类型转换到位字段

Question

I am trying to assign a value to a bit field in a struct. It is a demotion since the assigned variable is of a smaller size than the type used on the right hand side. Is there a way to force typecast it, understanding that there is some risk in demotion. This is test code, and I do not expect the right hand value to ever be greater than what my bit field can take.

Compiling using:

    gcc -Wconversion compileError.c

Code:

#include "stdio.h"

typedef unsigned long  int uint64_t;
typedef unsigned short int uint16_t;

typedef struct
{
    uint64_t  val:48;
    uint16_t  length;
}data_t;

static data_t testData[10] = {};

int main()
{
    data_t*   pData = (data_t*)&testData;
    uint64_t  var   = 0;

    pData->val = var + 1;

    printf("Just trying to compile this program\n");
    return 0;
}

Compiler Output:

compileError.c: In function main:
compileError.c:19:22: warning: conversion to long unsigned   
int:48 from uint64_t may alter its value [-Wconversion]
 pData->val = var + 1;

Answer 1

This blog shows how you can temporarily silence that warning.

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wconversion"
    pData->val = var + 1;
#pragma GCC diagnostic pop

There's an open GCC Enhancement Request to address this, and there's a comment showing a workaround using masking.

pData-> val = (var + 1) & 0xffffffffffffLU