C Library - fprintf() function

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The C library fprintf() function is used to write formatted data to a stream. It is part of the standard I/O library <stdio.h> and allows you to write data to a file stream as opposed to printf() which writes to the standard output stream.

Syntax

Following is the C library syntax of the fprintf() function −

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

Parameters

This function accepts the following parameters −

• stream :A pointer to the FILE object that identifies the stream where the output is to be written.
• format : A string that specifies the format of the output. It may contain format specifiers that are replaced by the values specified in the subsequent arguments.
• ... : Additional arguments that correspond to the format specifiers in the format string.

Return Value

The fprintf() function returns the number of characters written if successful, and a negative value if an error occurs.

Example 1: Writing to a File

This example opens a file named "output.txt" in write mode, writes "Hello, World!" to it using fprintf(), and then closes the file.

Below is the illustration of C library fprintf() function.

#include <stdio.h>

int main() {
   FILE *file_ptr;
   
   // Open file in write mode
   file_ptr = fopen("output.txt", "w"); 
   if (file_ptr == NULL) {
       printf("Error opening file!");
       return 1;
   }
   fprintf(file_ptr, "Hello, World!\n");
   
   // Close the file
   fclose(file_ptr); 
   return 0;
}

Output

The above code produces following result−

Hello, World!

Example 2: Writing Multiple Lines

This example shows how to write multiple lines to a file using successive calls to fprintf().

#include <stdio.h>

int main() {
   FILE *file_ptr;
   file_ptr = fopen("output.txt", "w");
   if (file_ptr == NULL) {
       printf("Error opening file!");
       return 1;
   }
   fprintf(file_ptr, "Line 1\n");
   fprintf(file_ptr, "Line 2\n");
   fprintf(file_ptr, "Line 3\n");
   fclose(file_ptr);
   return 0;
}

Output

After execution of above code, we get the following result

Line 1
Line 2
Line 3