This is going to be so helpful in the compress-pdf application :D

compress-pdf-gs.cpp

#ifndef __COMPRESS_H__
#define __COMPRESS_H__

#include "iapi.h"
#include <string>
#include <cstring>
#include <iostream>
#include <errno.h>

static int GSDLLCALL gs_stdout
(void *caller_handle, const char* buf, int len)
{
    return len;
}

static int GSDLLCALL gs_stderr
(void *caller_handle, const char* buf, int len)
{
    return len;
}

static int GSDLLCALL gs_stdin
(void *caller_handle, char *buf, int len)
{
    return len;
}

namespace PDFTool {
    int compress (char*);
}

int PDFTool::compress
(char *input_file, char *output_file)
{
    void *gs = nullptr;

    char *gsargs[] = {
        "",
        "-sDEVICE=pdfwrite",
        "-dPDFSETTINGS=/ebook",
        "-dNOPAUSE",
        "-dQUIET",
        "-dBATCH",
        "-sOutputFile=\"output.pdf\"",
        input_file
    };

    int ret_code = gsapi_new_instance (&gs, NULL);
    if (ret_code < 0)
        return EXIT_FAILURE;

    gsapi_set_stdio (gs, gs_stdin, gs_stdout, gs_stderr);
    ret_code = gsapi_set_arg_encoding (gs, GS_ARG_ENCODING_UTF8);
    if (ret_code)
        ret_code = gsapi_init_with_args (gs, 8, gsargs);
    
    gsapi_delete_instance(gs);
    gsapi_exit(gs);

    return ret_code;
}

#endif