如何用指针与offsetof()正确访问结构体？代码赋值报错求解

Let's break down what's going wrong here and fix your pointer arithmetic properly.

Why You're Seeing the Error #

That line *(ptr + offset) = data; is the root of the problem. Here's why:

• ptr is a struct osc_in_data* pointer. In C, pointer arithmetic with struct pointers uses the full size of the struct as the unit of measurement—not individual bytes.
• So ptr + offset doesn't point to the field inside your struct at offset bytes. Instead, it jumps to an entirely separate struct osc_in_data instance in the array. That's why the compiler complains about assigning an int32_t to a struct osc_in_data type.

The Correct Pointer Offset Approach #

To access a field using a byte offset (which is exactly what offsetof returns), you need to adjust your pointer arithmetic to work at the byte level:

1. Convert the struct pointer to a byte-based pointer (like uint8_t*) so arithmetic operations use single bytes as the unit.
2. Add your byte offset to this pointer to reach the exact memory address of the target field.
3. Convert the result back to a pointer matching the field's type (in your case, int32_t*) before assigning the value.

Here's the fixed line for your function:

*(int32_t*)((uint8_t*)ptr + offset) = data;

Full Modified Code #

Make sure you include the necessary headers for offsetof and fixed-size integers:

#include <stddef.h>
#include <stdint.h>

#define NumHarmonics 10
int32_t data1;
int32_t data2;

struct osc_in_data {
    int32_t LevelAttackRate;
    int64_t LevelPeakLevel;
    int32_t LevelDecayRate;
} OscControl[NumHarmonics];

void SetADSRvalues(int32_t offset, int32_t data) {
    int32_t harmonic;
    struct osc_in_data *ptr;
    for (harmonic = 0; harmonic < NumHarmonics; harmonic++) {
        ptr = &OscControl[harmonic];
        // Correct pointer arithmetic to target the desired field
        *(int32_t*)((uint8_t*)ptr + offset) = data;
    }
}

// Example usage
void initialize_adsr() {
    SetADSRvalues(offsetof(struct osc_in_data, LevelAttackRate), data1);
    SetADSRvalues(offsetof(struct osc_in_data, LevelDecayRate), data2);
}

Key Notes #

• This solution works because both LevelAttackRate and LevelDecayRate are int32_t types. If you wanted to set LevelPeakLevel (an int64_t), you'd need to cast to int64_t* instead and adjust the function's data parameter type to match.
• Always ensure the offset you pass aligns with the data type you're assigning—mismatched types will lead to undefined behavior.

内容的提问来源于stack exchange，提问作者Mike Bryant