Exiv2

You are going in the correct direction.

Here's what I would do with my favourite test file (which is only my web-site)

832 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/build $ bin/exiv2 -pa --grep date/i  http://clanmills.com/Stonehenge.jpg
Exif.Image.DateTime                          Ascii      20  2015:07:16 20:25:28
Exif.Photo.DateTimeOriginal                  Ascii      20  2015:07:16 15:38:54
Exif.Photo.DateTimeDigitized                 Ascii      20  2015:07:16 15:38:54
Exif.NikonWt.DateDisplayFormat               Byte        1  Y/M/D
Exif.GPSInfo.GPSDateStamp                    Ascii      11  2015:07:16
Xmp.xmp.ModifyDate                           XmpText    25  2015-07-16T20:25:28+01:00

833 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/build $ bin/exifvalue http://clanmills.com/Stonehenge.jpg Exif.Photo.DateTimeOriginal
2015:07:16 15:38:54
834 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/build $

main.c

// ***************************************************************** -*- C -*-
// main.c (samples/exifvalue.cpp written in C)

#include "wrapper.h" 
#include <stdio.h>

// To build:
// g++ -std=c++98 main.c wrapper.cpp -I/usr/local/include -L/usr/local/lib -lexiv2 -o main
//
// To test:
// ./main http://clanmills.com/Stonehenge.jpg Exif.Photo.DateTimeOriginal
// 2015:07:16 15:38:54
//
int main(int argc, const char* argv[])
{
    const char* prog = argv[0];
    const char* path = argv[1];
    const char* key  = argv[2];
    int         rc   = 0      ;

    if (argc != 3) {
        printf("Usage: %s path key\n",prog);
        rc = 1;
    } else {
       char buffer[100];
       if ( getMetadata(path,key,buffer,sizeof(buffer)) ) {
           printf("%s\n",buffer);
       } else {
           printf("key %s not in %s\n",key,path);
           rc = 2;
       }
    }
    return rc;
}

wrapper.h

#ifdef __cplusplus
extern "C" {
#endif

int getMetadata(const char* path,const char* key,char* buffer,const int buffer_l);

#ifdef __cplusplus
}
#endif

wrapper.cpp

// ***************************************************************** -*- C++ -*-
// wrapper.cpp

#include <exiv2/exiv2.hpp>
#include "wrapper.h" 

extern "C" 
{
    int getMetadata(const char* path,const char* key,char* buffer,int buffer_l) {
        Exiv2::Image::AutoPtr image = Exiv2::ImageFactory::open(path);
        assert(image.get() != 0);
        image->readMetadata();
        Exiv2::ExifData& exifData = image->exifData();
        strcpy(buffer,   exifData[key].toString().c_str());
        return 1;
    }
}

This was quickly written. I have not put exception handling and buffer overflow checks in wrapper.cpp

I'm really busy at the moment working on Exiv2 v0.27 RC1. If you want to discuss this in more detail, can I ask you to wait until next week. I will be happy to screen share with you if you need some one-to-one assistance.

My working code looks like this:

extern "C" 
{
#include "common_header.h" 
#include "lip_image_processing.h" 
#include "lip_image_processing_internal.h" 
}
void exiv2(const char *jpg_filename, IMAGE *jpg_image)
{
    char data[256];
    Exiv2::ExifData::const_iterator exif_data_ptr, begin, end;
    Exiv2::Image::AutoPtr image;

    image = Exiv2::ImageFactory::open(jpg_filename);

    if(!image.get()) 
    {
        image_lib_program_error(__FILE__, __LINE__, __FUNC__,
            "can't read <%s>", jpg_filename);
    }

    image->readMetadata();
    Exiv2::ExifData &exifData = image->exifData();

    if(exifData.empty()) 
    {
        image_lib_program_warning(__FILE__, __LINE__, __FUNC__,
            "Exif data is empty in file <%s>", 
            strip_path_from_filename(jpg_filename));
        return;
    }

    begin    = exifData.begin();
    end        = exifData.end();

    for(exif_data_ptr = begin; exif_data_ptr != end; exif_data_ptr++) 
    {
        strncpy(data, exifData[exif_data_ptr->key()].toString().c_str(), 255);

        switch(exif_data_ptr->tag())
        {
            case EXIF_CAMERA_MODEL_TAG:
                strncpy(jpg_image->CameraModel, data, CAMERA_MODEL_STRING_LENGTH);
            break;

            case EXIF_ORIENTATION_TAG:
                jpg_image->orientation = atoi(data);
            break;

            case EXIF_DATETIME_TAG:
                strncpy(jpg_image->DateTime, data, DATETIME_STRING_LENGTH);
            break;

            default:
            break;
        }
    }

    printf
    (
        "%s\n\tcamera=\"%s\" datetime=\"%s\" orientation=%d\n", 
        jpg_filename, 
        jpg_image->CameraModel, jpg_image->DateTime, 
        jpg_image->orientation
    );
}

I'm a little lost, here. I see you marked this "[Solved]", so I guess we're done. Thank you for using Exiv2.