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This article introduces the relevant knowledge of "how to understand the reentrant function". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

1 preface

Recently, I came across a bug that has been located for a long time in the project maintained by the company. when bug found it, it found that it had made a very low-level error-- the printf function was called in the interrupt handling function, because the interrupt handling function called the non-reentrant function, resulting in interrupt loss and system location errors, which directly led to the stop of all threads in the application process of the embedded linux system, which in turn led to the watchdog process not being fed to the dog. The device restarts.

So what is a non-reentrant function?

Why can't interrupt handlers call non-reentrant functions directly?

How to write a reentrant function?

Start a short essay on the above three questions:

2 what is a non-reentrant function?

A reentrant function is mainly used in a multitasking environment. A reentrant function is simply a function that can be interrupted, that is, it can be interrupted at any time when the function is executed, and it can be transferred to the OS schedule to execute another piece of code without any error when returning to control. The non-reentrant function may have problems if it is interrupted because it uses some system resources, such as global variable area, interrupt vector table and so on. This kind of function cannot be run in a multitasking environment.

Most functions that meet the following conditions are non-reentrant:

Static (static) data structures are used in the function body.

The malloc () or free () function is called inside the function body

The standard Istroke O function is called in the function body.

a. Reentrant function

Void strcpy (char * lpszDest, char * lpszSrc) {while (* lpszDest++=*lpszSrc++); / / < Local variable used * dest=0;}

b. Non-reentrant function 1

Char cTemp; / / < global variable void SwapChar1 (char * lpcX, char * lpcY) {cTemp=*lpcX; * lpcX=*lpcY; lpcY=cTemp; / / < accessed the global variable}

c. Non-reentrant function 2

Void SwapChar2 (char * lpcX,char * lpcY) {static char cTemp; / < static local variable cTemp=*lpcX; * lpcX=*lpcY; lpcY=cTemp; / / < uses static local variable}

3 Why can't interrupt handlers call non-reentrant functions directly?

In a multitasking system, interruptions may occur at any time during the execution of a task; if the environment on which the function depends remains unchanged from the time it is interrupted during the execution of a function to the process of returning to the breakpoint for execution, then the function is reentrant, otherwise it cannot be reentered.

Don't you have to save and restore the context before and after the interrupt? how can the environment on which the function depends has changed? We know that some context is saved during an interrupt, but it is limited to a small amount of context such as return address, cpu register, etc., and the internal use of the function, such as global or static variables, buffer, etc., is not on the list of protection, so if these values change during the interruption of the function, then when the function goes back to the breakpoint to continue execution, the result will be unpredictable.

If the global variable protected by mutex is called in the interrupt handler function, if the variable is being called by another thread, it will cause the interrupt handler function not to return in time, resulting in serious problems such as interrupt loss.

And when used in a multithreaded environment, concurrent reading and writing to the same memory block without locking will cause problems such as segmentfault/coredump.

All in all, the simpler the interrupt handler does, the better.

4 how to write a reentrant function?

Do not access those global variables in the function body

If you must access global variables, remember to protect global variables with mutex semaphores. Or turn off the interrupt before calling the function, and then turn on the interrupt after the call

Do not use static local variables

Insist on using only default state (auto) local variables

When interacting with the hardware, remember to turn off the hardware interrupt. To complete the interaction, remember to turn on the interrupt. In some series, this is called "enter / exit core" or described as OS_ENTER_KERNAL/OS_EXIT_KERNAL.

Cannot call any non-reentrant function

Use the stack carefully. It's best to OS_ENTER_KERNAL before using it.

This is the end of "how to understand reentrant functions". Thank you for your reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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