Opening Modes in Standard I/O in C/C++ with Examples
Last Updated :
23 Jul, 2025
Pre-requisites: File Handling in C++
So far the operations using the C program are done on a prompt/terminal that is not stored anywhere. But in the software industry, most of the programs are written to store the information fetched from the program. One such way is to store the fetched information in a file. Different operations that can be performed on a file are:
- Creation of a new file (fopen with attributes as “a” or “a+” or “w” or “w++”).
- Opening an existing file (fopen).
- Reading from file (fscanf or fgets).
- Writing to a file (fprintf or fputs).
- Moving to a specific location in a file (fseek, rewind).
- Closing a file (fclose).
The text in the brackets denotes the functions used for performing those operations.
Why files are needed?
- Data Preservation: Storing data in files helps to preserve data even if the program terminates.
- Easy Data Access: Accessing data becomes easy when there is a large amount of data and it is stored in the file, then this data can be accessed using the C commands.
- Portability: It becomes easier to move data from one computer to another without any changes.
Types of files
They are two types of files that everyone should be aware of-
- Text Files: Text files are the normal files that have an extension ".txt" in the file name. These can be simply created using editors like Notepad. Upon opening the files the text will be visible like simple plain text and the content can be easily edited or deleted. These are the lowest maintenance files, easy to read. But there are a few disadvantages of text files like they are the least secure files and take bigger storage space.
- Binary Files: Binary files are ".bin" extension files. Data in these files are stored in the binary form i.e. 0's and 1's. These files can hold a large amount of data and provide a higher level of security than text files but these files are not easily readable.
File Operations
There are four basic operations that can be performed on a file:
- Creating a new file.
- Opening an existing file.
- Reading from or writing information to the file.
- Closing the file.
Working with files
When working with the files, there is a need to declare a pointer of the type file. This file-type pointer is needed for communication between the file and the program.
File *fptr;
Opening a file
Opening a file is done using the fopen() function in the header file stdio.h.
Syntax:
fptr = fopen("file_name", "mode");
Example:
fopen("D:\\geeksforgeeks\\newprogramgfg.txt", "w"); fopen("D:\\geeksforgeeks\\oldprogramgfg.bin", "rb");
- Suppose the file newprogramgfg.txt doesn't exist in the location D:\geeksforgeeks. The first function creates a new file named newprogramgfg.txt and opens it for writing as per the mode 'w'. The writing mode allows you to create and edit (overwrite) the contents of the file.
- Suppose the second binary file oldprogramgfg.bin exists in the location D:\geeksforgeeks. The second function opens the existing file for reading in binary mode 'rb'. The reading mode only allows one to read the file, one cannot write into the file.
File Opening modes in C:
| 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. |
|---|
Closing a file
The file should be closed after reading or writing. Closing a file is performed using the fclose() function.
Syntax:
fclose(fptr);
Here, fptr is the file type pointer associated with the file to be closed.
Reading and writing to a text file
For reading and writing to a text file, the functions fprintf() and fscanf() are used. They are the file versions of the printf() and scanf() functions. The only difference is that the fprintf() and fscanf() expect the pointer to the structure file.
Write to a text file:
Syntax:
FILE *filePointer; filePointer = fopen(“filename.txt”, “w”)
Below is the C program to write to a text file.
C
// C program to implement
// the above approach
#include <stdio.h>
#include <string.h>
// Driver code
int main()
{
// Declare the file pointer
FILE* filePointer;
// Get the data to be written in file
char dataToBeWritten[50]
= "GeeksforGeeks-A Computer"
+ " Science Portal for Geeks";
// Open the existing file GfgTest.c using fopen()
// in write mode using "w" attribute
filePointer = fopen("GfgTest.txt", "w");
// Check if this filePointer is null
// which maybe if the file does not exist
if (filePointer == NULL) {
printf("GfgTest.txt file failed to open.");
}
else {
printf("The file is now opened.\n");
// Write the dataToBeWritten into the file
if (strlen(dataToBeWritten) > 0) {
// writing in the file using fputs()
fprintf(filePointer, dataToBeWritten);
fprintf(filePointer, "\n");
}
// Closing the file using fclose()
fclose(filePointer);
printf("Data successfully written"
+ " in file GfgTest.txt\n");
printf("The file is now closed.");
}
return 0;
}
Output:
Read from a file:
Syntax:
FILE * filePointer; filePointer = fopen(“fileName.txt”, “r”);
Below is the C program to read text file.
