Transcript BinaryFileHandling - MridulaSharma.com

C: Advanced Topics-II
Summer 2014
COMP 2130 Intro Computer Systems
Computing Science
Thompson Rivers University
File Handling
a.
There are different types of files:
Unformatted Files
a.
b.
Text Files
Formatted Files
a.
b.
TRU-COMP2130
Text Files with fprintf and fscanf
Binary Files
C: Advanced Topics
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Text Files
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We have discussed the text files (even with command line arguments)
The steps are:
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Open a file
Write or read
Close the file
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Formatted files
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The files created with fprintf, can be read using fscanf in the same
sequence
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The steps are again the same
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Open/create the file
Read/write on the file (using fscanf/fprintf)
Close the file
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Binary Files
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Binary files are very similar to arrays of structures, except the
structures are in a disk-file rather than an array in memory.
There are two features, that distinguish binary files from text files:
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You can instantly use any structure in the file.
You can change the contents of a structure anywhere in the file.
The steps are:
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Create/open the file
Write/read the structure
Close the file
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Operations on Binary File
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Open the file with “b” in the mode
After opening, one can read and write a structure on the file
It is also possible to seek a specific position in the file. The position is
at record 0 when the file is opened.
A read operation reads the structure where the file position indicator is
pointing to. After reading the structure the pointer is moved to point at
the next structure.
A write operation will write to the currently pointed-to structure. After
the write operation the file position indicator is moved to point at the
next structure.
The fseek function will move the file position indicator to the record
that is requested.
The ftell returns the current value of the file pointer
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Common File Access operations
#include <stdio.h>
FILE *in, *out; // FILE is defined in <stdio.h>
in = fopen(“in_filename”, “r”);
// mode: r, w, a, r+,
w+, a+
if (in == NULL) ...
out = fopen(“out_filename”, “w”);
fclose(in);
fprintf(out, “format ...”, variables...);
fscanf(...);
fgets(...);
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Opening and using binary files
The opening modes may be:
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Special binary file operations
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int fseek(FILE*, long, SEEK_SET or SEEK_CURRENT or
SEEK_END);
// move file position pointer
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int fwrite(void*, int memb_size, int no_memb, FILE*);
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int fread(void*, int memb_size, int no_memb, FILE*);
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int ftell ( FILE * stream );
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Defining the structure
The record or the structure is defined as:
struct member
{
int mem_id;
char mem_name[25];
int salary;
};
* We may use typedef to define the user defined datatype as well
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To create the file and add records
void create()
{
fp=fopen("Binary.txt", "ab");
struct member m;
int ans=1;
while (ans==1)
{
printf("\nEnter Member ID : "); scanf("%d", &m.mem_id);
printf("\nEnter Member Name : "); scanf("%s",m.mem_name);
printf("\nEnter Salary : "); scanf("%d", &m.salary);
fwrite(&m, sizeof(struct member),1,fp); // this is writing the address of the structure on the file
printf("\n\nPress '1' to add more and '0' to go back : ");
scanf("%d", &ans);
if (ans==0) {fclose(fp); return;}
if (ans!=0 && ans !=1) printf("Invalid option entered ");
}
fclose(fp);
}
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Reading all the records
void display() {
fp=fopen("Binary.txt", "rb");
struct member mem;
int a;
while(1)
{
fread(&mem, sizeof(struct member), 1, fp); // read a specific record at the record pointer
if (feof(fp)) break;
printf("\nMemebr ID : %d\nMember Name : %s\nSalary : %d\n", mem.mem_id,
mem.mem_name, mem.salary);
}
printf("Press '0' to go back ");
scanf("%d", &a);
fclose(fp);
}
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Reading a specific record

