Programming Forums > Application Development > C

tyros · Apr 19th, 2008, 10:44 AM

I m curious abt the header files used in C
i tried reading the code in stdio,h for the function scanf
but i cant understand anything suggest any source for some info regarding this

Jessehk · Apr 19th, 2008, 11:15 AM

Any header files used by C (stdio.h, math.h, stdlib.h, string.h, etc) are all written in C. Where you might be getting confused is that most implementations of the C standard library that I've seen (especially on POSIX systems where there is a lot of added functionality) are extremely complicated.

There are LOTS of macro tricks, compiler tricks (and extensions) and it is generally unreadable.
For example, the character class functions are fairly easy to use:

 c Syntax (Toggle Plain Text) 
#include <stdio.h>
#include <ctype.h>
 
int main( void ) {
    int ch = 'A';
 
    /* is ch an alphabetic character? */
    if ( isalpha( ch ) )
        puts( "\'A\' is alphabetic!" );
 
    return 0;
}
#include <stdio.h>
#include <ctype.h>

int main( void ) {
    int ch = 'A';
    
    /* is ch an alphabetic character? */
    if ( isalpha( ch ) )
        puts( "\'A\' is alphabetic!" );
    
    return 0;
}

But if you check its preprocessed output (with gcc -E in my particular case), you'll see a massive file where the entire contents of the stdio.h file and the ctype.h file (as well as some other stuff that I don't pretend to understand) have been included.

The actual call to isalpha() was replaced with this:

 c Syntax (Toggle Plain Text) 
int main( void ) {
    int ch = 'A';
 
 
    if ( ((*__ctype_b_loc ())[(int) ((ch))] & (unsigned short int) _ISalpha) )
        puts( "\'A\' is alphabetic!" );
 
    return 0;
}
int main( void ) {
    int ch = 'A';


    if ( ((*__ctype_b_loc ())[(int) ((ch))] & (unsigned short int) _ISalpha) )
        puts( "\'A\' is alphabetic!" );

    return 0;
}

I'm pretty sure that the implementation of the character class functions in glibc (the GNU standard C library) is actually a table lookup.

If you think that's bad, try looking at the C++ headers.

EDIT: I should also mention that the definition for functions will not be in the header files. Header files declare functions so that the compiler and linker will be aware of them. The actual source for a function like scanf will likely not be present on your system. Depending on which platform you're on, you can download the source to your C library and find it. It probably won't be easy to find, though.

Narue · Apr 19th, 2008, 12:15 PM

>suggest any source for some info regarding this
http://www.amazon.com/Standard-Libra.../dp/0131315099

If you'd like, I can also give you a sample implementation for Windows. It's greatly simplified just so that beginners will have an easier time than with vendor quality implementations.

tyros · Apr 21st, 2008, 10:12 AM

thnks a lot ppl
and narue plz giv me tht windows impementation u were talking abt

Narue · Apr 21st, 2008, 11:15 AM

Attached are the source files. Note that all of the standard names that I redefine have been prefixed with 'j' or 'J' to avoid conflicts with your compiler. Other parts of the standard library (such as malloc or the ctype functions) are used as-is with the implementation; I don't redefine them. Finally, I haven't had time to deck out the whole thing with detailed comments and documentation, so you'll have to suffer with the current minimal commenting.

If you don't understand the what, why, or how of any piece of code, feel free to start a thread on it.

Apr 19th, 2008, 10:44 AM	#1
tyros Newbie Join Date: Apr 2008 Posts: 2 Rep Power: 0	stdio.h what language is it I m curious abt the header files used in C i tried reading the code in stdio,h for the function scanf but i cant understand anything suggest any source for some info regarding this

Apr 19th, 2008, 11:15 AM	#2
Jessehk The Oblivious One Join Date: May 2005 Location: Ontario, Canada Posts: 630 Rep Power: 4	Re: stdio.h what language is it Any header files used by C (stdio.h, math.h, stdlib.h, string.h, etc) are all written in C. Where you might be getting confused is that most implementations of the C standard library that I've seen (especially on POSIX systems where there is a lot of added functionality) are extremely complicated. There are LOTS of macro tricks, compiler tricks (and extensions) and it is generally unreadable. For example, the character class functions are fairly easy to use: c Syntax (Toggle Plain Text) #include <stdio.h> #include <ctype.h> int main( void ) { int ch = 'A'; /* is ch an alphabetic character? / if ( isalpha( ch ) ) puts( "\'A\' is alphabetic!" ); return 0; } #include <stdio.h> #include <ctype.h> int main( void ) { int ch = 'A'; / is ch an alphabetic character? / if ( isalpha( ch ) ) puts( "\'A\' is alphabetic!" ); return 0; } But if you check its preprocessed output (with gcc -E* in my particular case), you'll see a massive file where the entire contents of the stdio.h file and the ctype.h file (as well as some other stuff that I don't pretend to understand) have been included. The actual call to isalpha() was replaced with this: c Syntax (Toggle Plain Text) int main( void ) { int ch = 'A'; if ( ((__ctype_b_loc ())[(int) ((ch))] & (unsigned short int) _ISalpha) ) puts( "\'A\' is alphabetic!" ); return 0; } int main( void ) { int ch = 'A'; if ( ((__ctype_b_loc ())[(int) ((ch))] & (unsigned short int) _ISalpha) ) puts( "\'A\' is alphabetic!" ); return 0; } I'm pretty sure that the implementation of the character class functions in glibc (the GNU standard C library) is actually a table lookup. If you think that's bad, try looking at the C++ headers. EDIT: I should also mention that the definition for functions will not be in the header files. Header files declare functions so that the compiler and linker will be aware of them. The actual source for a function like scanf will likely not be present on your system. Depending on which platform you're on, you can download the source to your C library and find it. It probably won't be easy to find, though. __________________ Dr. Zoidberg: [ecstatic] I'm going to a movie... with FRIENDS!

Apr 19th, 2008, 12:15 PM	#3
Narue Professional Programmer Join Date: Sep 2005 Posts: 419 Rep Power: 3	Re: stdio.h what language is it >suggest any source for some info regarding this http://www.amazon.com/Standard-Libra.../dp/0131315099 If you'd like, I can also give you a sample implementation for Windows. It's greatly simplified just so that beginners will have an easier time than with vendor quality implementations. __________________ Even if the voices aren't real, they have some pretty good ideas.

Apr 21st, 2008, 10:12 AM	#4
tyros Newbie Join Date: Apr 2008 Posts: 2 Rep Power: 0	Re: stdio.h what language is it thnks a lot ppl and narue plz giv me tht windows impementation u were talking abt

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Programming Forums > Application Development > C

stdio.h what language is it