Can I create a fileheader using printf?

Hi,

I´m trying to create a aiff to wave converter as a way to learn. So far I´m dealing with the header and have set up a couple of structs that holds both a set of names and variables such as file size, sample rate etc. At the moment I´m using printf to create the new header and pass the output to a new file that I can inspect in a hexeditor.

Code:

The FORM chunk represents the first 12 bytes in a aiff file.

Code:

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        // FORM chunk

        typedef struct {
                char                        *ckID;                // 'FORM'
                int                        ckDataSize;               
                char                        *formType;        // 'AIFF'
        } FormAIFFChunk;

        int size = 0x3E52; // for testing purpose only! *


        // collect size and print FORM chunk

        FormAIFFChunk form = {"FORM", 0x0000, "AIFF"};
        form.ckDataSize = size;
        printf("%s%x%s", form.ckID, form.ckDataSize, form.formType);

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The problem now is that 'size' shows up as a string in the hexeditor and occupy 4 whole bytes, Eventhough it´s a 2 byte value. Is there a way to get around this using printf?

I did an earlier atempt using arrays and memcpy, but using the structs got a lot more clear.

> Is there a way to get around this using printf?
It's binary data, so probably no.

You should be using fwrite() to a file, not printf to stdout.

Hmm, yeah that makes sense I suppose. I was hoping that I could test this part first without having to deal with files directly in the code. I´ll look into fwrite().

Thanks, Salem.

Quote:

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Originally Posted by Subsonics

Hi,

I´m trying to create a aiff to wave converter as a way to learn. So far I´m dealing with the header and have set up a couple of structs that holds both a set of names and variables such as file size, sample rate etc. At the moment I´m using printf to create the new header and pass the output to a new file that I can inspect in a hexeditor.

Code:

The FORM chunk represents the first 12 bytes in a aiff file.

Code:

---


        // FORM chunk

        typedef struct {
                char                        *ckID;                // 'FORM'
                int                        ckDataSize;               
                char                        *formType;        // 'AIFF'
        } FormAIFFChunk;

---

---

I suspect you want this:

Code:

---

                char                        ckID[4];                // 'FORM'
                int                        ckDataSize;               
                char                        formType[4];        // 'AIFF'

---

--
Mats

Quote:

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Originally Posted by matsp

I suspect you want this:

Code:

---

                char                        ckID[4];                // 'FORM'
                int                        ckDataSize;               
                char                        formType[4];        // 'AIFF'

---

--
Mats

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Yes I guess I could use arrays. I came across the use of pointers not so long ago to store strings this way, have no idea how that works though. Do any one of you guys know?

Also, if I type

Code:

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char *var = "var";
char array[4] = {"array"};

printf("%s %s", var, array);

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I get: var array? as a result. Is the questionmark a result of printf printing the closing character of the array?

It's the result of you trying to put 5 chars into a 4-char array, and losing an all important \0 to mark the end of the string.

Code:

---

char *var = "var";
char array[4] = {"array"};

printf("%s %s", var, array);

---

The array variable should not be allowed (at least not without warning) by the compiler, as it is a 5 character long array. Since you only have 4 elements in the array, that is longer than the space you have allowed for it - not a good idea. Since strings (in C) are defined as a sequence of characters followed by a zero-character (not the '0', but '\0' or 0) - since your 4 character array containing 5 characters does not have any space for this terminating zero, printf will keep going until the next zero that happens to be in memory. In this case you were lucky that it was only one extra character before that happens to be.

var is a pointer to a character. For use with printf, it makes little difference, but if you were to use it inside a struct, and then use fwrite to store the whole struct to a file, you would find that you are storing the address of the string, rather than the content of the string - which certainly won't work if you read it back with a different program, because that memory will absolutely not contain the string you want.

--
Mats

Quote:

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Originally Posted by Salem

It's the result of you trying to put 5 chars into a 4-char array, and losing an all important \0 to mark the end of the string.

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Oops, that is a typo I made here actually. I do get the same result with array[5] though.

Quote:

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Originally Posted by matsp

var is a pointer to a character. For use with printf, it makes little difference,

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Ok, but why does it work in this example? Where are the two extra characters stored? Is it safe to store strings this way?

Here is the complete code snippet that get this behaviour BTW.

Code:

---

#include <stdio.h>

int main(int argc, char *argv[])
{
        char *test = "test";
        char array[5] = {"array"};

        printf("test: %s, array: %s\n", test, array);

        return 0;
}

---

When I compile and run this I get:

test: test, array: array?

Quote:

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Originally Posted by Subsonics

Ok, but why does it work in this example? Where are the two extra characters stored? Is it safe to store strings this way?

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It works, because printf gets the address of array, and the address stored in var - both results in the address of a sequence of characters. And as there is a zero SOMEWHERE after the string value "array", the printf stops after writing some "rubbish" (which is the content that happened to be in that location in memory - change your code a bit, and you will get a different result).

