Here is source code of the C Program to join lines of two given files and store them in a new file. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

/*
* C Program to Join Lines of Two given Files and
* Store them in a New file
*/
#include <stdio.h>
#include <stdlib.h>

/* Function Prototype */
int joinfiles(FILE *, FILE *, FILE *);

char ch;
int flag;

void main(int argc, char *argv[])
{
FILE *file1, *file2, *target;

file1 = fopen(argv[1], "r");
if (file1 == NULL)
{
perror("Error Occured!");
}
file2 = fopen(argv[2], "r");
if (file2 == NULL)
{
perror("Error Occured!");
}
target = fopen(argv[3], "a");
if (target == NULL)
{
perror("Error Occured!");
}

joinfiles(file1, file2, target); /* Calling Function */

if (flag == 1)
{
printf("The files have been successfully concatenated\n");
}
}

/* Code join the two given files line by line into a new file */

int joinfiles(FILE *file1, FILE *file2, FILE *target)
{
while ((fgetc(file1) != EOF) || (fgetc(file2) != EOF))
{
fseek(file1, -1, 1);
while ((ch = fgetc(file1)) != '\n')
{
if (ch == EOF)
{
break;
}
else
{
fputc(ch, target);
}
}
while ((ch = fgetc(file2)) != '\n')
{
if (ch == EOF)
{
break;
}
else
{
fputc(ch, target);
}
}
fputc('\n', target);
}
fclose(file1);
fclose(file2);
fclose(target);
return flag = 1;
}


$ cc file7.c
$ ./a.out lines.c words.c final.c
The files have been successfully concatenated

/* FIRST FILE */

/*
Hello!!
This is a C Program File.
Consider a code to Add two numbers
*/

#include <stdio.h>
/* Function Prototype */
int sum(int, int);
void main()
{
int num1, num2;
printf("Enter Number1 and Number2:");
scanf("%d %d ", num1, num2);
sum(num1, num2);
}

int sum(int a, int b)
{
return a + b;
}

/* SECOND FILE */

/*
* this is temporary file for use in file handling
*/
#include <stdio.h>

int sqrt(int);
void main()
{
int num;
printf("enter the number:");
scanf("%d", &num);
sqrt(num);
printf("The square of the given number is:", num);
}
int sqrt(int num)
{
return num*num;
}

/* CONCATENATED FILE */
/*
Hello!! * this is temporary file for use in file handling
This is a C Program File. *
Consider a code to Add two numbers */
*/
#include <stdio.h>
#include <stdio.h>
int sqrt(int);
/* Function Prototype */void main()
{
int sum(int, int); int num;
void main() printf("enter the number:");
{ scanf("%d", &num);
int num1, num2; sqrt(num);
printf("Enter Number1 and Number2:"); printf("The square of the given number is:", num);
scanf("%d %d ", num1, num2);}
sum(num1, num2);int sqrt(int num)
}{
return num*num;
int sum(int a, int b)}
{
return a + b;
}