CODEWITHSUNDEEP
clinuxlinux-kernelkernel
Aman Deep Gautam
Aman Deep Gautam

Reputation: 8737

c syntax in kernel code and its working

I have a doubt about a syntax used in linux kernel code. I have an intuition of what it does but I want to know it more formally. I am using kernel v3.5.4

In file /include/linux/sched.h the following is defined

struct task_struct {
    volatile long state;
    //some more data members
};

and in file /include/linux/init_task.h file the following is defined:

#define INIT_TASK(tsk) {
    .state     = 0,               \
    //some more initializations

}

I am confused about two things:

a) I feel it is used for initialization but can anyone suggest some good read for this type of initialization for structures.

b) I do not understand how the following initialization works. Like how this #define and the corresponding task_struct structure are related.

[EDIT] I noticed the following things also: c) Is \ at the end of every line necessary.

d) There are many parts of kernel doe wrapped in #ifdef #endif. If you want to initialize a data member wrapped in #ifdef #endif can we use this form of initialization. I mean can we use #ifdef #endif inside INIT_TASK() like this

#define INIT_TASK(tsk) {
    .state     = 0,               \

    //some more initializations
    #ifdef CX 
    .tickets   = 5,               \
    #endif

}

Upvotes: 0

Views: 541

Answers (1)

ThiefMaster
ThiefMaster

Reputation: 318488

struct task_struct whatever = INIT_TASK(someTsk);

This results in the following code:

struct task_struct whatever = { .state = 0 };

which is valid C syntax to initialize fields in a struct via their name instead of their position. Doing so makes the code safe against struct members that are not added at the last position.

Regarding the backslashes: Yes, they are necessary so the preprocessor knows that the macro continues on the next line.

No, you cannot use #ifdef inside a macro.

Upvotes: 8

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