Thread: How does the signature of printf justify its use?

  1. #1
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    How does the signature of printf justify its use?

    For example the prototype of printf is the following.

    Code:
    int printf ( const char * format, ... );
    However, I don't understand what happens when we have the following code here.

    Code:
    char myString[50] = "Print this string";
    int myInt = 50;
    
    printf("This is a string: %s", myString);
    printf("This is a string: %s and an int: %d", myString, myInt);

    if myInt is an integer, doesn't that disobey the const char* format for printfs arguments? Is there some sort of casting that's happening somewhere?

  2. #2
    C++ Witch laserlight's Avatar
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    You are looking at the use of a variable argument list. There is a tutorial here that briefly covers its use.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
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    There's a kind of casting going on, but it's through the va_arg macro.
    Consider a simplified printf with only %d and %s (I've also cheated and used printf to actually print the ints!)
    Code:
    #include <stdio.h>
    #include <stdarg.h>
    
    int myprintf(const char *fmt, ...) {
      va_list va;
      va_start(va, fmt);
      for ( ; *fmt; fmt++) {
        switch (*fmt) {
        case '%':
          switch (fmt[1]) {
          case 'd':
            printf("%d", va_arg(va, int));  // interpret next arg as an int
            fmt++;
            break;
          case 's':
            printf("%s", va_arg(va, char*)); // interpret next arg as char*
            fmt++;
            break;
          default:
            ; // unknown format spec
          }
          break;
        case '\\':
          if (fmt[1]) putchar(fmt[1]);
          break;
        default:
          putchar(*fmt);
        }
      }
      va_end(va);
    }
    
    int main(void) {
      myprintf("Hello world!\n");
      myprintf("%d dollars and %d cents\n", 121, 12);
      myprintf("%d %s enter a %s\n", 3, "nuns", "bar");
      return 0;
    }

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