C File Pointers

C File Pointers

File pointer is actually a pointer to a structure, structure has been typedefed into FILE in the header file 'stdio.h'.

File pointer syntax,

FILE *<file-poiner1>, *<file-poiner2>, ...;

FILE structure,

typedef struct
{
  // fill/empty level of buffer
  int    level;
  // File status flags
  unsigned flags;
  // File descripter
  char fd;
  // ungetc char if no buffer
  unsigned char hold;
  // buffer size
  int bsize;
  // data transfer buffer
  unsigned char *buffer;
  // Current active pointer
  unsigned char *curp;
  //Temporary file indicator
  unsigned istemp;
  //Used for validity checking
  short token;
} FILE; // This is FILE object
 
    

Following steps happens when open a file using file pointer,

The content of the file are loaded into buffer.

A FILE structure is created in memory, address of this structure us returned once elements are assigned.

File Pointers C program

This c program shows how to use thefile pointers to read the number of characters in the input file.

Program opens a file, reads it character by character till the time its end is not encountered. On reaching tthe end of file 'EOF', the file is closed and prints the total number of characters in the file.

fp is the file pointer in the program.

#include<stdio.h>                                                          
                                                                                
void main()                                                                     
{                                                                               
  FILE *fp;                                                                     
  char ch;                                                                      
  int noc = 0;                                                                  
  fp = fopen("test.txt", "r");                                                  
  while(1)                                                                      
  {                                                                             
    ch = fgetc(fp);                                                             
    if(ch == EOF)                                                               
    {                                                                           
      break;                                                                    
    }                                                                           
    noc++;                                                                      
  }                                                                             
  fclose(fp);                                                                   
  printf("\nNumber of characters in file: %d", noc);                            
}   

Let us consider the input text.txt file content is,

$ cat test.txt 
Welcome to codingpointer.com!

Output:

$ cc file-pointer.c
$ ./a.out 

Number of characters in file: 30

Read File using FILE pointers

Another example to read the file content along with number of characters using FILE pointer,

#include<stdio.h>                                                               
                                                                                
void main()                                                                     
{                                                                               
  FILE *fp;                                                                     
  char ch;                                                                      
  int noc = 0;                                                                  
  fp = fopen("test.txt", "r");                                                  
  printf("File content: \n");                                                   
  while(1)                                                                      
  {                                                                             
    ch = fgetc(fp);                                                             
    if(ch == EOF)                                                               
    {                                                                           
      break;                                                                    
    }                                                                           
    printf("%c", ch);                                                           
    noc++;                                                                      
  }                                                                             
  fclose(fp);                                                                   
  printf("\nNumber of characters in file: %d", noc);                            
}  

Let us consider the input text.txt file content is,

$ cat test.txt 
Welcome to codingpointer.com!

Output:

$ cc file-pointer.c
$ ./a.out 
File content: 
Welcome to codingpointer.com!

Number of characters in file: 30