C
// C program to implement
// the above approach
#include <stdio.h>
#include <string.h>
// Driver code
int main()
{
// Declare the file pointer
FILE* filePointer;
// Declare the variable for the data
// to be read from file
char dataToBeRead[50];
// Open the existing file GfgTest.txt
// using fopen() in read mode using
// "r" attribute
filePointer = fopen("GfgTest.txt", "r");
// Check if this filePointer is null
// which maybe if the file does not exist
if (filePointer == NULL) {
printf("GfgTest.txt file failed to open.");
}
else {
printf("The file is now opened.\n");
// Read the dataToBeRead from the file
// using fgets() method
while (fgets(dataToBeRead, 50,
filePointer)
!= NULL) {
// Print the dataToBeRead
printf("%s", dataToBeRead);
}
// Closing the file using fclose()
fclose(filePointer);
printf("Data successfully read"
+ " from file GfgTest.txt\n");
printf("The file is now closed.");
}
return 0;
}
Output:
Reading and writing in binary file
Write a binary file:
Syntax:
FILE *filePointer; filePointer = fopen(“fileName.bin”, “wb”);
To write data to a binary file, fwrite() function is needed. This function takes four arguments:
- Address of data to be written in the disk.
- Size of data to be written on the disk.
- The number of such types of data.
- Pointer to the file where you want to write.
Syntax:
fwrite(addressData, sizeofData, numbersData, pointerToFile);
Below is the C program to implement the above approach:
C
// C program to implement
// the above approach
#include <stdio.h>
#include <stdlib.h>
struct threeNum {
int n1, n2, n3;
};
// Driver code
int main()
{
int n;
struct threeNum num;
// Declaring the file pointer
FILE* fptr;
if ((fptr = fopen("C:\\GfgTest.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);
}
printf("The file GfgTest.bin is"
+ " written successfully");
fclose(fptr);
return 0;
}
Read from a binary file:
Syntax:
FILE * filePointer; filePointer = fopen(“fileName.txt”, “rb”);
To read data from a binary file, fread(0 function is used. Similar to fwrite() function, this function also takes four arguments.
Syntax:
fread(addressData, sizeofData, numbersData, pointerToFile);
Below is the C program to implement the above approach:
C
// C program to implement
// the above approach
#include <stdio.h>
#include <stdlib.h>
struct threeNum {
int n1, n2, n3;
};
// Driver code
int main()
{
int n;
struct threeNum num;
// Declaring the file pointer
FILE* fptr;
if ((fptr = fopen("C:\\GfgTest.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);
printf("\n");
}
fclose(fptr);
return 0;
}
Output:
Append content in text file
Syntax:
FILE * filePointer; filePointer = fopen(“fileName.txt”, “a”);
Once file is opened in append mode, rest of the task is same as that to write content in a text file.
Below is the example to append a string to the file:
C
// C program to implement
// the above approach
#include <stdio.h>
#include <string.h>
// Driver code
int main()
{
// Declare the file pointer
FILE* filePointer;
// Get the data to be appended in file
char dataToBeWritten[100]
= "It is a platform for"
+ " learning language"
+ " tech related topics";
// Open the existing file GfgTest.txt using
// fopen() in append mode using "a" attribute
filePointer = fopen("GfgTest.txt", "a");
// Check if this filePointer is null
// which maybe if the file does not exist
if (filePointer == NULL) {
printf("GfgTest.txt file failed to open.");
}
else {
printf("The file is now opened.\n");
// Append the dataToBeWritten into the file
if (strlen(dataToBeWritten) > 0) {
// writing in the file using fputs()
fprintf(filePointer, dataToBeWritten);
fprintf(filePointer, "\n");
}
// Closing the file using fclose()
fclose(filePointer);
printf("Data successfully appended"
+ " in file GfgTest.txt\n");
printf("The file is now closed.");
}
return 0;
}
Output:
Append content in binary file
Syntax:
FILE * filePointer; filePointer = fopen(“fileName.bin”, “ab”);
Once file is opened in append mode, rest of the task is same as that to write content in a binary file.
C
// C program to implement
// the above approach
#include <stdio.h>
#include <stdlib.h>
struct threeNum {
int n1, n2, n3;
};
// Driver code
int main()
{
int n;
struct threeNum num;
// Declaring the file pointer
FILE* fptr;
// Opening the file in
// append mode
if ((fptr = fopen("C:\\GfgTest.bin",
"ab"))
== NULL) {
printf("Error! opening file");
// Program exits if the file pointer
// returns NULL.
exit(1);
}
for (n = 1; n < 10; ++n) {
num.n1 = n;
num.n2 = 5 * n;
num.n3 = 5 * n + 1;
fwrite(&num, sizeof(struct threeNum),
1, fptr);
}
printf("The file GfgTest.bin"
+ " is appended successfully");
fclose(fptr);
return 0;
}
Output:
Opening file for both reading and writing
Syntax:
FILE * filePointer; filePointer = fopen(“fileName.txt”, “r+”);
The file is opened using the mode "r+'" and the file is opened in both reading and writing mode.
