Let us take the first one as an example. We read in a $12-$byte TIFF tag and find that its identifier

field is $0$x$010$F$.$ Its data type field will be $2,$ which means that the data is encoded in an ASCII

string. The number of data items field will tell us how many bytes our string has.

The final field in the tag can contain the value of the data itself if it fits in $4$ bytes, or it can

contain an offset to the data elsewhere in the file. Since an arbitrary string cannot fit in $4$ bytes,

in our case this value is an offset. It$$s an offset from the beginning of the TIFF header, which

occurred at byte $12$ of the file. So we seek to $12+$ offset in the file and read each letter from

that position in the file, until we have read them all $($we$$ll encounter a NUL terminator at the

end$).$

At this point, we$$ve read the manufacturer string. We must seek back to the location in the file

where we were reading tags and continue on to the next one.

To make this concrete, say we encounter our Manufacturer String tag while reading bytes $22-34$

of our file. The tag would be:

$0$x$010$F $26158$

The $6$ tells us how many bytes the string will be $($including the NUL terminator$).$ The $158$ tells us

to seek to $158+12$ bytes from the start of the file $($the $12$ bytes is the offset of the TIFF header

from the start of the file$).$

When we seek to offset $170,$ we read in $$Canon$,$ the manufacturer of the camera.

We must now seek back to offset $34$ so that we can read the next tag in this section.

If we encounter the $0$x$8769$ identifier, there is an additional Exif block elsewhere in the file. We

can stop reading at this point even if we haven$$t read all count tags because the TIFF format

states that all identifiers must be in sorted order.

We will seek to the offset specified in this Exif sub block tag, again $+12$ bytes. There, we$$ll repeat

the above process one more time to get more specific information about the picture.

First, read in a new count as an unsigned short. Next, loop, reading more $12-$byte TIFF tags from

the file.

$4$

This time, we$$ll be concerned with the following fields:

Tag identifier Data Type Description

$0$xA$0024(32-$bit integer$)$ Width in pixels

$0$xA$0034(32-$bit integer$)$ Height in pixels

$0$x$88273(16-$bit integer$)$ ISO speed

$0$x$829$a $5($fraction of $232-$bit unsigned integers$)$ Exposure speed

$0$x$829$d $5($fraction of $232-$bit unsigned integers$)$ F$-$stop

$0$x$920$A $5($fraction of $232-$bit unsigned integers$)$ Lens focal length

$0$x$90032($ASCII String$)$ Date taken

The good news is that type $4$ means that the value is directly encoded in the last $4$ bytes of our

tag and no seeking needs to be done.

Type $5$ requires us to behave like we did with the string, but rather than reading several single$-$

byte characters, we will read $2$ unsigned ints. Display the ratio of the two numbers as shown in

the example below.

What To Do

For your project you will make a utility that can print the contents of an existing tag, if there.

Make a program called exifview and make it so that it runs with the following command line:

$./$exifview FILENAME

It should print the contents of the EXIF tag to the console if present, or give a message if not

present or readable by our program.

Output

$ $./$exifview img$1.$jpg

Manufacturer: Canon

Model: Canon EOS REBEL SL$1$

Exposure Time: $1/80$ second

F$-$stop: f$/2\mathrm{.}8$

ISO: ISO $2000$

Date Taken: $2013$:$08$:$2022$:$23$:$45$

Focal Length: $40$ mm

Width: $512$ pixels

Height: $768$ pixels

Hints and Requirements

$$ We need to treat these files as binary files rather than text files. Make sure to open the

file correctly, and to use fread for I$/$O$.$

$5$

$$ You must use a structure to represent a JPEG$/$TIFF$/$EXIF header and another struct to

represent a TIFF tag. Do NOT use a bunch of disjoint variables. Do your fread$()$ with the

whole structure at once. $($That is$,$ read an entire tag in one file operation.$)$

$$ If the header field does not contain the Exif string in the right place, print an error

message that the tag was not found. If the TIFF header contains MM instead of II$,$ print

an error message that we do not support the endianness.