Run a lambda in parallel processes

forkpool.cpp

#include <functional>
#include <set>
#include <string>
#include <vector>
#include <iostream>
#include <unistd.h>
#include <sys/wait.h>

void
run_all( const std::vector< std::string >& source_files,
         std::function< void( const std::string& ) > f )
{
    std::set< pid_t > pool;
    const size_t pool_max_size = 8;

    for ( size_t i = 0, total = source_files.size( ); i < total; i += 1 )
    {
        // Wait until there's some space in the pool
        while ( pool.size( ) >= pool_max_size )
        {
            int status = 0;
            pid_t exited_child = wait( &status );
            if ( exited_child <= 0 )
                exit( 1 );  // TODO: Shouldn't really happen

            pool.erase( exited_child );
        }

        pid_t pid = fork( );
        if ( pid == 0 )
        {
            // In the child process
            const std::string& file = source_files[ i ];
            std::cout << "[" << i + 1 << "/" << total << "] " << file << std::endl;

            f( file );

            // The child process should terminate here
            return;
        }
        else
        {
            // In the parent process
            pool.insert( pid );
        }
    }

    // Wait for the remaining children to terminate
    int status = 0;
    while (::wait( &status ) > 0 )
    {
    }
}

int
main( )
{
    run_all( {"1", "2", "3"}, []( const std::string& s ) { std::cout << s << std::endl; } );

    return 0;
}