C
// C program to implement
// the above approach
#include <stdio.h>
#include <string.h>
// Driver code
int main()
{
// Declare the file pointer
FILE* filePointer;
char dataToBeWritten[100]
= "It is a platform for"
+ " learning language"
+ " tech related topics.";
// Declare the variable for the data
// to be read from file
char dataToBeRead[50];
// Open the existing file GfgTest.txt
// using fopen() in read mode using
// "r+" attribute
filePointer = fopen("GfgTest.txt", "r+");
// Check if this filePointer is null
// which maybe if the file does not exist
if (filePointer == NULL) {
printf("GfgTest.txt file failed to open.");
}
else {
printf("The file is now opened.\n");
// Read the dataToBeRead from the file
// using fgets() method
while (fgets(dataToBeRead, 50,
filePointer)
!= NULL) {
// Print the dataToBeRead
printf("%s", dataToBeRead);
}
printf(
"\nData successfully read"
+ " from file GfgTest.txt");
if (strlen(dataToBeWritten) > 0) {
// writing in the file using fprintf()
fprintf(filePointer, dataToBeWritten);
fprintf(filePointer, "\n");
}
printf("\nData successfully"
+ " written to the file");
// Closing the file using fclose()
fclose(filePointer);
printf("\nThe file is now closed.");
}
return 0;
}
Output:
Opening file for both reading and writing in binary mode
Syntax:
FILE * filePointer; filePointer = fopen(“fileName.bin”, “rb+”);
C
// C program to implement
// the above approach
#include <stdio.h>
#include <stdlib.h>
struct threeNum {
int n1, n2, n3;
};
// Driver code
int main()
{
int n;
struct threeNum num;
// Declaring the file pointer
FILE* fptr;
if ((fptr = fopen("C:\\GfgTest.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);
printf("\n");
}
printf("Data successfully read from the file");
for (n = 1; n < 7; ++n) {
num.n1 = n;
num.n2 = 5 * n;
num.n3 = 5 * n + 1;
fwrite(&num, sizeof(struct threeNum),
1, fptr);
}
printf("The file GfgTest.bin"
+ " is written successfully");
fclose(fptr);
return 0;
}
Output:
Opening file for both reading and writing in text mode
In this mode, the file is opened for both reading and writing in text mode. If the file exists, then the content is overwritten in the file, and in case the file does not exist then in that case, a new file is created.
Syntax:
FILE * filePointer; filePointer = fopen(“fileName.txt”, “w+”);
C
// C program to implement
// the above approach
#include <stdio.h>
#include <string.h>
// Driver code
int main()
{
// Declare the file pointer
FILE* filePointer;
char dataToBeWritten[100]
= "It is a platform"
+ " for learning language"
+ " tech related topics.";
// Declare the variable for the data
// to be read from file
char dataToBeRead[50];
// Open the existing file GfgTest.txt
// using fopen() in read mode using
// "r+" attribute
filePointer = fopen("GfgTest.txt", "w+");
// Check if this filePointer is null
// which maybe if the file does not exist
if (filePointer == NULL) {
printf("GfgTest.txt file failed to open.");
}
else {
printf("The file is now opened.\n");
if (strlen(dataToBeWritten) > 0) {
// writing in the file using fprintf()
fprintf(filePointer, dataToBeWritten);
fprintf(filePointer, "\n");
}
printf("Data successfully"
+ " written to the file\n");
// Read the dataToBeRead from the file
// using fgets() method
while (fgets(dataToBeRead, 50,
filePointer)
!= NULL) {
// Print the dataToBeRead
printf("%s", dataToBeRead);
}
printf("\nData successfully read"
+ " from file GfgTest.txt");
// Closing the file using fclose()
fclose(filePointer);
printf("\nThe file is now closed.");
}
return 0;
}
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Jump statements in C++Jump statements are used to manipulate the flow of the program if some conditions are met. It is used to terminate or continue the loop inside a program or to stop the execution of a function.In C++, there is four jump statement:Table of Contentcontinue Statementbreak Statementreturn Statementgoto S
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C++ LoopsIn C++ programming, sometimes there is a need to perform some operation more than once or (say) n number of times. For example, suppose we want to print "Hello World" 5 times. Manually, we have to write cout for the C++ statement 5 times as shown.C++#include <iostream> using namespace std; int
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for Loop in C++In C++, for loop is an entry-controlled loop that is used to execute a block of code repeatedly for the given number of times. It is generally preferred over while and do-while loops in case the number of iterations is known beforehand.Let's take a look at an example:C++#include <bits/stdc++.h
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Range-Based for Loop in C++In C++, the range-based for loop introduced in C++ 11 is a version of for loop that is able to iterate over a range. This range can be anything that is iteratable, such as arrays, strings and STL containers. It provides a more readable and concise syntax compared to traditional for loops.Let's take
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C++ While LoopIn C++, the while loop is an entry-controlled loop that repeatedly executes a block of code as long as the given condition remains true. Unlike the for loop, while loop is used in situations where we do not know the exact number of iterations of the loop beforehand as the loop execution is terminate
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C++ do while LoopIn C++, the do-while loop is an exit-controlled loop that repeatedly executes a block of code at least once and continues executing as long as a given condition remains true. Unlike the while loop, the do-while loop guarantees that the loop body will execute at least once, regardless of whether the
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C++ Functions
Functions in C++A Function is a reusable block of code designed to perform a specific task. It helps break large programs into smaller, logical parts. Functions make code cleaner, easier to understand, and more maintainable.Just like in other languages, C++ functions can take inputs (called parameters), execute a b
