Why does C's printf format string have both %c and %s?

Probably to distinguish between null terminated string and a character. If they just had %s, then every single character must also be null terminated.

char c = 'a';

In the above case, c must be null terminated. This is my assumption though :)

%s prints out chars until it reaches a 0 (or '\0', same thing).

If you just have a char x;, printing it with printf("%s", &x); - you'd have to provide the address, since %s expects a char* - would yield unexpected results, as &x + 1 might not be 0.

So you couldn't just print a single character unless it was null-terminated (very inefficent).

EDIT: As other have pointed out, the two expect different things in the var args parameters - one a pointer, the other a single char. But that difference is somewhat clear.

The issue that is mentioned by others that a single character would have to be null terminated isn't a real one. This could be dealt with by providing a precision to the format %.1s would do the trick.

What is more important in my view is that for %s in any of its forms you'd have to provide a pointer to one or several characters. That would mean that you wouldn't be able to print rvalues (computed expressions, function returns etc) or register variables.

Edit: I am really pissed off by the reaction to this answer, so I will probably delete this, this is really not worth it. It seems that people react on this without even having read the question or knowing how to appreciate the technicality of the question.

To make that clear: I don't say that you should prefer %.1s over %c. I only say that reasons why %c cannot be replaced by that are different than the other answer pretend to tell. These other answers are just technically wrong. Null termination is not an issue with %s.

The printf function is a variadic function, meaning that it has variable number of arguments. Arguments are pushed on the stack before the function (printf) is called. In order for the function printf to use the stack, it needs to know information about what is in the stack, the format string is used for that purpose.

e.g.

printf( "%c", ch );    tells the function the argument 'ch' 
                       is to be interpreted as a character and sizeof(char)

whereas

printf( "%s", s );   tells the function the argument 's' is a pointer 
                     to a null terminated string sizeof(char*)

it is not possible inside the printf function to otherwise determine stack contents e.g. distinguishing between 'ch' and 's' because in C there is no type checking during runtime.

%s says print all the characters until you find a null (treat the variable as a pointer).

%c says print just one character (treat the variable as a character code)

Using %s for a character doesn't work because the character is going to be treated like a pointer, then it's going to try to print all the characters following that place in memory until it finds a null

Stealing from the other answers to explain it in a different way.

If you wanted to print a character using %s, you could use the following to properly pass it an address of a char and to keep it from writing garbage on the screen until finding a null.

char c = 'c';
printf('%.1s', &c);