Compiling char array with null terminators in between

Question

I am trying to make a small C utility that reads in a MP3 tag from a file and displays it. I'm trying to make it as efficient as possible, so I'm making all of the char arrays that store the name, title, etc of the song dynamic, so they're sized at run time. I find the size of each 'frame' (a frame is: name, title, etc) through the MP3 ID3 header.

I am starting with the title of the song first. The MP3 file buffer that I loaded with the fopen operation and put into a char buffer looks like this for the title:

T|0|I|0|T|0|I|0|l|0|E,

all in hex of course. As you can see, there are 0's in between the letters of the title, thus in C, meaning a string terminator when trying to store in a char array.

I am trying the follwing:

memcpy( bBuffFirstFrame, &bBuffTag[0x0A+10], iFrameSize );

Where bBuffTag is where all of the ID3 title information is located.

Reading the documentation, it says that memcpy should store everything from the original buffer, including null terminators. However, the char array I am creating (bBuffFirstFrame) only stores the first byte and gets terminated because of the null terminator. So instead of storing the song name "N|0|A|0|M|0|E" (0 is a null), it stores only N.

bBuffFirstFrame is being defined as:

bBuffFirstFrame = ( unsigned char* ) malloc( iFrameSize );

So my first question is, is this the best way even to approach this problem? Should I use malloc to create dynamic arrays or should I just create an array thats really big for each frame? For example:

bBuffFirstFrame[10000]?

As you probably know, the song title changes with each MP3. Another reason I don't want to create a pre-defined size array is that it might cause a buffer overflow for a real big song name.

Final question, is there a way to store null terminators inside of a char array without terminating the string?

Sorry if this sounds confusing. Thanks!

BTW, do the '\0' characters have anything to do with the strings being in UNICODE? — ThomasMcLeod
– ThomasMcLeod, Commented Dec 11, 2011 at 0:02
According to the documentation (id3.org/id3v2.4.0-structure), it says: If nothing else is said, strings, including numeric strings and URLs [URL], are represented as ISO-8859-1 [ISO-8859-1] Good point though. I should look into this. Really new to this stuff — brazc0re
– brazc0re, Commented Dec 11, 2011 at 0:10
@ThomasMcLeod: I think you mean UTF-16 or UCS-2, not Unicode. — R.. GitHub STOP HELPING ICE
– R.. GitHub STOP HELPING ICE, Commented Dec 11, 2011 at 7:06

Matt Lacey · Accepted Answer · 2011-12-11 00:00:52Z

2

The null terminators will only be respected by the string functions, so if you know the size of the strings you're reading etc. you should be fine — are you sure you're using memcpy and not strcpy? I know it sounds silly to ask given what you've posted, but memcpy shouldn't stop at a null terminator.

I would suggest that iFrameSize is actually not holding the value you expect and that is where the problem is coming from

answered Dec 11, 2011 at 0:00

Matt Lacey

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3 Comments

brazc0re Over a year ago

Your right. The whole array was actually stored. In my WATCH window, it only shows the first letter of the title because it's displaying it as a string. I can now manipulate the array to get rid of the null terminators and display the string properly. Thanks!

brazc0re Over a year ago

I checked this by: for( i = 0; i < iFrameSize; i++ ){ printf("%c\n", bBuffFirstFrame[i]); }

Matt Lacey Over a year ago

Ouch, that's a sneaky one! Sometimes with things like this I find viewing the memory contents useful as you can get a better idea of what's going on in there.

ThomasMcLeod · Accepted Answer · 2011-12-11 00:00:05Z

1

You can store data you like in a C array. The problem is that many C runtime function that operate iteratively on strings recognize the first '\0' in the array as the termination character.

answered Dec 11, 2011 at 0:00

ThomasMcLeod

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