process;
// This is equivalent to process.exec({"1s", "-l"))
process.launch({"ls", "-l"});
//
// ...
// Here you can do I/0 to and from the spawned process
// ...
process.waitForExitSync();
//--------------------------------------------------------------------------
// 4. Execute child process, filling its stdin with a StringView
// This is equivalent of shell command: `echo "child proc" | grep process`
Process().exec({"grep", "process"}, Process::StdOut::Inherit(), "child proc");
//--------------------------------------------------------------------------
// 5. Read process output using a pipe, using launch + waitForExitSync
Process process5;
PipeDescriptor outputPipe;
process5.launch({"executable.exe", "—argument1", "-argument2"}, outputPipe);
String output5 = StringEncoding::Ascii; // Could also use SmallString<N>
File(outputPipe.readPipe).readUntilEOF(output5);
process5.waitForExitSync(); // call process-getExitStatus() for status code
//--------------------------------------------------------------------------
// 6. Executes two processes piping p1 output to p2 input
Process p1, p2;
ProcessChain chain;
chain.pipe(p1, {"echo", "Salve\nDoctori"});
chain.pipe(p2, {"grep", "Doc"});
// Read the output of the last process in the chain
String output6;
chain.exec(output6);
SC_ASSERT_RELEASE(output == "Doctori\n");
//--------------------------------------------------------------------------
// 7. Set an environment var and current directory for child process 
Process process7;
// This child process7 will inherit parent environment variables plus NewEnvVar
SC_TEST_EXPECT(process7.setEnvironment("NewEnvVar", "SomeValue"));
// This child process7 will inherit parent environment variables but we re-define PATH
SC_TEST_EXPECT(process7.setEnvironment("PATH", "/usr/sane_cpp_binaries"));
// Set the current working directory
SC_TEST_EXPECT(process7.setWorkingDirectory("/usr/home"));

Features

Class	Description
SC::Process	Execute a child process with standard file descriptors redirection.
SC::ProcessChain	Execute multiple child processes chaining input / output between them.
SC::ProcessEnvironment	Reads current process environment variables.

Status

🟩 Usable
Library is being used in SC::Plugin and in SC::Tools.

Description

The SC::Process class is used when handling a process in isolation, while the SC::ProcessChain is used when there is need to chain inputs and outputs of multiple processes together.

Videos

This is the list of videos that have been recorded showing some of the internal thoughts that have been going into this library:

• Ep.06 - Posix fork

Blog

Some relevant blog posts are:

• March 2024 Update
• April 2024 Update

Process

Execute a child process with standard file descriptors redirection.
Features:

• Redirect standard in/out/err of a child process to a Pipe
• Inherit child process file descriptors from parent process
• Ignore (silence) child process standard file descriptor
• Wait for the child process exit code

Example: execute child process (launch and wait for it to fully execute)

    // Example: execute child process (launch and wait for it to fully execute)
    SC_TRY(Process().exec({"cmd.exe", "-h"}));

Example: execute child process, redirecting stdout to a string

    // Example: execute child process, redirecting stdout to a string
    SmallString<256> output; // could be also just String
    SC_TRY(Process().exec({"where.exe", "winver"}, output));
    // Output now contains "C:\Windows\System32\winver.exe\n"

Example: launch a child process and wait for it to finish execution

    // Example: launch a child process and explicitly wait for it to finish execution
    Process process;
    SC_TRY(process.launch({"ls", "-l"}));
    // ...
    // Here you can do I/O to and from the spawned process
    // ...
    SC_TRY(process.waitForExitSync());
    // This is equivalent to process.exec({"ls", "-l"})

Example: execute child process, filling its stdin with a StringView

    // Example: execute child process, filling its stdin with a StringView
    // This is equivalent of shell command:
    // `echo "child process" | grep process`
    SC_TRY(Process().exec({"grep", "process"}, Process::StdOut::Inherit(), "child proc"));

Example: read process output using a pipe, using launch + waitForExitSync

    // Example: read process output using a pipe, using launch + waitForExitSync
    Process        process;
    PipeDescriptor outputPipe;
    SC_TRY(process.launch({"executable.exe", "--argument1", "--argument2"}, outputPipe));
    String output = StringEncoding::Ascii; // Could also use SmallString<N>
    SC_TRY(File(outputPipe.readPipe).readUntilEOF(output));
    SC_TRY(process.waitForExitSync());
    // ... Do something with the 'output' string

Example: Add an environment variable

    Process process;
    // This child process will inherit parent environment variables plus NewEnvVar
    SC_TEST_EXPECT(process.setEnvironment("NewEnvVar", "SomeValue"));
    String output;
    // Spawn the child process writing all env variables as KEY=VALUE\n to stdout, redirected to output
    SC_TEST_EXPECT(spawnChildAndPrintEnvironmentVars(process, output));
    // We can check that the NewEnvVar has been set to SomeValue
    SC_TEST_EXPECT(output.view().containsString("NewEnvVar=SomeValue"));
    // PATH env var exists because we are inheriting environment
    SC_TEST_EXPECT(output.view().containsString("PATH="));

