1310_ Implementation analysis of a printf

All learning summary ： GitHub - GreyZhang/c_basic: little bits of c.

From this place we can see , Although we're calling printf, But the functions implemented in this module are actually printf_, It's just a clever transformation through the skill of macro definition . In this way, we can maintain the original habit of writing code , At the same time, it avoids possible printf Conflict .

About va_list、va_start、va_end Use , Let's take a look at... Through an example .

The example above , You can see the simple use of several macros . meanwhile , There is also a new one va_arg Use . First compile and run this code .

This is the code compilation above 、 The effect of operation . Actually , Here is a key place . Although these va_xxx The usage of function variable parameter realizes the call of function variable parameter , But in fact, the number of this parameter cannot be recognized automatically , It is controlled by the code itself . What does this mean ？ We can see such a conclusion by modifying the code .

Like the code above , I put count The parameter of is changed to 3, The following parameters are still 4 individual . that , What is the effect of such compilation ？

This is the effect of compiling and running the above code . From this result , Actually, I did 5 Finally, the program only recognizes 4 individual . this 4 individual , It could be interpreted as one 1 + 3 Result , And the first parameter in front count It determines how many parameters should be taken in the end . That is to say , The number identification of this parameter cannot be automated or intelligent , You can only make program level judgments by yourself . such , Continue with the previous analysis .

such , Let's look at the implementation of this function . In fact, the key point will shift to _vsnprintf The use of functions ._vsnprintf function , Called a character output function , A temporary one in front buffer, After the printf_ The passed in parameters of are completely inherited .

Take another look at the basic parameters of this function ： The first parameter is actually a function type , This definition is attached to the above ; The second parameter is a pointer to a character , From the name, it should be a buffer of characters , The third is the maximum length , The fourth and fifth parameters are from printf_ function .

The next judgment is to look at this character buffer Is it a valid character , If not, then you don't need to output , The output function called is also an output free function . above printf_ Defines a valid buffer, In fact, it also ensures that this place will not be effective out It's covered up .

Keep going , If the input parameter is not % At the beginning , That is to say, characters can be output directly . Otherwise , The processing of parameters moves back .

So much above , In fact, I just made a judgment on the format . If , To give this pritnf If you continue to lose weight , There are some parameters I don't usually use, such as ,j,z wait , I can remove it directly .

The code interception of this part is not very complete , But it is easy to understand . What we deal with here is all digital information , First of all, a module is judged , Know exactly how many decimal . Here, we can probably know his next handling method , I won't analyze the scattered small functions . Another clever place is the processing of positive and negative values , Directly through this, the processing logic is divided into two . After that , The data display format conversion and output are packaged together .

Here is an implementation support of floating point and scientific counting .

Here is the printing of characters and strings , In fact, format conversion is considered in every processing , This may result in output content ratio buffer More , The main function is filling and alignment . If only for debugging embedded software , In fact, such functions can also be accommodated , A part of the design can be eliminated .

There is not much to explain in the following output , Here is an output %, In fact, there is only % But when there is no data format , Then you can directly output one % end .

Look at it this way , In fact the whole printf The implementation of is very simple , The key point is actually this vsnprintf The implementation of the .

In the code I used , There are also several such functions . Actually , These functions are almost useless for my usual use , therefore , It can also be optimized directly . I directly try to do a code shielding , Then compile and test the whole project .

The result is the same as I expected , There are no compilation or link problems . look , The gadget I use can continue to be tailored in terms of resources .