C fprintf() & fscanf()

fprintf writes neatly formatted text to a file and fscanf reads it back, like printf and scanf but for a notebook instead of the screen. They match the values you save with the ones you load.

In this page:

Writing formatted text with fprintf()
Reading formatted text with fscanf()
Working with mixed data types
Appending data with fprintf()
Handling EOF in fscanf()

Syntax

fprintf(file_pointer, "format", values);

fscanf(file_pointer, "%specifier", &variable_name);

Writing formatted text with fprintf()

This means all the same format specifiers you use with printf() (%d, %s, %f, and so on) work identically with fprintf(), just directed at a file stream instead of the console -- useful for generating human-readable log files or reports.

Example: Writing formatted text with fprintf()

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	FILE *fp = fopen("data.txt", "w");
	fprintf(fp, "Age: %d", 25);
	fclose(fp);
	// Print formatted text to the screen
	printf("Formatted text written to file");
	// Return 0 to signal that the program finished successfully
	return 0;
}

Reading formatted text with fscanf()

fscanf() uses the same %d, %s, %f format specifiers as scanf(), and like scanf(), it requires addresses (using &) for non-string variables to know where to store what it reads.

Example: Reading formatted text with fscanf()

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	FILE *fp = fopen("data.txt", "w");
	fprintf(fp, "%d", 42);
	fclose(fp);
	int value;
	fp = fopen("data.txt", "r");
	fscanf(fp, "%d", &value);
	fclose(fp);
	// Print formatted text to the screen
	printf("%d", value);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Working with mixed data types

For example, you might write a CSV-style line with fprintf(fp, "%s,%d,%.2f\n", name, age, gpa) and later parse it back with a matching fscanf() call using the same format string structure.

Example: Working with mixed data types

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	FILE *fp = fopen("data.csv", "w");
	fprintf(fp, "%s,%d,%.2f\n", "Alice", 20, 3.5);
	fclose(fp);
	char name[20];
	int age;
	float gpa;
	fp = fopen("data.csv", "r");
	fscanf(fp, "%[^,],%d,%f", name, &age, &gpa);
	fclose(fp);
	// Print formatted text to the screen
	printf("%s %d %.2f", name, age, gpa);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Appending data with fprintf()

Because a mode positions the file pointer at the end before any writes happen, repeated fprintf() calls across multiple program runs build up a growing log file rather than overwriting previous entries each time.

Example: Appending data with fprintf()

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	FILE *fp = fopen("log.txt", "w");
	fprintf(fp, "First entry\n");
	fclose(fp);
	fp = fopen("log.txt", "a");
	fprintf(fp, "Second entry\n");
	fclose(fp);
	// Print formatted text to the screen
	printf("Log grows across runs");
	// Return 0 to signal that the program finished successfully
	return 0;
}

Handling EOF in fscanf()

fscanf() returns the number of items it successfully matched and assigned, and this drops below your expected count (or hits EOF entirely) once there's no more data left to read -- checking this is how you detect the end of variable-length input.

Example: Handling EOF in fscanf()

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	FILE *fp = fopen("data.txt", "w");
	fprintf(fp, "5");
	fclose(fp);
	int value;
	fp = fopen("data.txt", "r");
	// Declare result and set it to fscanf(fp, "%d", &value)
	int result = fscanf(fp, "%d", &value);
	fclose(fp);
	// Print formatted text to the screen
	printf("%d", result == 1);
	// Return 0 to signal that the program finished successfully
	return 0;
}

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