Write exif data to jpg using libexif

Question

I'm trying to use libexif to write exif data into a jpeg file. I'm pretty much following the example provided with it. I can understand all the Exif data blocks being written and everything, but how do I load the jpeg data like they suggested? (using libjpeg). I couldn't find a function that would directly let me import the jpeg as char. My code works fine if I include a dummy jpg with it like shown below.

/*
 * write-exif.c
 *
 * Placed into the public domain by Daniel Fandrich
 *
 * Create a new EXIF data block and write it into a JPEG image file.
 *
 * The JPEG image data used in this example is fixed and is guaranteed not
 * to contain an EXIF tag block already, so it is easy to precompute where
 * in the file the EXIF data should be. In real life, a library like
 * libjpeg (included with the exif command-line tool source code) would
 * be used to write to an existing JPEG file.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <libexif/exif-data.h>

/* this file will be unilaterally overwritten */
#define FILE_NAME "write-exif.jpg"

/* raw JPEG image data */
static const unsigned char image_jpg[] = {
  0xff, 0xd8, 0xff, 0xe0, 0x00, 0x10, 0x4a, 0x46, 0x49, 0x46, 0x00, 0x01,
  0x01, 0x01, 0x00, 0x48, 0x00, 0x48, 0x00, 0x00, 0xff, 0xdb, 0x00, 0x43,
  0x00, 0x14, 0x0e, 0x0f, 0x12, 0x0f, 0x0d, 0x14, 0x12, 0x10, 0x12, 0x17,
  0x15, 0x14, 0x18, 0x1e, 0x32, 0x21, 0x1e, 0x1c, 0x1c, 0x1e, 0x3d, 0x2c,
  0x2e, 0x24, 0x32, 0x49, 0x40, 0x4c, 0x4b, 0x47, 0x40, 0x46, 0x45, 0x50,
  0x5a, 0x73, 0x62, 0x50, 0x55, 0x6d, 0x56, 0x45, 0x46, 0x64, 0x88, 0x65,
  0x6d, 0x77, 0x7b, 0x81, 0x82, 0x81, 0x4e, 0x60, 0x8d, 0x97, 0x8c, 0x7d,
  0x96, 0x73, 0x7e, 0x81, 0x7c, 0xff, 0xc0, 0x00, 0x0b, 0x08, 0x00, 0x40,
  0x00, 0x40, 0x01, 0x01, 0x11, 0x00, 0xff, 0xc4, 0x00, 0x1b, 0x00, 0x00,
  0x03, 0x01, 0x01, 0x01, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x05, 0x06, 0x04, 0x03, 0x07, 0x02, 0x01, 0xff,
  0xc4, 0x00, 0x2f, 0x10, 0x00, 0x01, 0x03, 0x03, 0x02, 0x05, 0x03, 0x03,
  0x03, 0x05, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x02, 0x03, 0x11,
  0x00, 0x04, 0x05, 0x21, 0x31, 0x06, 0x12, 0x41, 0x51, 0x81, 0x61, 0x71,
  0x91, 0x13, 0x32, 0xa1, 0x14, 0x22, 0xc1, 0x15, 0x23, 0x52, 0xd1, 0xf0,
  0xff, 0xda, 0x00, 0x08, 0x01, 0x01, 0x00, 0x00, 0x3f, 0x00, 0xb3, 0xa2,
  0x8a, 0x28, 0xa2, 0x8a, 0x28, 0xa2, 0x97, 0x64, 0x72, 0xd6, 0x58, 0xd4,
  0x8f, 0xd5, 0x3d, 0xca, 0xa5, 0x7d, 0xa8, 0x4e, 0xaa, 0x3e, 0xb0, 0x3a,
  0x7a, 0x9d, 0x2b, 0x2d, 0x87, 0x13, 0xe3, 0xaf, 0x9f, 0x0c, 0xb6, 0xb5,
  0xb6, 0xe2, 0xb4, 0x48, 0x71, 0x30, 0x14, 0x7b, 0x02, 0x09, 0x14, 0xee,
  0x94, 0x64, 0x38, 0x87, 0x1f, 0x8d, 0x5f, 0xd3, 0x7d, 0xe2, 0xa7, 0x46,
  0xe8, 0x6c, 0x49, 0x1e, 0xfd, 0x07, 0xb1, 0x33, 0x5f, 0xb8, 0xdc, 0xf5,
  0x96, 0x51, 0x7c, 0x96, 0xee, 0x14, 0xba, 0x04, 0xf2, 0x2c, 0x42, 0xa3,
  0xd3, 0x70, 0x7c, 0x1a, 0x6d, 0x45, 0x2c, 0xcd, 0xe5, 0x11, 0x89, 0xb1,
  0x5b, 0xc4, 0x02, 0xe1, 0xd1, 0xb4, 0xcf, 0xdc, 0xa3, 0xfc, 0x0d, 0xcd,
  0x41, 0xde, 0xe3, 0xb2, 0x2e, 0xda, 0x1c, 0xbd, 0xe2, 0x4a, 0x90, 0xe9,
  0x04, 0x95, 0x2b, 0xf7, 0x41, 0xd0, 0x18, 0xe8, 0x36, 0x03, 0xc6, 0x91,
  0x4a, 0x81, 0x20, 0x82, 0x24, 0x10, 0x66, 0x45, 0x5a, 0xdd, 0xf1, 0x32,
  0xff, 0x00, 0xa2, 0x5a, 0x8b, 0x62, 0x57, 0x7d, 0x70, 0x80, 0x93, 0x1a,
  0x94, 0x10, 0x79, 0x4a, 
};