In order to read a specific record, the number of record is taken from
the user and then fseek is used to move the record pointer
void search() {
int a;
struct member mem;
fp=fopen("Binary.txt", "rb");
while(1)
{
printf("Enter the record number to search : "); scanf("%d", &a);
fseek(fp, sizeof(struct member)*(a-1), 0); // seeking the location from the beginning of the file
fread(&mem, sizeof(struct member),1, fp);
printf("\nMemebr ID : %d\nMember Name : %s\nSalary : %d\n", mem.mem_id, mem.mem_name,
mem.salary);
printf("Press '1' to search more and '0' to go back : "); scanf("%d", &a);
if (a==0) {fclose(fp); return;}
if (a!=0 && a!=1) printf("Invalid input ");
}
}
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Fseek()
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The fseek may be used to travel anywhere in the file:
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int fseek ( FILE * stream, long int offset, int origin );
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The origin may be:
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Offset would be negative with SEEK_END
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Rewind()
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This positions the record pointer to the beginning of the file
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void rewind ( FILE * stream );
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Update a record
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First the record is read and it may also be written back with the new
values
void update()
{
int a;
fp=fopen("Binary.txt", "r+"); //this would open in read as well as write mode
char name_temp[25]; int sal_temp; struct member mem;
while(1)
{
printf("Enter the record number to search : "); scanf("%d", &a);
fseek(fp, sizeof(struct member)*(a-1), 0); //this is searching the specific record
fread(&mem, sizeof(struct member),1, fp);
printf("\n\nEnter new name : "); scanf("%s", mem.mem_name); //inputting the new name and salary
printf("\n\nEnter new salary : "); scanf("%d", &mem.salary);
fseek(fp, -sizeof(struct member), SEEK_CUR); //we need to go back to the original position to write back
fwrite(&mem,sizeof(struct member), 1, fp);
printf("\n\n Record Updated ");
}
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Display the record
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After reading the record, the simple display is:
printf("\nMemebr ID : %d\nMember Name : %s\nSalary : %d\n",
mem.mem_id, mem.mem_name, mem.salary);
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If we wish to define a function to display the members values:
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showOne(&mem);
It is passing the address and pointer would receive the value
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Using pointer values
void showOne(struct member *ptr)
{
printf("\nMember ID : %d", ptr->mem_id); //we use -> in spite of .
printf("\nMember Name : %s", ptr->mem_name);
printf("\nSalary : %d\n\n", ptr->salary);
}
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void print_rcd(struct student_rcd rcd)
{
printf(“Number: %d\n”, rcd.student_number);
printf(“Name: %s\n”, rcd.name); // name is an array.
// not &(rcd.name)
}
void read_rcd(struct student_rcd *rcd)
{
printf(“Enter number: “);
scanf(“%d”, &(rcd->student_number)); // reference rqd
printf(“Enter name: “);
scanf(“%s”, rcd->name);
// name is an array.
// not &(rcd->name)
}
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C: Advanced Topics
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Delete a record
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Once created, we may edit the data, but cannot delete
Read all records and transfer to other file except the one’s you wish to
delete
Rename the new file
while (1)
{
fread(&mem,sizeof(struct member),1,fp); //reading from fp
if (feof(fp)) break;
if (strcmp(naam, mem.mem_name) == 0) {
printf("A record with requested name found and deleted.\n\n");
found=1;
} else {
fwrite(&mem, sizeof(struct member), 1, fp_tmp); //writing on fp_tmp
}
}
remove("Binary.txt");
rename("tmp.bin", "Binary.txt");
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Some extra methods:
void menu() //to show all the menu options
{
system("clear");
printf("\n\n\t\tM A I N M E N U \n");
printf("\t1. Create the Binary File \n");
printf("\t2. Display the contents of the file \n");
printf("\t3. Seek a specific record \n");
printf("\t4. Update the contents of a record \n");
printf("\t5. Delete a record for the specific name \n");
printf("\t6. Exit \n");
}
int input() //to take user input
{int a;
printf("\n\t\tPlease Enter your choice .... ");
scanf("%d", &a);
return(a);
}
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Screen shots
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Formatted files
To create a file
void create()
{
fp=fopen("format.txt", "a");
int mem_id;
char mem_name[25];
int salary;
int ans=1;
while (ans==1)
{
printf("\nEnter Member ID : "); scanf("%d", &mem_id);
printf("\nEnter Member Name : "); scanf("%s",mem_name);
printf("\nEnter Salary : "); scanf("%d", &salary);
fprintf(fp, "%d %s %d\n",mem_id, mem_name, salary);
printf("\n\nPress '1' to add more and '0' to go back : ");
scanf("%d", &ans);
if (ans==0) {fclose(fp); return;}
if (ans!=0 && ans !=1) printf("Invalid option entered ");
}
fclose(fp);
}
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Read

To read the file
void display() {
fp=fopen("format.txt", "r");
int mem_id;
char mem_name[25];
int salary;
int a;
while(!feof(fp))
{
fscanf(fp, "%d %s %d\n", &mem_id, mem_name, &salary);
printf("\n\t Member ID : %d \n\tMember Name : %s \n\tSalary : %d\n", mem_id, mem_name, salary);
}
printf("Press '0' to go back ");
scanf("%d", &a);
fclose(fp);
}
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Search a member_id
while(1)
{
found=0;
printf("Enter the record number to search : "); scanf("%d", &a);
rewind(fp);
while(!feof(fp))
{
fscanf(fp, "%d %s %d\n", &mem_id, mem_name, &salary);
if (mem_id==a)
{
found=1;
printf("\nMemebr ID : %d\nMember Name : %s\nSalary : %d\n", mem_id,mem_name, salary);
}
}
if (!found) printf("This Member_ID is not present ");
printf("Press '1' to search more and '0' to go back : "); scanf("%d", &a);
if (a==0) {fclose(fp); return;}
if (a!=0 && a!=1) printf("Invalid input ");
}
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