C fopen() & fclose()

In this page:

Opening a file with fopen()
Closing a file with fclose()
Reading modes ("r", "rb")
Writing modes ("w", "wb", "a")
Managing multiple open files

Opening a file with fopen()

Common modes include "r" (read), "w" (write, truncating existing content), and "a" (append) -- always check the returned pointer against NULL before using it, since a missing file or insufficient permissions will cause fopen() to fail silently otherwise.

Example: Opening a file with fopen()

#include <stdio.h>
int main() {
	FILE *fp = fopen("data.txt", "w");
	if (fp == NULL) {
		printf("Failed to open");
		return 1;
	}
	fprintf(fp, "Hello");
	fclose(fp);
	return 0;
}

Closing a file with fclose()

Beyond just flushing buffered writes, forgetting to close files can eventually exhaust the operating system's limit on how many files a single process can have open simultaneously, causing later fopen() calls to start failing.

Example: Closing a file with fclose()

#include <stdio.h>
int main() {
	FILE *fp = fopen("data.txt", "w");
	fprintf(fp, "Hello");
	fclose(fp);
	printf("Closed properly");
	return 0;
}

Reading modes ("r", "rb")

Text mode (r) may perform platform-specific newline translation (like converting \r\n to \n on Windows), which corrupts binary data such as images -- binary mode (rb) disables this translation and reads bytes exactly as stored.

Example: Reading modes ("r", "rb")

#include <stdio.h>
int main() {
	FILE *fp = fopen("data.txt", "w");
	fprintf(fp, "Hello");
	fclose(fp);
	fp = fopen("data.txt", "r");
	char buffer[10];
	fgets(buffer, 10, fp);
	printf("%s", buffer);
	fclose(fp);
	return 0;
}

Writing modes ("w", "wb", "a")

w mode silently destroys any existing file content the moment you open it, even before you write anything -- if you need to preserve existing data while adding to it, a mode is the safer choice.

Example: Writing modes ("w", "wb", "a")

#include <stdio.h>
int main() {
	FILE *fp = fopen("data.txt", "w");
	fprintf(fp, "First");
	fclose(fp);
	fp = fopen("data.txt", "a");
	fprintf(fp, "Second");
	fclose(fp);
	printf("Appended without erasing");
	return 0;
}

Managing multiple open files

Each FILE pointer independently tracks its own read/write position within its file, so operations on one open file have no effect on another -- just make sure every fopen() call you make has a corresponding fclose() eventually.

Example: Managing multiple open files

#include <stdio.h>
int main() {
	FILE *fp1 = fopen("a.txt", "w");
	FILE *fp2 = fopen("b.txt", "w");
	fprintf(fp1, "File A");
	fprintf(fp2, "File B");
	fclose(fp1);
	fclose(fp2);
	printf("Both files written independently");
	return 0;
}

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