File reading & writing in C with different modes

1. Opening a file

fopen() function is used for opening a file.
Syntax:

FILE pointer_name = fopen ("file_name", "Mode");




pointer_name can be anything of your choice.
file_name is the name of the file, which you want to open. Specify the full path here like “C:\\myfiles\\newfile.txt”.

While opening a file, you need to specify the mode. The mode that we use to read a file is “r” which is “read only mode”.
for example:

FILE  *fp;
fp = fopen("C:\\myfiles\\newfile.txt", "r");

The address of the first character is stored in pointer fp.

How to check whether the file has opened successfully?
If file does not open successfully then the pointer will be assigned a NULL value, so you can write the logic like this:
This code will check whether the file has opened successfully or not. If the file does not open, this will display an error message to the user.

..
FILE fpr;
fpr = fopen("C:\\myfiles\\newfile.txt", "r");
if (fpr == NULL)
{
    puts("Error while opening file");
    exit();
}

Various File Opening Modes:

The file is opened using fopen() function, while opening you can use any of the following mode as per the requirement.
Mode “r”: It is a read only mode, which means if the file is opened in r mode, it won’t allow you to write and modify content of it. When fopen() opens a file successfully then it returns the address of first character of the file, otherwise it returns NULL.

Mode “w”: It is a write only mode. The fopen() function creates a new file when the specified file doesn’t exist and if it fails to open file then it returns NULL.

Mode “a”: Using this mode Content can be appended at the end of an existing file. Like Mode “w”, fopen() creates a new file if it file doesn’t exist. On unsuccessful open it returns NULL.
File Pointer points to: last character of the file.

Mode “r+”: This mode is same as mode “r”; however you can perform various operations on the file opened in this mode. You are allowed to read, write and modify the content of file opened in “r+” mode.
File Pointer points to: First character of the file.

Mode “w+”: Same as mode “w” apart from operations, which can be performed; the file can be read, write and modified in this mode.
Mode “a+”: Same as mode “a”; you can read and append the data in the file, however content modification is not allowed in this mode.

2. Reading a File

To read the file, we must open it first using any of the mode, for example if you only want to read the file then open it in “r” mode. Based on the mode selected during file opening, we are allowed to perform certain operations on the file.

C Program to read a file

fgetc( ): This function reads the character from current pointer’s position and upon successful read moves the pointer to next character in the file. Once the pointers reaches to the end of the file, this function returns EOF (End of File). We have used EOFin our program to determine the end of the file.

#include <stdio.h>
int main()
{
     /* Pointer to the file */
     FILE *fp1;
     /* Character variable to read the content of file */
     char c;

     /* Opening a file in r mode*/
     fp1= fopen ("C:\\myfiles\\newfile.txt", "r");

     /* Infinite loop –I have used break to come out of the loop*/
     while(1)
     {
        c = fgetc(fp1);
        if(c==EOF)
            break;
        else
            printf("%c", c);
     }
     fclose(fp1);
     return 0;
}

3. Writing to a file

To write the file, we must open the file in a mode that supports writing. For example, if you open a file in “r” mode, you won’t be able to write the file as “r” is read only mode that only allows reading.

Example: C Program to write the file

This program asks the user to enter a character and writes that character at the end of the file. If the file doesn’t exist then this program will create a file with the specified name and writes the input character into the file.

#include <stdio.h>
int main()
{
   char ch;
   FILE *fpw;
   fpw = fopen("C:\\newfile.txt","w");

   if(fpw == NULL)
   {
      printf("Error");   
      exit(1);             
   }

   printf("Enter any character: ");
   scanf("%c",&ch);

   /* You can also use fputc(ch, fpw);*/
   fprintf(fpw,"%c",ch);
   fclose(fpw);

   return 0;
}

4. Closing a file

fclose(fp);

The fclose( ) function is used for closing an opened file. As an argument you must provide a pointer to the file that you want to close.

An example to show Open, read, write and close operation in C

#include <stdio.h>
int main()
{
    char ch;

    /* Pointer for both the file*/
    FILE *fpr, *fpw;
    /* Opening file FILE1.C in “r” mode for reading */
    fpr = fopen("C:\\file1.txt", "r");

    /* Ensure FILE1.C opened successfully*/
    if (fpr == NULL)
    {
         puts("Input file cannot be opened");
    }

    /* Opening file FILE2.C in “w” mode for writing*/
    fpw= fopen("C:\\file2.txt", "w");

    /* Ensure FILE2.C opened successfully*/


    if (fpw == NULL)
    {
       puts("Output file cannot be opened");
    }

    /*Read & Write Logic*/
    while(1)
    {
        ch = fgetc(fpr);
        if (ch==EOF)
            break;
        else
            fputc(ch, fpw);
    }



    /* Closing both the files */
    fclose(fpr);
    fclose(fpw);

    return 0;
}