Read write append to file using file pointers in C

File Pointer : There is a declaration of FILE structure inside stdio.h , we can declare a pointer like this

FILE *fpt;

fopen() : Opens file in different modes and return the pointer to its memory location.

fpt=fopen("data.csv","r");

fprintf() Writting to file through file pointer

fprintf(fpt,"welcome to my file");

Different modes of file opening are given here.

Mode	Details	Remarks
r	Read	File opned & content can be collected, no updating , adding is allowed
w	write	File created if not available, content are overwritten
a	append	File created if not available, content written at the end of the file
r+	Read	File opened for reading and writing
w+	write	File opened for reading and writing, a new file created
a+	append	File opened for reading and appending

getc() : Get a single character from the file pointer

c=getc(fpt);

putchar() : prints the single character to screen

putchar(c);
putchar(c=getc(fpt));// getc reads from file pointer fpt

fgets() : Getting string from the file pointer and place in the variable str of a fixed buffer size

fgets(str,99,fpt)

fputs() : Writting string to file.

fputs(str,fpt);

EOF : End of file is defined inside stdio.h . Reading a special character whose ASCII value is 26, signifies end of the file.

Reading a file and displaying content

#include <stdio.h>
int main(void){
FILE *fpt; // file pointer
char c;
fpt=fopen("data.csv","r"); // file already exit for reading
if(fpt==NULL)
    printf("Can't open file ");
else {
    while(c!='\n'){
       putchar(c=getc(fpt));

}
    fclose(fpt);
}
return 0;
}

In the above code while loop will execute as long as the pointer not reached the first line break . Line breaks are identified by '\n'

while(c!='\n')

We can change this to read the complete file by checking EOF ( End of File )

while(c!=EOF){
       putchar(c=getc(fpt));
}

Counting number of line breaks and space inside a file

Reading string from file

In above code we were reading each char from the file pointer, we can read one string by using fgets(), this is required when one line of data is to be collected. Check the exercise at the end of this page.

#include <stdio.h>
int main(void){
FILE *fpt;
char str[100];
fpt=fopen("data4.dat","r");
if(fpt==NULL)
    printf("Can't open file ");
else {

while(fgets(str,99,fpt) != NULL){
    printf("%s",str);
}
}
fclose(fpt);
return 0;
}

Writting to File

#include <stdio.h>
int main(void){
FILE *fpt; // File pointer 
char c;
fpt=fopen("data1.dat","w"); // Opening file in write mode
fprintf(fpt,"welcome to my file"); 
fprintf(fpt,"\n Next Line ");
  fclose(fpt);
return 0;
}

Writting string to File

We can enter one line of code at a time and hit enter to complete the line. Each line is checked for its length by using strlen() and once we enter twice enter key the length of line will be equal to 0 and the while loop will terminate.

So hit enter twice to end the entry of line of text to file.

#include <stdio.h>
#include <string.h>
int main(void){
FILE *fpt;
char str[100];
fpt=fopen("data4.dat","w");
if(fpt==NULL)
    printf("Can't open file ");
else {
 printf("Enter one line of text\n");
while(strlen(gets(str))>0){
    fputs(str,fpt);
    fputs("\n",fpt);
}
}
fclose(fpt);
return 0;
}

File Append

We can add text and end of the file by opening the file in appending mode.

#include <stdio.h>
int main(void){
FILE *fpt;
char c;
fpt=fopen("data1.dat","a");
fprintf(fpt,"\n my new line ");
fprintf(fpt,"\n my new Next Line ");
  fclose(fpt);
return 0;
}

Writting to a file through structure

#include <stdio.h>
struct my_class{
int id;
char name[50];
int mark;
float av;
};
int main(void){
FILE *fpt;
fpt=fopen("emp_data.dat","w");
struct my_class emp;

if(fpt==NULL){
    puts("Cant open file");
   // exit();
}
printf("\nEnter id:");
scanf("%d",&emp.id);
fflush ( stdin ) ;
printf("\nEnter Name:");
gets(emp.name);
printf("\n mark:");
scanf("%d",&emp.mark);
printf("\n Average :");
scanf("%f",&emp.av);
printf("\n**************\n");
fprintf(fpt,"%d,%s,%d,%f \n",emp.id,emp.name,emp.mark,emp.av);
printf("\n name:%s",emp.name);
printf("\n mark:%d",emp.mark);
printf("\n id:%f",emp.av);
return 0;
}

Reading line data by looping

Basics of C String Array C- Tutorials

File Handling in C