👀 Views: 93 💬 Answers: 1 📅 Created: 2025-06-05

c structs bit-fields C

I'm facing a puzzling issue with struct alignment and bit-fields in my C program. I defined a struct to manage some flags and statuses, but I'm experiencing unexpected results when I try to read the values of these fields. The struct is defined as follows: ```c struct Status { unsigned int flag1 : 1; unsigned int flag2 : 1; unsigned int flag3 : 1; unsigned int reserved : 5; unsigned int statusCode : 8; }; ``` In this struct, I intended for `flag1`, `flag2`, and `flag3` to be used as binary flags, while `statusCode` should take values ranging from 0 to 255. However, when I set these fields and read them back, the values seem to be corrupted or not what I expect. For instance, I initialize the struct as follows: ```c struct Status s; s.flag1 = 1; s.flag2 = 0; s.flag3 = 1; s.statusCode = 255; ``` But when I try to print out the values: ```c printf("flag1: %u, flag2: %u, flag3: %u, statusCode: %u\n", s.flag1, s.flag2, s.flag3, s.statusCode); ``` I get output like this: ``` flag1: 1, flag2: 0, flag3: 1, statusCode: 0 ``` The `statusCode` field doesn't hold the value I'm assigning. I've tried adding `#pragma pack` directives to control the alignment, but it didn't yield any improvement. My compiler is GCC 11.2 on Ubuntu 20.04, and I'm compiling with the `-O2` optimization flag. Could there be issues related to how the compiler aligns these fields or optimizes the struct? Any insights into proper usage of bit-fields in this context would be greatly appreciated!