C Strings

A string in C is a row of letters that ends with a special stop marker, like a train with a last car. It is how C holds text.

In this page:

What is a String?
Declaring and Initializing
The Null-Terminator
Accessing Characters
Reading Strings safely with fgets

Syntax

char string_name[size];

char string_name[] = "text";  // ends with a hidden '\0'

printf("%s", string_name);

What is a String?

This null-terminator convention is what lets functions like strlen() know where a string ends without being told its length separately -- but it also means the underlying char array must always be at least one byte larger than the visible text to hold that trailing zero.

Example: What is a String?

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	char name[] = "Hi";
	// Print formatted text to the screen
	printf("%s", name);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Declaring and Initializing

char name[20] reserves a fixed 20-byte buffer regardless of how much text you actually store in it, while char name[] = "Hi" lets the compiler size the array to exactly fit the initializer plus its null terminator (3 bytes here).

Example: Declaring and Initializing

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	char name[20] = "Alex";
	// Print formatted text to the screen
	printf("%s", name);
	// Return 0 to signal that the program finished successfully
	return 0;
}

The Null-Terminator

Because \0 has the integer value 0, it's often described as falsy in conditional checks -- a common idiom like while (*str) loops until it hits the terminator, since any other character has a nonzero ASCII value.

Example: The Null-Terminator

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	char str[] = "Hi";
	// Declare i and set it to 0
	int i = 0;
	// Repeat while str[i] != '/0' is true
	while (str[i] != '\0') {
		i++;
	}
	// Print formatted text to the screen
	printf("%d", i);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Accessing Characters

Since a C string is just a char array, name[0] = J changes only the first character; there's no dedicated string-replace function, and any modification that shortens the visible text still needs its own \0 placed at the new end.

Example: Accessing Characters

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	char name[] = "Jill";
	name[0] = 'J';
	// Print formatted text to the screen
	printf("%s", name);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Reading Strings safely with fgets

fgets() takes a maximum buffer size as an argument, so it can never write past the end of your array the way gets() can -- the one gotcha is that fgets() keeps the trailing newline character if there's room, which gets() never did.

Example: Reading Strings safely with fgets

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	char buffer[20] = "Sample\n";
	// Print formatted text to the screen
	printf("%s", buffer);
	// Return 0 to signal that the program finished successfully
	return 0;
}

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