C-传递结构成员的地址以返回错误的函数

Question

Answer :

The culprit was the keyword restrict. I missed a cast with it Thanks

Context

Linux.

Question

Could you help me find a classy solution to write my parameter ?

Why I fail

I fail because I don't want to take the normal path (see code).

My need

Something quite classy like &(data->mmap) as a parameter to avoid creating a dummy variable like exposed in my way to circumvent the problem..

This actually doesn't work.

I have to do that way because that function may be called with different meanings. The struct is much more complex and according to my position in the code, the second parameter is not the same.

The code

struct mystruct {
 char * mmap;
}
void test(mystruct * _input, char * my_char){

 int fd;
 ...
 // WORKS FINE. THE ADDRESS IS PRINTED IN mySecondFunction CORRECTLY.
 // THAT IS THE OBVIOUS WAY
 //_input->_mmap = (char*)mmap( 0 , 128 , PROT_READ | PROT_WRITE , MAP_SHARED , fd , 0 );

 // FAILS TO PRINT THE RIGHT ADDRESS IN mySecondFunction
  my_char = (char*)mmap( 0 , 128 , PROT_READ | PROT_WRITE , MAP_SHARED , fd , 0 );
 ...

 printf("in function address %p\n", my_char);

}
void mySecondFunction(mystruct * data){

 test(data, data->mmap);
 printf("data->mmap %p\n",data->mmap);

}
int main(void){

 mystruct test;

 mySecondFunction(&test);


 return 0;
}

My way to deal with this situation

void test(mystruct * _input, char ** my_char){

   ...
   * my_char = (char*)mmap( 0 , 128 , PROT_READ | PROT_WRITE , MAP_SHARED , fd , 0 );
   ...
}
void mySecondFunction(mystruct * data){

 char * _tmp;
 // This is where i would like something like &data->mmap to replace &_tmp in order to get the right address directly.
 test(data, &_tmp);
 data->mmap = tmp;
 printf("data->mmap %p\n",data->mmap);

}

Thanks

Answer 1

In test , you set the local my_char variable to the address you get from mmap . This doesn't affect the _tmp variable in main .

If you want to change the variable you pass to the function you have to instead pass a pointer to it, like this:

void test(mystruct * _input, char ** my_char){
   *my_char = (vhar*)mmap(...);
}

test(data, &data->mmap);

Answer 2

Here's a complete fix:

void test(mystruct * _input, char ** my_char){
    //Update the reference that was passed
    *my_char = (char*)mmap( 0 , 128 , PROT_READ | PROT_WRITE , MAP_SHARED , fd , 0 );

    printf("in function address %p\n", my_char);
}

void mySecondFunction(mystruct * data){
    test(data, &(data->mmap));  // Pass by reference
    printf("data->mmap %p\n",&(data->mmap)); // Prints address of data->mmap
}