C's fprintf() function shares similarities to the printf() function, in that they are both used to output text. 

The key difference between them is that fprintf() shares formatted output to a file stream rather than on the stdout console.

In this brief guide, we will help you understand what fprintf() is and how to use it.

What is the fprintf() Function?

Programmers sometimes find the need to write data into files rather than the terminal, and this is where the fprintf() function comes in. 

In C, the function is known as the format print function. You can find the function's definition in the stdio.h header file. 

The interesting thing about this function is that it can send a formatted output string to a stream. It counts the number of characters successfully written to the file and returns the count. If the function fails, it returns an EOF.

Syntax of the fprintf() Function 

Declaring the fprint() function in a C program is quite straightforward. You can use the syntax:

int fprintf (FILE * stream, const char * format, ...)

In C, when using the fprintf function, you provide a stream, which is a pointer to a file object where you want to write data and a format containing the text you want to write to the stream. 

If the operation is successful, fprintf() returns an integer indicating the number of characters written to the file.

Parameters Supported by fprintf()

The fprintf() function can accept two arguments: stream and format.

The stream argument is the pointer to a FILE object. It helps identify the sequence of file objects where data should be written. 

The C string represents the text from the file indicated by the stream pointer. 

You can include format tags, which will be substituted with the values provided in the subsequent arguments.

The blueprint of a format tag is as follows:

% [flags][width][.precision][length]specifier.

Let's understand the components of this tag one after another, beginning with the flags.

Flags

Width

Precision

Specifiers

Length

Depending on the format string passed to the fprintf() function, it may expect some additional arguments. Additionally, these extra arguments expect a value for every "%" tag present in the format string.

The Return Value of the fprintf() Function

This function returns an integer value representing the number of characters printed into a file.

If the function encounters some issue when writing to the file stream, it stops and returns a negative value. 

Example of fprintf() Function

Before you use the fprintf() function in C, you must open the text file in write mode in a specific position. Of course, you will need to use a file pointer. 

Next, you must write the string into the file by passing the pointer and the string to the fprintf() function. Here's an example:

#include <stdio.h>

int main() {

  char input[100];

  FILE *filePointer;

  int numberOfInputs, i;

  filePointer = fopen("output.txt", "w");

  if (filePointer == NULL) {

    printf("Error: Unable to open file!");

    return 0;

  }

  printf("Enter number of inputs to write to the file: ");

  scanf("%d", &numberOfInputs);

  for (i = 1; i <= numberOfInputs; i++) {

    printf("Enter input %d: ", i);

    scanf("%s", input);

    fprintf(filePointer, "Input %d: %s\n", i, input);

  }

  fclose(filePointer);

  return 0;

}

This program prompts the user for a certain number of inputs, writes those inputs to a file along with their indices, and then closes the file.

Let's assume a user gives this program the following inputs:

Enter number of inputs to write to the file: 3

Enter input 1: Apple

Enter input 2: Banana

Enter input 3: Orange

After the user enters these inputs, the program writes them into a file named "output.txt" in the following format:

Another Example of fprintf() Function

The program below prompts the user for student information and then writes it to a file named "student_info.txt" before closing the file.

#include <stdio.h>

int main() {

  // Initializing file pointer.

  FILE *file_pointer;

  file_pointer = fopen("student_info.txt", "w");

  // If file_pointer is a null pointer, the file opening failed.

  if (file_pointer == NULL) {

    printf("Failed to open file!");

    return 0;

  }

  int roll_number;

  char name[50];

  float gpa;

  // Input student info from the user.

  printf("Enter student's name: ");

  scanf("%s", name);

  printf("Enter student's roll number: ");

  scanf("%d", &roll_number);

  printf("Enter student's GPA: ");

  scanf("%f", &gpa);

  // Writing data into the file.

  fprintf(file_pointer, "Name: %s\n", name);

  fprintf(file_pointer, "Roll Number: %d\n", roll_number);

  fprintf(file_pointer, "GPA: %.2f\n", gpa);

  // Close opened file.

  fclose(file_pointer);

  return 0;

}

Let's assume that the user gives the program these inputs:

After the user enters this information, the program creates a file named "student_info.txt" and writes the following content into it:

If the file creation or writing fails, the program will output "Failed to open file!" instead of creating the file and writing the information.