C Library - fopen() function

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The C library function FILE *fopen(const char *filename, const char *mode) opens the filename pointed to, by filename using the given mode. It opens a file and returns a pointer to a FILE object that can be used to access the file.

Syntax

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

FILE *fopen(const char *filename, const char *mode);

Parameter

This function accepts the following parameters −

• filename: A string representing the name of the file to be opened. This can include an absolute or relative path.
• mode: A string representing the mode in which the file should be opened. Common modes include:
• "r": Open for reading. The file must exist.
• "w": Open for writing. Creates an empty file or truncates an existing file.
• "a": Open for appending. Writes data at the end of the file. Creates the file if it does not exist.
• r+": Open for reading and writing. The file must exist.
• "w+": Open for reading and writing. Creates an empty file or truncates an existing file.
• a+": Open for reading and appending. The file is created if it does not exist.

Return Value

The fopen function returns a FILE pointer if the file is successfully opened. If the file cannot be opened, it returns NULL.

Example 1: Reading a File

This example opens a file named example.txt in read mode and prints its contents to the console character by character.

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

#include <stdio.h>

int main() {
   FILE *file = fopen("example.txt", "r");
   if (file == NULL) {
       perror("Error opening file");
       return 1;
   }

   char ch;
   while ((ch = fgetc(file)) != EOF) {
       putchar(ch);
   }

   fclose(file);
   return 0;
}

Output

The above code produces following result−

(Contents of example.txt printed to the console)

Example 2: Appending to a File

Here, we open a file named log.txt in append mode, adds a new log entry to the end of the file, and then closes the file.

#include <stdio.h>

int main() {
   FILE *file = fopen("log.txt", "a");
   if (file == NULL) {
       perror("Error opening file");
       return 1;
   }

   const char *log_entry = "Log entry: Application started";
   fprintf(file, "%s\n", log_entry);

   fclose(file);
   return 0;
}

Output

After execution of above code, we get the following result −

(log.txt contains the new log entry "Log entry: Application started" appended to any existing content)