Example: Redefine an environment variable

    Process process;
    // This child process will inherit parent environment variables but we re-define PATH
    SC_TEST_EXPECT(process.setEnvironment("PATH", "/usr/sane_cpp_binaries"));
    String output;
    // Spawn the child process writing all env variables as KEY=VALUE\n to stdout, redirected to output
    SC_TEST_EXPECT(spawnChildAndPrintEnvironmentVars(process, output));
    // PATH env var has been re-defined
    SC_TEST_EXPECT(output.view().containsString("PATH=/usr/sane_cpp_binaries"));

Example: Disable environment variable inheritance

    Process process;
    process.inheritParentEnvironmentVariables(false);
    String output;
    // Spawn the child process writing all env variables as KEY=VALUE\n to stdout, redirected to output
    SC_TEST_EXPECT(spawnChildAndPrintEnvironmentVars(process, output));
    // PATH env var doesn't exist because of Process::inheritParentEnvironmentVariables(false)
    SC_TEST_EXPECT(not output.view().containsString("PATH="));

ProcessChain

Execute multiple child processes chaining input / output between them.
Chains multiple child processes together, so that the output of a process becomes input of another (similar to what happens wit the pipe (|) operator on Posix shells).

SC::PipeDescriptor from File library is used to chain read / write endpoints of different processes together.

Example: Inherit stdout file descriptor

    // Executes two processes piping output of process p1 to input of process p2.
    // Then reads the output of the last process in the chain and check its correctness.
    ProcessChain chain;
    Process      p1, p2;
    // Print "Salve\nDoctori" on Windows and Posix and then grep for "Doc"
    StringView expectedOutput;
    switch (HostPlatform)
    {
    case Platform::Windows: {
        expectedOutput = "Doctori\r\n";
        SC_TEST_EXPECT(chain.pipe(p1, {"cmd", "/C", "echo", "Salve", "&", "echo", "Doctori"}));
        SC_TEST_EXPECT(chain.pipe(p2, {"findstr", "Doc"}));
    }
    break;
    default: { // Posix
        expectedOutput = "Doctori\n";
        SC_TEST_EXPECT(chain.pipe(p1, {"echo", "Salve\nDoctori"}));
        SC_TEST_EXPECT(chain.pipe(p2, {"grep", "Doc"}));
    }
    break;
    }
    String output;
    SC_TEST_EXPECT(chain.exec(output));
    SC_TEST_EXPECT(output == expectedOutput);

Example: Read stderr and stdout into a string

    // Executes two processes piping output of process p1 to input of process p2.
    // Reads p2 stdout and stderr into a pair of Strings.
    ProcessChain chain;
    Process      p1;
    StringView   expectedOutput;
    switch (HostPlatform)
    {
    case Platform::Windows: {
        expectedOutput = "C:\\Windows\\System32\\where.exe\r\n";
        SC_TEST_EXPECT(chain.pipe(p1, {"where", "where.exe"}));
    }
    break;
    default: { // Posix
        expectedOutput = "DOCTORI\n";
        SC_TEST_EXPECT(chain.pipe(p1, {"echo", "DOCTORI"}));
    }
    break;
    }
 
    String stdOut(StringEncoding::Ascii);
    String stdErr(StringEncoding::Ascii);
    SC_TEST_EXPECT(chain.exec(stdOut, Process::StdIn::Inherit(), stdErr));
    SC_TEST_EXPECT(stdOut == expectedOutput);
    SC_TEST_EXPECT(stdErr.isEmpty());

Example: Read standard output into a string using a Pipe

    // Chain two processes and read the last stdout into a String (using a pipe)
    ProcessChain chain;
 
    String  output(StringEncoding::Ascii);
    Process p1, p2;
 
    StringView expectedOutput;
    switch (HostPlatform)
    {
    case Platform::Windows: {
        expectedOutput = "WHERE [/R dir] [/Q] [/F] [/T] pattern...\r\n";
        SC_TEST_EXPECT(chain.pipe(p1, {"where", "/?"}));
        SC_TEST_EXPECT(chain.pipe(p2, {"findstr", "dir]"}));
    }
    break;
    default: { // Posix
        expectedOutput = "sbin\n";
        SC_TEST_EXPECT(chain.pipe(p1, {"ls", "/"}));
        SC_TEST_EXPECT(chain.pipe(p2, {"grep", "sbin"}));
    }
    break;
    }
    PipeDescriptor outputPipe;
    SC_TEST_EXPECT(chain.launch(outputPipe));
    SC_TEST_EXPECT(File(outputPipe.readPipe).readUntilEOF(output));
    SC_TEST_EXPECT(chain.waitForExitSync());
    SC_TEST_EXPECT(output.view().startsWith(expectedOutput));

ProcessEnvironment

Reads current process environment variables. Example: Print all environment variables to stdout

    SC::ProcessEnvironment environment;
    for (size_t idx = 0; idx < environment.size(); ++idx)
    {
        StringView name, value;
        (void)environment.get(idx, name, value);
        if (value.isEmpty())
        {
            report.console.printLine(name);
        }
        else
        {
            report.console.print(name);
            report.console.print("=");
            report.console.printLine(value);
        }
    }

Roadmap

🟦 Complete Features:

• To be defined

💡 Unplanned Features:

• None so far