C 中的格式说明符有什么意义？

Question

What is the point of format specifier in C if we have allready set the type of variable before printf?

For example:

#include<stdio.h>
int main(void)
{
    int a=7
    printf("%d", a);
} 

Like, it's allready stated what a is, it's integer(int). So what is the point of adding %d to specify that it's an integer?

Answer 1

The answer to this question really only makes sense in the context of C's history.

C is, by now, a pretty old language. Though undoubtedly a "high level language", it is famously low-level as high-level languages go. And its earliest compiler was deliberately and self-consciously small and simple.

In its first incarnation, C did not enforce type safety during function calls. For example, if you called sqrt(144) , you got the wrong answer, because sqrt expects an argument of type double , but 144 is an int . It was the programmer's responsibility to call a function with arguments of the correct types: the compiler did not know (did not even attempt to keep track of) the arguments expected by each function, so it did not and could not perform automatic conversions. (A separate program, lint , could check that functions were called with the correct arguments.)

C++ corrected this deficiency, by introducing the function prototype . These were inherited by C in the first ANSI C standard in 1989. However, a function prototype only works for a function that expects a single, fixed argument list, meaning that it can't help for functions that accept a variable number of arguments, the premier example being: printf .

The other thing to remember is that, in C, printf is a more or less ordinary function. ("Ordinary" other than accepting a variable number of arguments, that is.) So the compiler has no direct mechanism to notice the types of the arguments and make that list of types available to printf. printf has no way of knowing, at run time, what types were passed during any given call; it can only rely (it must rely) on the clues provided in the format string. (This is by contrast to languages, many of them, where the print statement is an explicit part of the language parsed by the compiler, meaning that the compiler can do whatever it needs to do in order to treat each argument properly according to its known type.)

So, by the rules of the language (which are constrained by backwards compatibility and the history of the language), the compiler can't do anything special with the arguments in a printf call, other than performing what is called the default argument promotions . So the compiler can't fix things (can't perform the "correct" implicit conversion) if you write something like

int a = 7;
printf("%f", a);

This is, admittedly, an uncomfortable situation. These days, programmers are used to the protections and the implicit promotions provided for by function prototypes. If, these days, you can call

int x = sqrt(144);

and have the right thing happen, why can't you similarly call

printf("%f\n", 144);

Well, you can't, although a good, modern compiler will try to help you out anyway. Although the compiler doesn't have to inspect the format string (because that's printf 's job to do, at run time), and the compiler isn't allowed to insert any implicit conversions (other than the default promotions, which don't help here), a compiler can duplicate printf 's logic, inspect the format string, and issue strong warnings if the programmer makes a mistake. For example, given

printf("%f\n", 144);

gcc prints "warning: format '%f' expects argument of type 'double', but argument 2 has type 'int", and clang prints "warning: format specifies type 'double' but the argument has type 'int'".

In my opinion, this is a fine compromise, balancing C's legacy behavior with modern expectations.

Answer 2

printf() accepts a variable number of arguments. To process those variable arguments it ( va_start() ) needs to know the last fixed argument is. It ( va_arg() ) also needs to know the type of each argument so it figure how much data to read.

The format specifier is also a compact template (or DSL) to express how text and variables should be formatted including field width, alignment, precision, encoding.

Answer 3

what is the point of adding %d to specify that it's an integer?

printf() is a function which receives a variable number of arguments of various type after the format argument. It does not directly know the number nor the type of arguments passed nor received.

The callers knows the argument count and types it gives to printf() .

To pass the arguments count and type information, the format argument is used by the caller to encodes the argument count and types. printf() uses that format and decodes it to know the argument count and type. It is very important that the format and following arguments passed are consistent.