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I need to recode the printf function using only a handful of functions from the libc library, namely write, malloc and free and a few other basic ones. The only formatting tab that I am having trouble with is %p as I have no idea how to display it using the write function, as passing the address of my pointer to a function that prints strings resulted in empty output. I have also tried typecasting it to a char * and unsigned char * but that did not work either. I have looked online but only found solutions invloving forbidden functions so I am not even sure where to begin.
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I rarely log on and am very new to the C language, therefore I apologize if this is a duplicate question or if this is a silly query.
I'm currently learning C and am hitting a wall with strings. I understand that char arrays are used in place of strings in the language. My question is, is there a better way than to assign an arbitrary value when declaring a char[] for user input(i.e setting the size of the array to one value when the user might enter more or less than that amount of characters)?
If you're on a POSIX system (basically anything that's not Windows), you can use getline(3) to do this. It will automatically allocate a buffer of the right size for you. Otherwise, you'll have to guess a length, allocate that, then read the input up to that length, and if you guessed wrong, use realloc to increase your guess and try again.
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I saw this special macro when I read a source code. If I remember correctly, it is defined in the standard library.
The name of this macro is related to the buffer size, and in my machine its implementation is 1024.
Now I want to use it to initialize the buffer but I forgot what it is called.
So is there any one who can help me make my code look more professional?
If I don't know what I am looking for specifically, how can I clearly say what I need?
Are you talking about BUFSIZ? It's a macro provided by <stdio.h> and it expands to the size of the buffer used by setbuf().
I'm not sure what use it has in your own code.
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Need to read everything until say *** comes up:
Input:
Hey there
how are
you
***
Output:
Hey there
how are
you
Would have used scanf("%[^***]s) but can't read all lines at once.
Only having basic C knowledge
The way I would do this is read one line at a time (with a funcion such as fgets instead of scanf) and then see if the line that you last read is equal to ***. You can use use strcmp to do that but you can also do it by hand if you are not allowed to use strcmp for some reason.
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Can anybody explain the variations in printf syntax..i am confused in some of it
like
printf(5+"hello my friend");//i have problem this addition of 5
printf("hello""my""friend");//i have problem with double apostrophe
what kind of printf prototype do these follows ?
Is this have anything to do with dynamic linking?
Can anybody show some other weird printfs and explain them.
A string in C is accessed via a pointer to a char (see H2CO3's comment for a more precise definition). If you add 5 to a pointer to a char, you start the string 5 characters later. So 5+"hello my friend" points to " my friend", skipping "hello".
When a C compiler sees two strings with nothing (except possibly whitespace) in between, it treats them as a single string. This makes it easier to break long strings in multiple lines.
So "hello""my""friend" compiles into exactly the same thing as "hellomyfriend" or
"hello"
"my"
"friend"
None of these has anything to do with printf, and a lot to do with strings.
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I am writing a system call function (in FreeBSD). As you may know the return type of such a function is an int which only indicates whether the syscall has been successful or not and the value which the caller has asked for is returned through setting:
td->td_retval[0]= my_requires_value;
where td is of type thread* and td_retval[0] is an int.
My problem is that my required value is a char []. How can I manage this? What should I set td->td_retval[0] as here in the syscall and how can I retrieve it back in the caller function?
I'm not very familiar with C and its pointers but I feel like this can be accomplished through using the right pointers and addresses.
If you want to return a string, your function should have an argument that is a pointer to a pre-allocated buffer in user space that will take the string (and another argument with the maximum size). The return value is only used to report errors, e.g. that the provided buffer is too small.
See this example, it may help you to design your syscall.