Double quotes vs single quotes in C

In C, single quotes and double quotes mean two different things. Single quotes create a character constant, a single char value. Double quotes create a string literal, a sequence of characters terminated by a 0.

In some languages there’s no difference between the two. In C there is, and picking the wrong one gives you compiler warnings or bugs.

Single quotes create a character

Single quotes are used to identify a single character (char value):

char letter = 'a';

A character constant is really a number. 'a' is the code of the letter a in the character set, which is 97 in ASCII:

char letter = 'a';
printf("%c\n", letter); // a
printf("%d\n", letter); // 97

Since it’s a number, you can do math with it. 'a' + 1 gives you 'b':

printf("%c\n", letter + 1); // b

One detail that surprises many people: in C, character constants have type int, not char. So sizeof('a') is 4 on most systems, not 1. The value fits in a char when you assign it, but the constant itself is an int.

Double quotes create a string literal

Double quotes are used to create a string literal:

char *name = "Flavio";

A string is composed by the characters of the string, plus a 0 character at the end, called the null terminator. That’s how C knows where the string ends.

So "Flavio" takes 7 bytes of memory, not 6:

printf("%zu\n", sizeof("Flavio")); // 7

Note that you can create a single-letter string literal:

char *letter = "a";

But this is not the same as 'a'. The string "a" is two bytes (the letter plus the terminator), and it’s a pointer to memory, not a number.

What happens if you mix them up?

If you write this, the compiler warns you:

char letter = "a"; // warning: assigning a pointer to a char

The same goes for comparisons. if (letter == "a") compares a character against a memory address, which is never what you want. Use 'a' to compare single characters, and strcmp() to compare strings.

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