Learn one of the most powerful programming languages in the world and become a rockstar developer.
A file is a container in computer storage devices used for storing data.
When dealing with files, there are two types of files you should know about:
Text files are the normal .txt files. You can easily create text files using any simple text editors such as Notepad.
When you open those files, you'll see all the contents within the file as plain text. You can easily edit or delete the contents.
They take minimum effort to maintain, are easily readable, and provide the least security and takes bigger storage space.
Binary files are mostly the .bin files in your computer.
Instead of storing data in plain text, they store it in the binary form (0's and 1's).
They can hold a higher amount of data, are not readable easily, and provides better security than text files.
In C, you can perform four major operations on files, either text or binary:
When working with files, you need to declare a pointer of type file. This declaration is needed for communication between the file and the program.
FILE *fptr;
Opening a file is performed using the fopen()
function defined in the stdio.h
header file.
The syntax for opening a file in standard I/O is:
ptr = fopen("fileopen","mode");
For example,
fopen("E:\\cprogram\\newprogram.txt","w"); fopen("E:\\cprogram\\oldprogram.bin","rb");
newprogram.txt
doesn't exist in the location E:\cprogram
. The first function creates a new file named newprogram.txt
and opens it for writing as per the mode 'w'.oldprogram.bin
exists in the location E:\cprogram
. The second function opens the existing file for reading in binary mode 'rb'.Mode | Meaning of Mode | During Inexistence of file |
---|---|---|
r |
Open for reading. | If the file does not exist, fopen() returns NULL. |
rb |
Open for reading in binary mode. | If the file does not exist, fopen() returns NULL. |
w |
Open for writing. | If the file exists, its contents are overwritten. If the file does not exist, it will be created. |
wb |
Open for writing in binary mode. | If the file exists, its contents are overwritten. If the file does not exist, it will be created. |
a |
Open for append. Data is added to the end of the file. |
If the file does not exist, it will be created. |
ab |
Open for append in binary mode. Data is added to the end of the file. |
If the file does not exist, it will be created. |
r+ |
Open for both reading and writing. | If the file does not exist, fopen() returns NULL. |
rb+ |
Open for both reading and writing in binary mode. | If the file does not exist, fopen() returns NULL. |
w+ |
Open for both reading and writing. | If the file exists, its contents are overwritten. If the file does not exist, it will be created. |
wb+ |
Open for both reading and writing in binary mode. | If the file exists, its contents are overwritten. If the file does not exist, it will be created. |
a+ |
Open for both reading and appending. | If the file does not exist, it will be created. |
ab+ |
Open for both reading and appending in binary mode. | If the file does not exist, it will be created. |
The file (both text and binary) should be closed after reading/writing.
Closing a file is performed using the fclose()
function.
fclose(fptr);
Here, fptr
is a file pointer associated with the file to be closed.
For reading and writing to a text file, we use the functions fprintf()
and fscanf().
They are just the file versions of printf()
and scanf()
. The only difference is that fprint()
and fscanf()
expects a pointer to the structure FILE.
#include <stdio.h> #include <stdlib.h> int main() { int num; FILE *fptr; // use appropriate location if you are using MacOS or Linux fptr = fopen("C:\\program.txt","w"); if(fptr == NULL) { printf("Error!"); exit(1); } printf("Enter num: "); scanf("%d",&num); fprintf(fptr,"%d",num); fclose(fptr); return 0; }
This program takes a number from the user and stores in the file program.txt
.
After you compile and run this program, you can see a text file program.txt
created in C drive of your computer. When you open the file, you can see the integer you entered.
#include <stdio.h> #include <stdlib.h> int main() { int num; FILE *fptr; if ((fptr = fopen("C:\\program.txt","r")) == NULL){ printf("Error! opening file"); // Program exits if the file pointer returns NULL. exit(1); } fscanf(fptr,"%d", &num); printf("Value of n=%d", num); fclose(fptr); return 0; }
This program reads the integer present in the program.txt
file and prints it onto the screen.
If you successfully created the file from Example 1, running this program will get you the integer you entered.
Other functions like fgetchar()
, fputc()
etc. can be used in a similar way.
Functions fread()
and fwrite()
are used for reading from and writing to a file on the disk respectively in case of binary files.
To write into a binary file, you need to use the fwrite()
function. The functions take four arguments:
fwrite(addressData, sizeData, numbersData, pointerToFile);
#include <stdio.h> #include <stdlib.h> struct threeNum { int n1, n2, n3; }; int main() { int n; struct threeNum num; FILE *fptr; if ((fptr = fopen("C:\\program.bin","wb")) == NULL){ printf("Error! opening file"); // Program exits if the file pointer returns NULL. exit(1); } for(n = 1; n < 5; ++n) { num.n1 = n; num.n2 = 5*n; num.n3 = 5*n + 1; fwrite(&num, sizeof(struct threeNum), 1, fptr); } fclose(fptr); return 0; }
In this program, we create a new file program.bin
in the C drive.
We declare a structure threeNum
with three numbers - n1, n2 and n3, and define it in the main function as num.
Now, inside the for loop, we store the value into the file using fwrite()
.
The first parameter takes the address of num and the second parameter takes the size of the structure threeNum
.
Since we're only inserting one instance of num, the third parameter is 1
. And, the last parameter *fptr
points to the file we're storing the data.
Finally, we close the file.
Function fread()
also take 4 arguments similar to the fwrite()
function as above.
fread(addressData, sizeData, numbersData, pointerToFile);
#include <stdio.h> #include <stdlib.h> struct threeNum { int n1, n2, n3; }; int main() { int n; struct threeNum num; FILE *fptr; if ((fptr = fopen("C:\\program.bin","rb")) == NULL){ printf("Error! opening file"); // Program exits if the file pointer returns NULL. exit(1); } for(n = 1; n < 5; ++n) { fread(&num, sizeof(struct threeNum), 1, fptr); printf("n1: %d\tn2: %d\tn3: %d", num.n1, num.n2, num.n3); } fclose(fptr); return 0; }
In this program, you read the same file program.bin
and loop through the records one by one.
In simple terms, you read one threeNum
record of threeNum
size from the file pointed by *fptr into the structure num.
You'll get the same records you inserted in Example 3.
If you have many records inside a file and need to access a record at a specific position, you need to loop through all the records before it to get the record.
This will waste a lot of memory and operation time. An easier way to get to the required data can be achieved using fseek()
.
As the name suggests, fseek()
seeks the cursor to the given record in the file.
fseek(FILE * stream, long int offset, int whence);
The first parameter stream is the pointer to the file. The second parameter is the position of the record to be found, and the third parameter specifies the location where the offset starts.
Whence | Meaning |
---|---|
SEEK_SET |
Starts the offset from the beginning of the file. |
SEEK_END |
Starts the offset from the end of the file. |
SEEK_CUR |
Starts the offset from the current location of the cursor in the file. |
This program will start reading the records from the file program.bin
in the reverse order (last to first) and prints it.