Unknown escape sequences

K&R Exercise 1-2: put a backslash pair that is not a real escape — \c is the usual first try — inside printf, then compile and see what the compiler does.

Escapes you already know

printf strings can include escapes: \n for a newline, \t for a tab, \\ for a backslash, \" for a quote. Hello, world already used \n. The question is what happens when you invent one.

Unicorns do not exist. Neither does \c.

Write the experiment

Create a file called unknown-escape.c:

#include <stdio.h>

int main(void)
{
    printf("hello, world\c");
    return 0;
}

Same program as hello, world, except the newline is now a \c.

Compile and read the warning

gcc unknown-escape.c -o unknown-escape

Clang does not refuse the file. It warns:

unknown-escape.c:5:25: warning: unknown escape sequence '\c' [-Wunknown-escape-sequence]
    printf("hello, world\c");
                        ^~
1 warning generated.

That caret sits under \c. This is a warning, not an error — you still get a binary.

Run it

./unknown-escape

You should see:

hello, worldc

The backslash vanished. The c stayed. There is no newline, so your shell prompt often sticks to the same line.

The compiler did not invent a new escape. It dropped the \ and kept the letter.

Try another letter

Change \c to \q (or any letter that is not a real escape) and compile again. You should get the same kind of warning and the same kind of output: the letter, no backslash.

Stick to real escapes when you mean them: \n, \t, \b, \", and \\ are the ones this chapter has used so far.

What you should take away

An unknown escape is not a secret feature. On Clang and GCC it is a warning, the program still runs, and \c becomes a plain c. Read the diagnostics — they point at the exact pair.

Next

→ Fahrenheit to Celsius table — Exercise 1-3