C Program to Store a array of structures in a file and modify the existing file...

Points to Note:
void rewind(FILE *stream) - Sets the file position to the beginning of the file of the given stream.
void fseek(FILE *stream, long int offset, int whence) - Sets the file position of the stream to the given offset.. ( More informally , it moves the file position to particular offset from the whence. )
whence is nothing but the starting point, it can take values SEEK_SET - Beginning of the file,
SEEK_CUR - Current position of the file, SEEK_END - End of the file

#include<stdio.h>

struct student{
    int roll;
    char name[100];
};

void main()
{
    FILE *f;
    struct student s[100];
    f=fopen("student.txt","w+");
    fprintf(f,"Roll NO:\tName:\n");
    int n,i;
    printf("Enter the no of students..");
    scanf("%d",&n);
    printf("\nEnter the roll no and the name of each student one by one..");
    // Writing to a file...
    for(i=0;i<n;i++){
        scanf("%d",&s[i].roll);
        scanf("%s",s[i].name);
        fprintf(f,"%d\t%s\n",s[i].roll,s[i].name);
    }
    // Reading from a file...
    rewind(f);
    char t;
    while(!feof(f)){
            t=fgetc(f);
            printf("%c",t);
    }
    // Modifying a file...
    char no[50],newname[100];
    printf("\nEnter  the roll no of the student to be modified..");
    scanf("%s",no);
    printf("\nEnter the new name of the student..");
    scanf("%s",newname);
    rewind(f);
    char temp[100];
    while(!feof(f)){
       fscanf(f,"%s",temp);
       if(strcmp(temp,no)==0){
        break;
       }
    }
    fseek(f,1,SEEK_CUR);
    fputs(newname,f);
    // Reading a modified file...
    printf("\nThe modified file is...\n");
    rewind(f);
    while(!feof(f)){
            t=fgetc(f);
            printf("%c",t);
    }
    fclose(f);
}

Corner Case :
        Sorry for the inconvenience,  this program allows you to modify the existing student name with a new student name only with same length.. I'll try handle this case in my future posts..