How to edit EXIF tag in C?

Question

I have a JPG file that was taken using BlackBerry 10 Dev Alpha. This code (a slightly modified version of this example) prints the result correctly

static char* read_tag(ExifData *ed, ExifIfd eid, ExifTag tag){
static char result[1024];
ExifEntry *entry = exif_content_get_entry(ed->ifd[eid], tag);

if (entry){
    char buf[1024];

    exif_entry_get_value(entry, buf, sizeof(buf));
    trim_spaces(buf);

    if (*buf) strcpy(result, buf);
    else strcpy(result, "NULL");
}
else strcpy(result, "NULL");

return result;
}

Which means the output of:

printf("Model : %s\n", read_tag(ed, EXIF_IFD_0, EXIF_TAG_MODEL));

is:

Model : BlackBerry 10 Dev Alpha

Now I wonder how to replace "BlackBerry 10 Dev Alpha" (EXIF_TAG_MODEL) with another value, e.g "Nokia 3330". I already take a look at another example . Unfortunately I found it quite hard to read. Maybe someone has a shorter/straightforward code?

Answer 1

libexif doesn't support directly loading JPG's in. You'll need another package to read in the JPG and extract the EXIF header (or you could write something yourself).

Note that in the example it simply creates a new exif header, then saves it to file using fwrite, and then appends the raw JPG data without exif information on the end in this part of the code here:

 /* 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;
            }

There is an excellent Github project called exifyay that uses libexif and has two extra libs that handle reading in JPGS. It is a python project but the sources for the libraries are C. You can find exifyay here (note I am not involved in any way with exifyay or libexif)

I have just recently compiled libexif and merged sources from exifyay into a VS2010 project here. There is an example in the folder 'contrib\examples\LibexifExample'. If you don't like downloading random links here is a sample of the code I got working:

/*
 * 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>
#include <libjpeg/jpeg-data.h>
#include <JpegEncoderEXIF/JpegEncoderEXIF.h>


/* 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"

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 = (unsigned char*)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)
{

    ExifEntry *entry;

    //Input JPG
    char mInputFilename[]="example.jpg";

    //Load JPG
    JPEGData * mJpegData = jpeg_data_new_from_file(mInputFilename);

    //Load Exif data from JPG
    ExifData * mExifData = jpeg_data_get_exif_data(mJpegData);

    //Set some Exif options
    exif_data_set_option(mExifData, EXIF_DATA_OPTION_FOLLOW_SPECIFICATION);
    exif_data_set_data_type(mExifData, EXIF_DATA_TYPE_COMPRESSED);
    exif_data_set_byte_order(mExifData, FILE_BYTE_ORDER);

    entry = create_tag(mExifData, 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(mExifData, EXIF_IFD_EXIF, EXIF_TAG_SUBJECT_AREA,
               4 * exif_format_get_size(EXIF_FORMAT_SHORT));
    entry->format = EXIF_FORMAT_SHORT;
    entry->components = 4;

    //Write back exif data
    jpeg_data_set_exif_data(mJpegData,mExifData);

    //Save to JPG
    jpeg_data_save_file(mJpegData,"test.jpg");

    return 0;
}