Updating in a sequential file?

ok it seems im in a bit of distress right now, i can't seem to figure out how to update/replace data in my file. Maybe having a glance at what i have may help so here it is below:

Code:

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_FIRSTNAME_LENGTH  25
#define MAX_LASTNAME_LENGTH  25
#define TRUE                  1
#define FALSE                  0


/* Student Structure */
struct stdntbio
{
  int age;
  float gpa;
  char firstname[MAX_FIRSTNAME_LENGTH];
  char lastname[MAX_LASTNAME_LENGTH];
};

/* Course grade structures */
struct CourseGrade
{
  struct stdntbio bio;
  float mth;
  float eng;
  float sub1;
  float sub2;
  float sub3;
};

/* Vars */
FILE *ptrFile = NULL;                               
char *filename = "studentdb.txt";               
char choice = 'n';                           

/* Function prototypes */
struct CourseGrade input_studentinfo();
struct stdntbio input_bio(); 
void show_student(struct CourseGrade student);
struct CourseGrade find_student(struct stdntbio bio);
void add_student();
void delete_student(struct stdntbio bio);
void list_students();
void display_menu();
int main()
{
    printf("\n      ************************************");
    printf("\n          WELCOME TO THE STUDENT DATABASE");
    printf("\n      ************************************");
   
  display_menu();
 
  for(;;)
  {
    printf("\nEnter a number to select an option: ");
    scanf(" %c", &choice);
    switch(choice)
    {
      case '1':                                   
        add_student();
        break;
      case '2':                                   
        delete_student(input_bio());
        break;
      case '3':                                   
        find_student(input_bio());
        break;
      case '4':                                   
        list_students();
        break;
      case '5':                                   
        return choice;
      default:
        printf("\nBad Input!");
        display_menu();
        break;
    }
  }
 }
 
struct CourseGrade input_studentinfo()
{
  struct CourseGrade student;   
  student.bio = input_bio();
    printf("Enter Grade for MTH: ");
  scanf("%f",&student.mth);
    printf("\nEnter Grade for ENG: ");
  scanf("%f",&student.eng);
    printf("\nEnter Grade for SUB1: ");
  scanf("%f",&student.sub1);
    printf("\nEnter Grade for SUB2: ");
  scanf("%f",&student.sub2);
    printf("\nEnter Grade for SUB3: ");
  scanf("%f",&student.sub3);
  return student;
}

/* Displays all student data, bio and grades */
void display_student(struct CourseGrade student)
{
  printf("\n%s %s %d  %.0f %.0f %.0f %.0f %.0f",
student.bio.firstname,student.bio.lastname,student.bio.age, student.mth,student.eng,
student.sub1,student.sub2,student.sub3);
}

/* Insert a student and their grades */
void add_student()
{
  struct CourseGrade student;

  if((ptrFile = fopen(filename, "a+")) == NULL)
  {
    printf("%s Failed to open for writing.", filename);
    system("exit");
  }
  student = input_studentinfo();               
  fwrite(&student, sizeof student, 1, ptrFile);
  fclose(ptrFile);                           
  printf("\nSuccess! Student Added");
}

/* take in a student from the user and make a structure for it */
struct stdntbio input_bio()
{
  struct stdntbio bio;
    printf("Enter a First name: ");
    scanf(" %s", &bio.firstname);
    printf("Enter a Last name: ");
    scanf(" %s", &bio.lastname);
    printf("Enter an Age: ");
    scanf(" %d", &bio.age);
  return bio;
}

void delete_student(struct stdntbio bio)
{
  FILE *pNewFile = NULL;
  char *pnewfilename = NULL;
  struct CourseGrade student;

  if((ptrFile = fopen(filename, "r")) == NULL)      /* Open current file to read it */
  {
    printf("File failed to open for reading.", filename);
    abort();
  }
    pnewfilename = tmpnam(NULL);
                                                    /* Create temporary file name      */
  if((pNewFile = fopen(pnewfilename, "w")) == NULL)
  {
    printf("%s Failed to open for writing.", pnewfilename);
    fclose(ptrFile);
    abort();
  }
 
    /* Copy existing file contents to temporary file, leaving deleted students */
  for(;;)
  {
    fread(&student, sizeof student, 1, ptrFile);   
    if(feof(ptrFile))                               
      break;                                       

    if(equals(bio, student.bio))             
    {
      printf("\nFound a record:");             
      display_student(student);
      printf("\nAre you sure you want to delete? (y or n)? ");
      scanf(" %s", &choice);
      if((choice) == 'y')             
        continue;                             
    }
    fwrite(&student, sizeof student, 1, pNewFile); 
  }
  fclose(ptrFile);
  fclose(pNewFile);

  if((pNewFile = fopen(pnewfilename, "r")) == NULL)
  {
    printf("Error opening %s for reading.", pnewfilename);
    abort();
  }
  if((ptrFile = fopen(filename, "w"))==NULL)         
  {
    printf("Error opening %s for writing.", filename);
    abort();
  }