/* length of data in image_jpg */
static const unsigned int image_jpg_len = sizeof(image_jpg);

/* dimensions of image */
static const unsigned int image_jpg_x = 64;
static const unsigned int image_jpg_y = 64;

/* start of JPEG image data section */
static const unsigned int image_data_offset = 20;
#define image_data_len (image_jpg_len - image_data_offset)

/* raw EXIF header data */
static const unsigned char exif_header[] = {
  0xff, 0xd8, 0xff, 0xe1
};
/* length of data in exif_header */
static const unsigned int exif_header_len = sizeof(exif_header);

/* byte order to use in the EXIF block */
#define FILE_BYTE_ORDER EXIF_BYTE_ORDER_INTEL

/* comment to write into the EXIF block */
#define FILE_COMMENT "libexif demonstration image"

/* special header required for EXIF_TAG_USER_COMMENT */
#define ASCII_COMMENT "ASCII\0\0\0"


/* Get an existing tag, or create one if it doesn't exist */
static ExifEntry *init_tag(ExifData *exif, ExifIfd ifd, ExifTag tag)
{
    ExifEntry *entry;
    /* Return an existing tag if one exists */
    if (!((entry = exif_content_get_entry (exif->ifd[ifd], tag)))) {
        /* Allocate a new entry */
        entry = exif_entry_new ();
        assert(entry != NULL); /* catch an out of memory condition */
        entry->tag = tag; /* tag must be set before calling
                 exif_content_add_entry */

        /* Attach the ExifEntry to an IFD */
        exif_content_add_entry (exif->ifd[ifd], entry);

        /* Allocate memory for the entry and fill with default data */
        exif_entry_initialize (entry, tag);

        /* Ownership of the ExifEntry has now been passed to the IFD.
         * One must be very careful in accessing a structure after
         * unref'ing it; in this case, we know "entry" won't be freed
         * because the reference count was bumped when it was added to
         * the IFD.
         */
        exif_entry_unref(entry);
    }
    return entry;
}

/* Create a brand-new tag with a data field of the given length, in the
 * given IFD. This is needed when exif_entry_initialize() isn't able to create
 * this type of tag itself, or the default data length it creates isn't the
 * correct length.
 */
static ExifEntry *create_tag(ExifData *exif, ExifIfd ifd, ExifTag tag, size_t len)
{
    void *buf;
    ExifEntry *entry;

    /* Create a memory allocator to manage this ExifEntry */
    ExifMem *mem = exif_mem_new_default();
    assert(mem != NULL); /* catch an out of memory condition */

    /* Create a new ExifEntry using our allocator */
    entry = exif_entry_new_mem (mem);
    assert(entry != NULL);

    /* Allocate memory to use for holding the tag data */
    buf = exif_mem_alloc(mem, len);
    assert(buf != NULL);

    /* Fill in the entry */
    entry->data = buf;
    entry->size = len;
    entry->tag = tag;
    entry->components = len;
    entry->format = EXIF_FORMAT_UNDEFINED;

    /* Attach the ExifEntry to an IFD */
    exif_content_add_entry (exif->ifd[ifd], entry);

    /* The ExifMem and ExifEntry are now owned elsewhere */
    exif_mem_unref(mem);
    exif_entry_unref(entry);

    return entry;
}

int main(int argc, char **argv)
{
    int rc = 1;
    FILE *f;
    unsigned char *exif_data;
    unsigned int exif_data_len;
    ExifEntry *entry;
    ExifData *exif = exif_data_new();
    if (!exif) {
        fprintf(stderr, "Out of memory\n");
        return 2;
    }

