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More How to Program in C

### 7.0 How to use libraries

    Libraries are collections of often used procedures. Even more, these procedures can do very difficult tasks for us, which to implement by ourselves would take hours (days,weeks,months....).

    So, why not to write a example program, which uses a library procdedure, and header files? And with this example we will get an error, which will help us to understand some special context.

    At first, let's have a closer look to a stripped version of our first program (I have left of the code to detect the kind of compiler, you use...)


int main(int argc, char *argv[] )
    printf( "Hello, Hackers!\n" );

    Do you see, what is special with this one? No? Remember the text above!    No clue?  "Before using a function, you have to write down a prototype of it." There are two functions used in this program:


    The prototype of "printf" is defined in the header file


    If you give the command

grep printf /usr/include/stdio.h

    You will see a couple of prototypes, not only of printf itself, but of similar functions.

    What's with main() ?

    This one is special!!! It is the mother/the father of all functions. It is the point where execution of the program starts. There can be only one function of this name but it must be there. So, to be convenient, the prototype of this special function is directly "burned in" the compiler itself.

    Let's change the program to use more than main().  Let's put in the use of header files defined by ourselves and a call to a library function. Change the above source code to:

#include "myprocs.h"

int main(int argc, char *argv[] )
    showme( "How to hack!" );

void showme( char *mywish )
    printf( "%s\n", mywish );
    printf( "%f\n", sin(35.0));

    Save this as "hh2.c" with your editor.    Next use your editor to create a file with the contents:

void showme( char *mystring );

    Save it as "myprocs.h".    Now give the command:

cc hh2.c -o hh2

Time to learn! There are errors.... The output of the compiler looks something like :

    hh2.c: In function `showme':
    hh2.c:14: warning: type mismatch in implicit declaration for
                  built-in function `sin'
    /tmp/cca008671.o: In function `showme':
    /tmp/cca008671.o(.text+0x36): undefined reference to `sin'
    collect2: ld returned 1 exit status

    Let's see what the compiler is trying to tell us.  First line: The compiler tells us that there is something wrong inside the function showme().

    Second line: There is a warning! Be warned! DON'T think: "Oh, this is only a warning, no error, so: proceed!" But a warning is the error of tomorrow. But it is more difficult to find those warning based errors. This warning said, that there is a type mismatch in the implicit call of the built-in function sin.

    Type mismatch?  A type mismatch is if you have ordered a really nice strawberry ice cream and will get a hot dog instead. Or in other words: If a function wants to get a text and you call it with a number. The compiler said, this happened to "sin". Let's have a look. Remember school days. What was it? "sin" stands for sine. And the argument for sine was a floating point number. So we are right here. "It is a compiler error!" Ah, wait! WE are hackers, so WE want to learn by making errors, not the compiler! What happens to the ice cream example above? They gave you a hot dog instead. The reasons? First: You have ordered an ice cream, they gave you a hot dog. They have made an error, and you have learned nothing. Better case: You have mumbled "Strawberry ice cream, please." They have understood "hot dog" and gave you what they think, you want. Your error, you have the chance to learn ;-)

    In this case, we have mumbled "sin(), please". The compiler doesn't understand that correctly, because: There is no prototype for it.

    (Note: An "implicit call" to a function is a call without having informed the compiler before, how it looks like (no prototype ofr it). This goes often hand in hand with "type mismatch" errors or warnings.)

    Where should I get the prototype definition?

    Simply, as in most cases it is absolutely sufficient to type in the command:

man sin

    My Linux box produces this answer:


SIN(3)              Linux Programmer's Manual              SIN(3)

       sin - sine function

       #include <math.h>

       double sin(double x);

       The sin() function returns the sine of x, where x is given
       in radians.

       The sin() function returns a value between -1 and 1.

       SVID 3, POSIX, BSD 4.3, ISO 9899

       acos(3), asin(3), atan(3), atan2(3), cos(3), tan(3)

                           June 8, 1993                         1

    Look at the text after SYNOPSIS.  There is a line with "#include <math.h>" !!! Tada! This instructs the compiler to read the header file "math.h" from the system directory /usr/include/ (or something like that, use the "whereis "math.h" command to get the correct path on your computer for this header) before proceeding.

    Insert such a line after the other #include"-lines in our example and compile it again.

    Now, the compiler don't produces that "mismatch error" again, this one is fixed. But the other one remains:

    /tmp/cca009281.o: In function `showme':
    /tmp/cca009281.o(.text+0x36): undefined reference to `sin'

    Looks very strange....

    While the compiler is translating source code for us, it needs some temporary files to write down things to remember. These files have names like "/tmp/cca009281.o". When the compiler rereads its notes, it realizes that there is a CALL to sin(), but no one has defined this function. And now?

    DON'T PANIC! sin() is part of the math library. Someone else has written down and precompiled the source code for a sine function for us and has included it into the math library.

    Where can I get that library???

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