  /* Copy contents of new temporary file back to old file          */
 
  for(;;)
  {
    fread(&student, sizeof student, 1, pNewFile);
    if(feof(pNewFile))                           
      break;                                     

    fwrite(&student, sizeof student, 1, ptrFile);
  }
  fclose(ptrFile);                   
  fclose(pNewFile);                   
  remove(pnewfilename);             
  printf("Delete complete.");
}

/* List all the records in the file */
void list_students()
{
  struct CourseGrade student;
  int file_empty = TRUE;

  if((ptrFile = fopen(filename, "r")) == NULL)     
  {
    printf("Error opening %s for reading.", filename);
    abort();
  }
 
  /* Listing the file contents */
  for(;;)
  {
    fread(&student, sizeof student, 1, ptrFile);
    if(feof(ptrFile))
      break;
    file_empty = FALSE;             
    display_student(student);       
  }
  fclose(ptrFile);               

  /* Check whether there were any students */
  if(file_empty)
    printf("The file has no students.\n");
  else
    printf("\n");
}

/* Displays the options that can be done */
void display_menu()
{ 
    printf("\n    Please select an option from the menu below:\n\n"
    "\n1 - Add a new student."
    "\n2 - Delete a student."
    "\n3 - Find a student."
    "\n4 - Display all students stored."
    "\n5 - Exit.");
}

/* Find students corresponding to a given name */
struct CourseGrade find_student(struct stdntbio bio)
{
  struct CourseGrade student;
  int student_found = FALSE;                 

  if((ptrFile = fopen(filename, "r")) == NULL)     
  {
    printf("Error opening %s for reading.", filename);
    abort();
  }
 
/* Search the students from the file */
  for(;;)
  {
    fread(&student, sizeof student, 1, ptrFile);
    if(feof(ptrFile))
      break;
  if(equals(bio,student.bio))           
  {
    if(!student_found)                       
    {
      student_found = TRUE;                     
      printf("Credit(s) for this student are:");
    }
    printf("\n MTH: %.1f \n Eng: %.1f \n Sub1: %.1f \n Sub2: %.1f \n Sub3: %.1f\n\n",
student.mth,student.eng,student.sub1,student.sub2,student.sub3); 
  }
  }
  fclose(ptrFile);
                                 
  if(!student_found)
    printf("Student was not found.\n");
  else
    printf("\n");
}

/* Compare two sets of data */
int equals(struct stdntbio bio1, struct stdntbio bio2)
{
  return (strcmp(bio1.firstname, bio2.firstname)==0) && (strcmp(bio1.lastname, bio2.lastname)==0);
}

---

Trying to update the data of a student, i know i need to update the structure but i don't know where to start.
As you can see i've gotten quite a bit done and alot more to go this is just a stepping stone i've yet to cross.

Code:

---

struct CourseGrade input_studentinfo()
{
  struct CourseGrade student;   
  student.bio = input_bio();
    printf("Enter Grade for MTH: ");
  scanf("%f",&student.mth);
    printf("\nEnter Grade for ENG: ");
  scanf("%f",&student.eng);
    printf("\nEnter Grade for SUB1: ");
  scanf("%f",&student.sub1);
    printf("\nEnter Grade for SUB2: ");
  scanf("%f",&student.sub2);
    printf("\nEnter Grade for SUB3: ");
  scanf("%f",&student.sub3);
  return student;

//You can't return a whole struct. Instead, return a pointer to that struct.
}

/* Displays all student data, bio and grades */
void display_student(struct CourseGrade student)
{
  printf("\n%s %s %d  %.0f %.0f %.0f %.0f %.0f",
student.bio.firstname,student.bio.lastname,student.bio.age, student.mth,student.eng,
student.sub1,student.sub2,student.sub3);
}

/* Insert a student and their grades */
void add_student()
{
  struct CourseGrade student;
  student * ptrS = &student;

  if((ptrFile = fopen(filename, "a+")) == NULL)
  {
    printf("%s Failed to open for writing.", filename);
    system("exit");
  }
  student = input_studentinfo();               
//you want to replace "student" in the line immediately above this one, with a
//pointer to student, so:
  ptrS = input_studentinfo();
  fwrite(ptrS, sizeof student1, ptrFile); 

// fwrite(&student, sizeof student, 1, ptrFile);
  fclose(ptrFile);                           
  printf("\nSuccess! Student Added");
}

/* take in a student from the user and make a structure for it */
struct stdntbio input_bio()
{
  struct stdntbio bio;
    printf("Enter a First name: ");
    scanf(" %s", &bio.firstname);
    printf("Enter a Last name: ");
    scanf(" %s", &bio.lastname);
    printf("Enter an Age: ");
    scanf(" %d", &bio.age);
  return bio;
//same here. bio is a struct, not a pointer. Make a pointer and return it, instead.
}

---

Use a pointer to bring back what you want from an updated or new student struct function. Then use that pointer to fwrite the struct to the file.