该语言如何与 %s 和实际字符串“%s”之类的格式代码区分开来？

Question

For example, in the code printf("hello world, %s", variable) it would print "hello world stack", but what if I actually wanted to print "hello world %s"?

How does the computer/language know the difference? From my perspective, it seems like the format code (%s) is left inside the quotes, therefore it should print the whole thing as a string.

Answer 1

You are right about the string part - a format string is just any ordinary string, and the '%' is stored in the memory just as a percentage sign. In fact, if you feed the very same string to puts , it just prints %s as is.

However, the interpretation of the format string of printf -family functions is done at runtime: it actually scans the format string character by character, and whenever it encounters a '%' , it parses the neighbouring few char s as a format specifier, and formats the corresponding parameter.

To print a literal '%' however, you just use %% and printf will know to output a percentage sign.

You can read the glibc implementation of printf -family functions here . Or a much tidier musl implementation of strftime here .

Answer 2

You can escape the '%' character with another '%':

printf("Hello: %%s\r\n");

will print "Hello: %s "

Answer 3

You can use printf("%%s") to print "%s", but you should really only use the pattern string for patterns. It would be better to do something like printf("%s", "%s") - that is, the string you want to print is the second, it will not interpret "%s", "%d", or any other pattern so you don't have to worry about how to express them.

It's also kind of useless and confusing for a single string, so better to use puts("%s") .