0xA0Lesson 11 of 20

Strings

C has no string type - just arrays of characters ending in a zero. Here is how to work with them safely.

15 min 5-question quiz 2 code exercises

By the end of this lesson you can

Explain how C stores strings with a null terminator
Use strlen, strcpy/snprintf and strcmp correctly
Read a line of text safely with fgets

A string in C is an array of char whose last character is the null terminator '\0' - a byte with value zero. Every string function relies on it: that zero is the only way to know where the string ends.

hello_chars.c

1#include <stdio.h>
2
3int main(void) {
4    char word[] = "Hi!";   // 4 bytes: 'H' 'i' '!' '\0'
5    printf("%s has %zu bytes\n", word, sizeof word);
6    for (int i = 0; word[i] != '\0'; i++) {
7        printf("word[%d] = '%c'\n", i, word[i]);
8    }
9    return 0;
10}

Output

Hi! has 4 bytes
word[0] = 'H'
word[1] = 'i'
word[2] = '!'

The string "Hi!" has 3 visible characters but takes 4 bytes. Forgetting to leave room for the terminator is one of the most common C bugs. When you size a buffer, always think "length + 1".

String literals and char pointers

char name[] = "Ada"; creates your own modifiable array, copied from the literal. const char *name = "Ada"; instead points at the literal itself, which lives in read-only memory - trying to modify it is undefined behavior. That is why the pointer is declared const: use it for strings you only read.

The <string.h> toolkit

Function	Does
`strlen(s)`	number of characters before the `'\0'` (not counting it)
`strcmp(a, b)`	compares: 0 if equal, negative if `a` sorts first, positive otherwise
`strcpy(dest, src)`	copies `src` into `dest` - no size check
`strcat(dest, src)`	appends `src` to `dest` - no size check
`snprintf(dest, size, fmt, ...)`	printf into a buffer, never writing more than `size` bytes

compare.c

1#include <stdio.h>
2#include <string.h>
3
4int main(void) {
5    const char *password = "hunter2";
6    char attempt[] = "hunter2";
7
8    if (password == attempt) {
9        printf("== says equal\n");
10    }
11    if (strcmp(password, attempt) == 0) {
12        printf("strcmp says equal\n");
13    }
14    printf("length: %zu\n", strlen(attempt));
15    return 0;
16}

Output

strcmp says equal
length: 7

Reading a line with fgets

scanf("%s", buf) stops at the first space and has no idea how big buf is. To read a whole line safely, use fgets(buf, sizeof buf, stdin): it reads at most sizeof buf - 1 characters and adds the terminator. It keeps the newline character, which you usually want to remove:

greet_line.c

1#include <stdio.h>
2#include <string.h>
3
4int main(void) {
5    char name[64];
6    if (fgets(name, sizeof name, stdin) == NULL) {
7        return 1;
8    }
9    name[strcspn(name, "\n")] = '\0';   // chop the newline
10    printf("Hello, %s!\n", name);
11    return 0;
12}

strcspn(name, "\n") returns the index of the first newline (or the string length if there is none), so writing '\0' there cleanly trims it.

Key takeaways

A C string is a char array ending in '\0'; it needs length + 1 bytes.
Compare with strcmp(a, b) == 0, never ==. Measure with strlen (which excludes the terminator).
Prefer bounded functions: snprintf over strcpy/strcat, and fgets over scanf("%s").

Lesson quiz

5 questions · pass with 4 correct · up to 50 XP

Passing this quiz completes the lesson and keeps your streak going. Questions you miss come back in review sessions later.

Practice: write C

These run for real. Write the program, press Run tests, and read what the compiler tells you - learning to read compiler messages is half of learning C.

Exercise 1

Count the vowels

+25 XP

Read a line of text (use fgets) and print how many vowels it contains - a, e, i, o, u in either case.

For Hello World print 3.

Hello World
Upper case
No vowels

main.c

Compiler Explorer

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Your code is compiled with gcc 14 (-Wall -Wextra) and run on Compiler Explorer (godbolt.org), a free public service.

Exercise 2

Reverse a word

+25 XP

Write void reverse(char *s) that reverses a string in place (swap characters from both ends toward the middle). main reads one word and prints it reversed.

For stressed print desserts.

stressed
Odd length
Single character

main.c

Compiler Explorer

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Your code is compiled with gcc 14 (-Wall -Wextra) and run on Compiler Explorer (godbolt.org), a free public service.

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