Develop a RAW image by using ctypes to call libraw.

raw_develop.py

# This file was my first attempt at using libraw with ctypes. For a more
# complete project check out https://github.com/photoshell/rawkit
# Usage: python raw_develop.py some/raw/file.cr2 some/destination/file.ppm


from ctypes import *
import sys


libraw = cdll.LoadLibrary('libraw.so.10')


class ph1_t(Structure):
    _fields_ = [
        ('format', c_int),
        ('key_off', c_int),
        ('t_black', c_int),
        ('black_off', c_int),
        ('split_col', c_int),
        ('tag_21a', c_int),
        ('tag_210', c_float),
    ]


class libraw_iparams_t(Structure):
    _fields_ = [
        ('make', c_char * 64),
        ('model', c_char * 64),
        ('raw_count', c_uint),
        ('dng_version', c_uint),
        ('is_foveon', c_uint),
        ('colors', c_int),
        ('filters', c_uint),
        ('xtrans', c_char * 6 * 6),
        ('cdesc', c_char * 5),
    ]


class libraw_image_sizes_t(Structure):
    _fields_ = [
        ('raw_height', c_ushort),
        ('raw_width', c_ushort),
        ('height', c_ushort),
        ('width', c_ushort),
        ('top_margin', c_ushort),
        ('left_margin', c_ushort),
        ('iheight', c_ushort),
        ('iwidth', c_ushort),
        ('raw_pitch', c_uint),
        ('pixel_aspect', c_double),
        ('flip', c_int),
        ('mask', c_int * 8 * 4),
    ]


class libraw_colordata_t(Structure):
    _fields_ = [
        ('curve', c_ushort * 0x10000),
        ('cblack', c_uint * 4),
        ('black', c_uint),
        ('data_maximum', c_uint),
        ('maximum', c_uint),
        ('white', c_ushort * 8 * 8),
        ('cam_mul', c_float * 4),
        ('pre_mul', c_float * 4),
        ('cmatrix', c_float * 3 * 4),
        ('rgb_cam', c_float * 3 * 4),
        ('cam_xyz', c_float * 4 * 3),
        ('phase_one_data', ph1_t),
        ('flash_used', c_float),
        ('canon_ev', c_float),
        ('model2', c_char * 64),
        ('profile', c_void_p),
        ('profile_length', c_uint),
        ('black_stat', c_uint * 8),
    ]


class libraw_imgother_t(Structure):
    _fields_ = [
        ('iso_speed', c_float),
        ('shutter', c_float),
        ('aperture', c_float),
        ('focal_len', c_float),
        ('timestamp', c_uint), # time_t
        ('shot_order', c_uint),
        ('gpsdata', c_uint * 32),
        ('desc', c_char * 512),
        ('artist', c_char * 64),
    ]


class libraw_thumbnail_t(Structure):
    _fields_ = [
        ('tformat', c_uint), # LibRaw_thumbnail_formats
        ('twidth', c_ushort),
        ('theight', c_ushort),
        ('tlength', c_uint),
        ('tcolors', c_int),
        ('thumb', POINTER(c_char)),
    ]


class libraw_internal_output_params_t(Structure):
    _fields_ = [
        ('mix_green', c_uint),
        ('raw_color', c_uint),
        ('zero_is_bad', c_uint),
        ('shrink', c_ushort),
        ('fuji_width', c_ushort),
    ]


class libraw_rawdata_t(Structure):
    _fields_ = [
        ('raw_alloc', c_void_p),
        ('raw_image', POINTER(c_ushort)),
        ('color4_image', POINTER(c_ushort * 4)),
        ('color3_image', POINTER(c_ushort * 3)),
        ('ph1_black', POINTER(c_short * 2)),
        ('iparams', libraw_iparams_t),
        ('sizes', libraw_image_sizes_t),
        ('ioparams', libraw_internal_output_params_t),
        ('color', libraw_colordata_t),
    ]


class libraw_output_params_t(Structure):
    _fields_ = [
        ('greybox', c_uint * 4),
        ('cropbox', c_uint * 4),
        ('aber', c_double * 4),
        ('gamm', c_double * 6),
        ('user_mul', c_float * 4),
        ('shot_select', c_uint),
        ('bright', c_float),
        ('threshold', c_float),
        ('half_size', c_int),
        ('four_color_rgb', c_int),
        ('highlight', c_int),
        ('use_auto_wb', c_int),
        ('use_camera_wb', c_int),
        ('use_camera_matrix', c_int),
        ('output_color', c_int),
        ('output_profile', POINTER(c_char)),
        ('camera_profile', POINTER(c_char)),
        ('bad_pixels', POINTER(c_char)),
        ('dark_frame', POINTER(c_char)),
        ('output_bps', c_int),
        ('output_tiff', c_int),
        ('user_flip', c_int),
        ('user_qual', c_int),
        ('user_black', c_int),
        ('user_cblack', c_int * 4),
        ('sony_arw2_hack', c_int),
        ('user_sat', c_int),
        ('med_passes', c_int),
        ('auto_bright_thr', c_float),
        ('adjust_maximum_thr', c_float),
        ('no_auto_bright', c_int),
        ('use_fuji_rotate', c_int),
        ('green_matching', c_int),
        ('dcb_iterations', c_int),
        ('dcb_enhance_fl', c_int),
        ('fbdd_noiserd', c_int),
        ('eeci_refine', c_int),
        ('es_med_passes', c_int),
        ('ca_correc', c_int),
        ('cared', c_float),
        ('cablue', c_float),
        ('cfaline', c_int),
        ('linenoise', c_float),
        ('cfa_clean', c_int),
        ('lclean', c_float),
        ('cclean', c_float),
        ('cfa_green', c_int),
        ('green_thresh', c_float),
        ('exp_correc', c_int),
        ('exp_shift', c_float),
        ('exp_preser', c_float),
        ('wf_debanding', c_int),
        ('wf_deband_treshold', c_float * 4),
        ('use_rawspeed', c_int),
        ('no_auto_scale', c_int),
        ('no_interpolation', c_int),
        ('straw_ycc', c_int),
        ('force_foveon_x3f', c_int),
    ]


class libraw_data_t(Structure):
    _fields_ = [
        ('image', POINTER(c_ushort * 4)),
        ('sizes', libraw_image_sizes_t),
        ('idata', libraw_iparams_t),
        ('params', libraw_output_params_t),
        ('progress_flags', c_uint),
        ('process_warnings', c_uint),
        ('color', libraw_colordata_t),
        ('other', libraw_imgother_t),
        ('thumbnail', libraw_thumbnail_t),
        ('rawdata', libraw_rawdata_t),
        ('parent_class', c_void_p),
    ]


# Load the RAW file
libraw.libraw_init.restype = POINTER(libraw_data_t)
data = libraw.libraw_init(0)
libraw.libraw_open_file(data, bytes(sys.argv[1], 'utf-8'))


# Develop the RAW file
libraw.libraw_unpack(data)
libraw.libraw_dcraw_process(data)
libraw.libraw_dcraw_ppm_tiff_writer(data, bytes(sys.argv[2], 'utf-8'))

I added a python convenience layer on top. See here:
https://github.com/paroj/libraw.py/blob/master/libraw.py

@paroj: Check out rawkit (docs); it's what we expanded this Gist into. We'd love to have your help if you'd like to contribute!