Access struct within struct in different functions

Question

I have nested structs and I'm having trouble assigning values to the inner struct in different functions. My structs are defined as:

typedef struct {
    double reading;
    float readingTime;
    int readingNum;
} Measurement;

typedef struct {
    Measurement vref;
    Measurement vout;
    Measurement voutShort; 
} Unit;

In the function (measureUnit()) where I declare Unit unit;I call takeMeasurement(unit) (which I simplified for clarity below) and where I try to assign values to some values in the inner Measurement struct :

takeMeasurement(Unit unit){
    int readingNum = 42;
    unit.vout.readingNum = readingNum;
}

When I then try to access these values in printData(Unit unit) which is called from measureUnit(),

`printf("%i", unit.vout.readingNum)`

always prints 0.

Shouldn't these values "follow" unit around through different functions?

hmjd · Accepted Answer · 2013-06-25 20:07:57Z

5

C passes arguments by value resulting in the changes being made to unit in takeMeasurement() being applied to a copy of the argument supplied to the function. Pass the address of a Unit to the function to ensure changes are visible to the caller:

void takeMeasurement(Unit* unit){
    int readingNum = 42;
    unit->vout.readingNum = readingNum;
}

To invoke:

Unit u = { 0.0 };
takeMeasurement(&u);

It is also desirable to pass a pointer to a const struct to a function that only reads the values of the struct to avoid making a copy of the struct, particularly if the struct is large:

void printData(const Unit* unit)
{
    printf("%i", unit->vout.readingNum);
}

edited Jun 25, 2013 at 20:07

answered Jun 25, 2013 at 20:00

hmjd

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MByD · Accepted Answer · 2013-06-25 19:59:47Z

2

You pass the struct itself to the function, which means you pass a copy of your struct to the function and modify this copy, not the original structure. You might want to pass a pointer to the struct instead:

takeMeasurement(Unit* unit){
    int readingNum = 42;
    unit->vout.readingNum = readingNum;
}

// call it something like that
takeMeasurement(&my_unit_struct);
//              ^ this means "the address of my_unit_struct"

answered Jun 25, 2013 at 19:59

MByD

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