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return Statement in C++In C++, the return statement returns the flow of the execution to the function from where it is called. This statement does not mandatorily need any conditional statements. As soon as the statement is executed, the flow of the program stops immediately and returns the control from where it was calle
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Parameter Passing Techniques in CIn C, passing values to a function means providing data to the function when it is called so that the function can use or manipulate that data. Here:Formal Parameters: Variables used in parameter list in a function declaration/definition as placeholders. Also called only parameters.Actual Parameters
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Difference Between Call by Value and Call by Reference in CFunctions can be invoked in two ways: Call by Value or Call by Reference. These two ways are generally differentiated by the type of values passed to them as parameters.The following table lists the differences between the call-by-value and call-by-reference methods of parameter passing.Call By Valu
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Default Arguments in C++A default argument is a value provided for a parameter in a function declaration that is automatically assigned by the compiler if no value is provided for those parameters in function call. If the value is passed for it, the default value is overwritten by the passed value.Example:C++#include <i
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Inline Functions in C++In C++, inline functions provide a way to optimize the performance of the program by reducing the overhead related to a function call. When a function is specified as inline the whole code of the inline function is inserted or substituted at the point of its call during the compilation instead of us
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Lambda Expression in C++C++ 11 introduced lambda expressions to allow inline functions which can be used for short snippets of code that are not going to be reused. Therefore, they do not require a name. They are mostly used in STL algorithms as callback functions.Example:C++#include <iostream> using namespace std; i
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C++ Pointers and References
Pointers and References in C++In C++ pointers and references both are mechanisms used to deal with memory, memory address, and data in a program. Pointers are used to store the memory address of another variable whereas references are used to create an alias for an already existing variable. Pointers in C++ Pointers in C++ are a
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C++ PointersA pointer is a special variable that holds the memory address of another variable, rather than storing a direct value itself. Pointers allow programs to access and manipulate data in memory efficiently, making them a key feature for system-level programming and dynamic memory management. When we acc
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Dangling, Void , Null and Wild Pointers in CIn C programming pointers are used to manipulate memory addresses, to store the address of some variable or memory location. But certain situations and characteristics related to pointers become challenging in terms of memory safety and program behavior these include Dangling (when pointing to deall
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Applications of Pointers in CPointers in C are variables that are used to store the memory address of another variable. Pointers allow us to efficiently manage the memory and hence optimize our program. In this article, we will discuss some of the major applications of pointers in C. Prerequisite: Pointers in C. C Pointers Appl
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Understanding nullptr in C++Consider the following C++ program that shows problem with NULL (need of nullptr) CPP // C++ program to demonstrate problem with NULL #include <bits/stdc++.h> using namespace std; // function with integer argument void fun(int N) { cout << "fun(int)"; return;} // Overloaded fun
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References in C++In C++, a reference works as an alias for an existing variable, providing an alternative name for it and allowing you to work with the original data directly.Example:C++#include <iostream> using namespace std; int main() { int x = 10; // ref is a reference to x. int& ref = x; // printing v
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Can References Refer to Invalid Location in C++?Reference Variables: You can create a second name for a variable in C++, which you can use to read or edit the original data contained in that variable. While this may not sound appealing at first, declaring a reference and assigning it a variable allows you to treat the reference as if it were the
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Pointers vs References in C++Prerequisite: Pointers, References C and C++ support pointers, which is different from most other programming languages such as Java, Python, Ruby, Perl and PHP as they only support references. But interestingly, C++, along with pointers, also supports references. On the surface, both references and
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Passing By Pointer vs Passing By Reference in C++In C++, we can pass parameters to a function either by pointers or by reference. In both cases, we get the same result. So, what is the difference between Passing by Pointer and Passing by Reference in C++?Let's first understand what Passing by Pointer and Passing by Reference in C++ mean:Passing by
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When do we pass arguments by pointer?In C, the pass-by pointer method allows users to pass the address of an argument to the function instead of the actual value. This allows programmers to change the actual data from the function and also improve the performance of the program. In C, variables are passed by pointer in the following ca
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