libexif 不支持直接加载 JPG。您需要另一个包来读取 JPG 并提取 EXIF 标头（或者您可以自己编写一些东西）。
请注意，在该示例中，它只是创建了一个新的 exif 标头，然后使用 fwrite 将其保存到文件中，然后在此处的这部分代码的末尾附加了没有 exif 信息的原始 JPG 数据：
/* 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;
}
有一个名为 exifyay 的优秀 Github 项目，它使用 libexif 并有两个额外的库来处理 JPGS 中的读取。这是一个 python 项目，但库的源代码是 C。你可以在这里找到 exifyay（注意我与 exifyay 或 libexif 没有任何关系）
我刚刚编译了 libexif 并将 exifyay 的源代码合并到 VS2010 项目中。文件夹“contrib\examples\LibexifExample”中有一个示例。如果您不喜欢下载随机链接，这里是我工作的代码示例：
/*
* 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;
}