Writing and Compiling C Program in Linux Part 2: Using Custom Library

• Instead of building custom function in a C program, we can include any custom function into a custom library. In this way, we could reuse the custom function between different program.
• First, we maintain the same main(), but we remove all custom financial functions. Instead we included a statement included “fin.h” so that the compiler will search for the header file.

#include <stdio.h>

#include "fin.h"

int main (void)

{

double myrate;

int myterm;

double myprincipal;

double myreturn;

printf ("Please enter the following:\n");

printf ("Rate of return:(In decimal 50%% is 0.5)");

scanf ("%lf", &myrate);

printf ("\n");

printf ("Number of terms:(No decimals): ");

scanf ("%d", &myterm);

printf ("\n");

printf ("The principal amount: ");

scanf ("%lf", &myprincipal);

printf ("\n");

myreturn = tvm (myrate, myterm, myprincipal);

printf ("Your return after %d term is $%.2lf.\n", myterm, myreturn);

return 0;

}

• Next, we create a header file (“fin.h”) that will include all financial functions in the future.

double tvm (double rate, int terms, double principal);

• Finally, we create a new program call finfn.c (as in financial function). The custom function will be place here.

#include <math.h>

#include "fin.h"

double tvm (double rate, int terms, double principal)

{

double dn;

double pp;

dn = terms;

pp = pow (1+rate, dn);

return principal * pp;

}

• The purpose splitting the program is such that tvm function can be reuse as a custom library in another program.
• We can compile all the program #gcc -Wall tvm.c finfn.c -o tvm –lm
• However, we don’t have to compile finfn.c every time of there is no changes.
• One way is to create an object file during compilation instead of create executable.
• We compile finfn.c first: #gcc -Wall -c finfn.c -o finlib.o
• With the object file available, we can create another program tvm2 using the same tvm function:

#include <stdio.h>

#include "fin.h"

int main (void)

{

double rate = 0.025;

int term = 10;

double principal = 1000;

double result = 0;

result = tvm (rate, term, principal);

printf ("The return of $1000 investment with a rate of 2.5 percent in 10 years will give you $%.2lf.\n", result);

return 0;

}

• Then we compile tvm2.c first before linking:

#gcc -Wall -c tvm2.c -o tvm2.o

• We can link both program as follows:

#gcc tvm.o finlib.o -lm -o tvm

#gcc tvm2.o finlib.o -lm -o tvm2

• In the example, we compile 2 programs using the custom function in one object file.
• Alternatively, we can compile straight from tvm2.c to executable like below:

#gcc -Wall tvm2.c finlib.o -lm -o tvm2

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