What is the difference between printf() and puts() in C?

Question

I know you can print with printf() and puts(). I can also see that printf() allows you to interpolate variables and do formatting.

Answer 1

(This is pointed out in a comment by Zan Lynx, but I think it deserves an aswer - given that the accepted answer doesn't mention it).

The essential difference between puts(mystr); and printf(mystr); is that in the latter the argument is interpreted as a formatting string. The result will be often the same (except for the added newline) if the string doesn't contain any control characters (%) but if you cannot rely on that (if mystr is a variable instead of a literal) you should not use it.

So, it's generally dangerous -and conceptually wrong- to pass a dynamic string as single argument of printf:

  char * myMessage;
  // ... myMessage gets assigned at runtime, unpredictable content
  printf(myMessage);  // <--- WRONG! (what if myMessage contains a '%' char?) 
  puts(myMessage);    // ok
  printf("%s\n",myMessage); // ok, equivalent to the previous, perhaps less efficient

The same applies to fputs vs fprintf (but fputs doesn't add the newline).

Answer 2

Besides formatting, puts returns a nonnegative integer if successful or EOF if unsuccessful; while printf returns the number of characters printed (not including the trailing null).

Answer 3

In simple cases, the compiler converts calls to printf() to calls to puts().

For example, the following code will be compiled to the assembly code I show next.

#include <stdio.h>
main() {
    printf("Hello world!");
    return 0;
}

push rbp
mov rbp,rsp
mov edi,str.Helloworld!
call dword imp.puts
mov eax,0x0
pop rbp
ret

In this example, I used GCC version 4.7.2 and compiled the source with gcc -o hello hello.c.

Answer 4

Right, printf could be thought of as a more powerful version of puts. printf provides the ability to format variables for output using format specifiers such as %s, %d, %lf, etc...