I just converted some delphi c to c, in the hope of helping one of my programmers write it in asp. that was disturbingly fun. :)

#include <stdio.h>

bail(unsigned char a, unsigned char b) {
	if (a != b) {
		printf("Invalid JFIF: %x != %x\n",a,b);
		exit(-1);
	}
}

int isin(unsigned char a, const unsigned char* parameterless) {
	int i;
	int found=0;
	for (i=0;i<parameterless[0] && !found;i++) {
		if (a == parameterless[i+1]) found = 1;
	}
	return found;
}

int readword(FILE* foo) {
	unsigned char b; int word=0;
	fread(&b,1,1,foo);
	word = 256 * b;
	fread(&b,1,1,foo);
	word += b;
	return word;
}

main(int argc, char** argv) {
	int i, len, width=-1, height=-1;
	FILE *foo;
	unsigned char b;
	static const unsigned char parameterless[]={8,0x01,0xd1,0xd2,0xd3,0xd4,0xd5,0xd6,0xd7};
	static const unsigned char sig[]={2,0xff,0xd8};
	if (argc != 2) {
		printf("sizeof takes a single argument -- a jpeg filename\n");
		exit(0);
	}
	printf("attempting to open: %s\n",argv[1]);
	foo = fopen(argv[1],"rb");
	if (!foo) {
		printf("ERROR OPENING FILE\n");
	}

	for (i=0;i<sig[0];i++) {
		fread(&b,1,1,foo); 
		printf("%x\n",b);
		bail(b,sig[i+1]);
	}

	while (fread(&b,1,1,foo)) {
		while (b == 0xff) {
			fread(&b,1,1,foo);
			if (b == 0xc0 || b == 0xc1) {
				fseek(foo,3,SEEK_CUR);
				height = readword(foo);
				width = readword(foo);
			} else {
				if (!isin(b,parameterless)) {
					len = readword(foo);
					fseek(foo,len-2,SEEK_CUR);
					fread(&b,1,1,foo);
				} else {
					b = 0xff; // keep going :)
				}
			}
		}
	}
	printf("width: %d, height: %d\n",width,height);
}

All that to get the width/height of a jpeg. I spent close to four hours trying to figure it out on my own, from the JFIF specs (I don't know if this handles the SPIFF spec, but I don't seem to have any JPEGs in SPIFF). And that was after an hour or two just _digging_ for the JFIF spec (not realizing I wanted JFIF as opposed to JPEG, for instance...)