C++ fopen()

fopen() prototype

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

The fopen() function takes a two arguments and returns a file stream associated with that file specified by the argument filename.

It is defined in <cstdio> header file.

Different types of file access mode are as follows:

fopen() Parameters

• filename: Pointer to the string containing the name of the file to be opened.
• mode: Pointer to the string that specifies the mode in which file is opened.

fopen() Return value

• If successful, the fopen() function returns a pointer to the FILE object that controls the opened file stream.
• On failure, it returns a null pointer.

Example 1: Opening a file in write mode using fopen()

#include <cstdio>
#include <cstring>

using namespace std;

int main()
{
	int c;
	FILE *fp;
	fp = fopen("file.txt", "w");
	char str[20] = "Hello World!";
	if (fp)
	{
		for(int i=0; i<strlen(str); i++) 
			putc(str[i],fp);
	}
	fclose(fp);
}

When you run the program, it will not generate any output but will write "Hello World!" to the file "file.txt".

Example 2: Opening a file in read mode using fopen()

#include <cstdio>

using namespace std;

int main()
{
	int c;
	FILE *fp;
	fp = fopen("file.txt", "r");
	if (fp)
	{
		while ((c = getc(fp)) != EOF)
			putchar(c);
		fclose(fp);
	}
	return 0;
}

When you run the program, the output will be [Assuming the same file as in Example 1]:

Hello World!

Example 3: Opening a file in append mode using fopen()

#include <cstdio>
#include <cstring>

using namespace std;

int main()
{
	int c;
	FILE *fp;
	fp = fopen("file.txt", "a");
	char str[20] = "Hello Again.";
	if (fp)
	{
		putc('\n',fp);
		for(int i=0; i<strlen(str); i++) 
			putc(str[i],fp);
	}
	fclose(fp);
}

When you run the program, it will not generate any output but will append "Hello Again" in a newline to the file "file.txt".