fseek() function

The fseek() function is used to set the file pointer to the specified offset. It is used to write data into file at desired location.
Syntax:
int fseek(FILE *stream, long int offset, int whence)
There are 3 constants used in the fseek() function for whence: SEEK_SET, SEEK_CUR and SEEK_END.
Example: 
#include <stdio.h>  
void main() {  
FILE *fp;  
fp = fopen("myfile.txt","w+");  
fputs("This is catchmecoder", fp);  
fseek( fp, 7, SEEK_SET );  
fputs("Pramesh gupta", fp);  
fclose(fp);  
 }  
myfile.txt
This is Pramesh gupta

rewind() function

The rewind() function sets the file pointer at the beginning of the stream. It is useful if you have to use stream many times.
Syntax:

void rewind(FILE *stream)
Example:
File: file.txt
this is a general text
rewind.c 
#include <stdio.h>  
void main() {  
FILE *fp;  
char c;   
fp=fopen("file.txt","r");  
while((c=fgetc(fp))!=EOF){  
printf("%c",c);  
 }  
rewind(fp);  //moves the file pointer at beginning of the file  
while((c=fgetc(fp))!=EOF){  
printf("%c",c);  
  }  
fclose(fp);        
  }
Output:
Hello catchmecoder...
Hello catchmecoder...
As you can see, rewind() function moves the file pointer at beginning of the file that is why "this is simple text" is printed 2 times. If you don't call rewind() function, "this is simple text" will be printed only once.

ftell() function

The ftell() function returns the current file position of the specified stream. We can use ftell() function to get the total size of a file after moving file pointer at the end of file. We can use SEEK_END constant to move the file pointer at the end of file
Syntax:

long int ftell(FILE *stream)
File: ftell.c
Example: 
#include <stdio.h>  
void main () {  
FILE *fp;  
int length;  
fp = fopen("file.txt", "r");  
fseek(fp, 0, SEEK_END);  
length = ftell(fp);  
fclose(fp);  
printf("Size of file: %d bytes", length);  
   }  
Output:
Size of file: 23 bytes