escaping - Why is percentage character not escaped with backslash in C?

Question

The printf() documentation says that if someone wants to print % in C, he can use:

printf("%%")

Why is it not:

printf("\%")

as with other special characters?

Answer 1

The backslash is processed by the compiler when interpreting the source text of the program. So the common result is that the source text "\%" produces a string containing ”%”.

The format string is interpreted by the printf routine, so it processes the characters passed to it. By this time, the backslash is no longer present, so printf never sees it.

Technically, \% is not legal in a string literal. The character starts an escape sequence, and the only legal escape sequences are listed in C 2011 6.4.4.4 1. They are followed by ', ", ?, , a, b, f, n, r, t, v, one to three octal digits, x and hexadecimal digits, u and four hexadecimal digits, or U and eight hexadecimal digits.

If printf had been designed so that a backslash would escape a percent, then you would have to pass it a backslash by escaping the backslash in the source text, so you would have to write:

printf("\%");