    /* Set the image options */
    exif_data_set_option(exif, EXIF_DATA_OPTION_FOLLOW_SPECIFICATION);
    exif_data_set_data_type(exif, EXIF_DATA_TYPE_COMPRESSED);
    exif_data_set_byte_order(exif, FILE_BYTE_ORDER);

    /* Create the mandatory EXIF fields with default data */
    exif_data_fix(exif);

    /* All these tags are created with default values by exif_data_fix() */
    /* Change the data to the correct values for this image. */
    entry = init_tag(exif, EXIF_IFD_EXIF, EXIF_TAG_PIXEL_X_DIMENSION);
    exif_set_long(entry->data, FILE_BYTE_ORDER, image_jpg_x);

    entry = init_tag(exif, EXIF_IFD_EXIF, EXIF_TAG_PIXEL_Y_DIMENSION);
    exif_set_long(entry->data, FILE_BYTE_ORDER, image_jpg_y);

    entry = init_tag(exif, EXIF_IFD_EXIF, EXIF_TAG_COLOR_SPACE);
    exif_set_short(entry->data, FILE_BYTE_ORDER, 1);

    /* Create a EXIF_TAG_USER_COMMENT tag. This one must be handled
     * differently because that tag isn't automatically created and
     * allocated by exif_data_fix(), nor can it be created using
     * exif_entry_initialize() so it must be explicitly allocated here.
     */
    entry = create_tag(exif, EXIF_IFD_EXIF, EXIF_TAG_USER_COMMENT, 
            sizeof(ASCII_COMMENT) + sizeof(FILE_COMMENT) - 2);
    /* Write the special header needed for a comment tag */
    memcpy(entry->data, ASCII_COMMENT, sizeof(ASCII_COMMENT)-1);
    /* Write the actual comment text, without the trailing NUL character */
    memcpy(entry->data+8, FILE_COMMENT, sizeof(FILE_COMMENT)-1);
    /* create_tag() happens to set the format and components correctly for
     * EXIF_TAG_USER_COMMENT, so there is nothing more to do. */

    /* Create a EXIF_TAG_SUBJECT_AREA tag */
    entry = create_tag(exif, EXIF_IFD_EXIF, EXIF_TAG_SUBJECT_AREA,
               4 * exif_format_get_size(EXIF_FORMAT_SHORT));
    entry->format = EXIF_FORMAT_SHORT;
    entry->components = 4;
    exif_set_short(entry->data, FILE_BYTE_ORDER, image_jpg_x / 2);
    exif_set_short(entry->data+2, FILE_BYTE_ORDER, image_jpg_y / 2);
    exif_set_short(entry->data+4, FILE_BYTE_ORDER, image_jpg_x);
    exif_set_short(entry->data+6, FILE_BYTE_ORDER, image_jpg_y);

    /* Get a pointer to the EXIF data block we just created */
    exif_data_save_data(exif, &exif_data, &exif_data_len);
    assert(exif_data != NULL);

    f = fopen(FILE_NAME, "wb");
    if (!f) {
        fprintf(stderr, "Error creating file %s\n", FILE_NAME);
        exif_data_unref(exif);
        return rc;
    }
    /* Write EXIF header */
    if (fwrite(exif_header, exif_header_len, 1, f) != 1) {
        fprintf(stderr, "Error writing to file %s\n", FILE_NAME);
        goto errout;
    }
    /* Write EXIF block length in big-endian order */
    if (fputc((exif_data_len+2) >> 8, f) < 0) {
        fprintf(stderr, "Error writing to file %s\n", FILE_NAME);
        goto errout;
    }
    if (fputc((exif_data_len+2) & 0xff, f) < 0) {
        fprintf(stderr, "Error writing to file %s\n", FILE_NAME);
        goto errout;
    }
    /* Write EXIF data block */
    if (fwrite(exif_data, exif_data_len, 1, f) != 1) {
        fprintf(stderr, "Error writing to file %s\n", FILE_NAME);
        goto errout;
    }
    /* Write JPEG image data, skipping the non-EXIF header */
    if (fwrite(image_jpg+image_data_offset, image_data_len, 1, f) != 1) {
        fprintf(stderr, "Error writing to file %s\n", FILE_NAME);
        goto errout;
    }
    printf("Wrote file %s\n", FILE_NAME);
    rc = 0;

errout:
    if (fclose(f)) {
        fprintf(stderr, "Error writing to file %s\n", FILE_NAME);
        rc = 1;
    }
    /* The allocator we're using for ExifData is the standard one, so use
     * it directly to free this pointer.
     */
    free(exif_data);
    exif_data_unref(exif);

    return rc;
}

Answer 1

You can just use the fopen function to load the specific jpg. and then write the exif data into it.