I'm not sure which "two extra characters" you are talking about - do you mean the 'y' from the "array" - it is store immediately after the 4 that you have given. Note however for printf to print "array", you really need 6 characters in your array, since you also need to have space for the terminating zero character - otherwise you will still have the problem that printf continues to print after the end of your string, whatever happens to be at that location, until the next zero it can find - in extreme cases, that could be megabytes of data!!

Note also that for your AIFF header, you DO NOT want a terminating zero - it is four characters that contain 'A', 'I', 'F' and 'F' - it is technically (from a C standpoint) not a string, and you should not try to print that with %s in printf [without copying to a longer char array (at least 5 long) and putting a zero at position [4] (fifth character)]

It is safe to store strings in char arrays, as long as they have the right size.

--
Mats

Quote:

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Originally Posted by matsp

.
I'm not sure which "two extra characters" you are talking about - do you mean the 'y' from the "array" - it is store immediately after the 4 that you have given.

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I´m talking abot the characters 'a' and 'r' in var, since *var is a pointer to a char I guess there should only be reserved room for one character in this case 'v', still however I´m able to store three characters and print them. The array was a typo I made here, in my code it is 5 char long.

Quote:

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Originally Posted by Subsonics

Oops, that is a typo I made here actually. I do get the same result with array[5] though.

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You don't have room for any characters there, so all of them are overwriting somebody else's memory.

Quote:

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Originally Posted by Subsonics

I´m talking abot the characters 'a' and 'r' in var, since *var is a pointer to a char I guess there should only be reserved room for one character in this case 'v', still however I´m able to store three characters and print them. The array was a typo I made here, in my code it is 5 char long.

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But a string in C is a sequence of characters that end with a zero - so pointer to char will point to the FIRST of those characters. In the case of "var" in your code, it will be stored somewhere in the program's memory as a sequence of four characters, 'v', 'a', 'r', '\0' - where this is stored is entirely up to the compiler, but generally strings are (nowadays) stored in a read-only data part of your executable file, and loaded into memory just like the code and any pre-initialized data.

Note that "char *var" in itself does not allow even ONE character to be stored. It is a place where you can store the ADDRESS of a character - when you initialize that string with "var", it is given the address that the compiler put those characters in.

--
Mats

Quote:

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Originally Posted by tabstop

You don't have room for any characters there, so all of them are overwriting somebody else's memory.

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How come? If I create an array to store 5 chars is there another way of doing it? I did like this

Code:

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char array[5];

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Quote:

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Originally Posted by Subsonics

How come? If I create an array to store 5 chars is there another way of doing it? I did like this

Code:

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char array[5];

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Yes, that allows 5 characters to be stored. I think tabstop was referring to your pointer variable, but I'm not sure.

--
Mats

Quote:

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Originally Posted by matsp

But a string in C is a sequence of characters that end with a zero - so pointer to char will point to the FIRST of those characters. In the case of "var" in your code, it will be stored somewhere in the program's memory as a sequence of four characters, 'v', 'a', 'r', '\0' - where this is stored is entirely up to the compiler, but generally strings are (nowadays) stored in a read-only data part of your executable file, and loaded into memory just like the code and any pre-initialized data.

Note that "char *var" in itself does not allow even ONE character to be stored. It is a place where you can store the ADDRESS of a character - when you initialize that string with "var", it is given the address that the compiler put those characters in.

--
Mats

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Ok, thanks for clarifying Mats.

Whats your oppinion about the use of char pointers to point to a complete string? Is it considered kosher generally or is it best avoided?

Quote:

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Originally Posted by Subsonics

Ok, thanks for clarifying Mats.

Whats your oppinion about the use of char pointers to point to a complete string? Is it considered kosher generally or is it best avoided?

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Char pointers that point to constant strings is fine (generally speaking - however, for the structure you described in the initial post here, you DO need to have a char array). You should really use "const char *var = "Some string";" - that way the compiler will tell you if you try to modify such a string - which will most likely cause some sort of problem - strange results later on or a crash.

Char pointers to variable content is more complex, as you need to somehow allocate memory, and both that and char arrays requires that you know the maximum length of the final content (and that you validate that you are not going beyond that maximum length, as that will store data into "something elses memory" which leads to corrupted data content and/or a crash - or if you are really "lucky" crash the program directly as you go beyond the limit).

--
Mats

Quote:

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Originally Posted by matsp

Yes, that allows 5 characters to be stored. I think tabstop was referring to your pointer variable, but I'm not sure.

--
Mats

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Yes indeed I was. Somehow I thought I saw code where you declared the pointer on one line and then assigned it on the next, but I don't see that now, which means I must have been hallucinating. Sorry.

Thanks for the help guys. Mats I have updated my code to use arrays instead of pointers now and it seems like a good habit to use 'const char' when using pointers this way as you mentioned, thanks.