#include<stdio.h>
#include<string.h>
#define size 5           


typedef struct {
	char last[10];                        /* Structure of type student*/
	char first[10];              
    double gpa;
} stud;


typedef struct {
	char last[10];
	char first[10];                      /* structure of type employee*/
	double salary;
}  emp;


void check_add_salary (emp *e, stud *s);




main(){ 

FILE *stud_file=NULL;
FILE *employ_file=NULL;
FILE *employ_file2=NULL;
int i=0 , j=0;
stud arr_stud[size];                                                  /* An array of structures (students)*/
emp arr_emp[size];                                                    /*An array of structures (employees)*/
                

if((stud_file=fopen("student.txt","r"))==NULL) 
   printf("\nThe file could not be opened\n\n");

else
   {   fseek(stud_file,50,SEEK_SET);
       while(!feof(stud_file))
   	   {fscanf(stud_file,"%s%s%lf\n", arr_stud[i].last,arr_stud[i].first,&arr_stud[i].gpa);
   
       i++;
   	   }
   
       
       
     
     
      fclose(stud_file);
   }
   /*end of else*/
   
   i=0;
   
   if((employ_file=fopen("employees.txt","r"))==NULL)
{  
   printf("\nThe file could not be opened\n\n");
}

else 
   {
     fseek(employ_file,51,SEEK_SET);
   	 while(!feof(stud_file))
	   	{
   	    fscanf(employ_file,"%s%s%lf\n", arr_emp[i].last,arr_emp[i].first,&arr_emp[i].salary);
 	   
        i++;   
        }
   
   fclose(employ_file);
   
   
   
   
   }
   
   /***end of reading from file employess*/
   
   
   check_salary(arr_emp,arr_stud);
   
   
   
   if((employ_file2=fopen("update.txt","w"))==NULL)
{  
   printf("\nThe file could not be opened\n\n");
}

else 
   {  fputs("\t\tThe new list of employees(after salary change)\n\n",employ_file2);
       fputs("Last name\tFirst name\t\tGPA\n",employ_file2);
       fputs("************************************************\n",employ_file2);
       
       for(i=0;i<size;i++)
         fprintf(employ_file2,"%-10s      %-10s               %-10.2f\n", arr_emp[i].last,arr_emp[i].first,arr_emp[i].salary);   /* Display the list of employees sorted by alphabetical order*/
	   
       fputs("\n************************************************\n\n\n\n",employ_file2);
       fclose(employ_file2);
   
   
   
  
  printf("\n The file of employees with the new salary has been created n\" );
   


   
}
   void check_salary (emp *r, stud *s)
   	{
   	
   		int i , j;
        for(i=0;i<size;i++)
          for(j=0;j<size;j++)
            if( strcmp (e[i].last,s[j].last)==0 && strcmp(e[i].first,s[j].first)==0 )
              if(s[i].gpa>3.0)                                                                                             /* The if statement is to add 10% to the salary*/
                {																														/*of employees whose students have GPA>3*/
		        	r[i].salary = r[i].salary + ((r[i].salary)*0.1); j=size-1;
		     	}       
			
   }

Example:
#include <stdio.h>

int main(void)
{
   FILE *stream;
   int i = 100;
   char c = 'C';
   float f = 1.234;

   /* open a file for update */
   stream = fopen("DUMMY.FIL", "w+");

   /* write some data to the file */
   fprintf(stream, "%d %c %f", i, c, f);

   /* close the file */
   fclose(stream);
   return 0;
}