Casting structures to buffers in embedded code - c

Question

It's sometimes necessary to cast a data structure into a pointer so that the data can be sent, for example, over an interface, or written out to some other stream. In these cases, I usually do something like this:
typedef struct {
int field1;
char field2;
} testStruct;
int main()
{
char *buf;
testStruct test;
buf = (char *)&test;
// write(buf, sizeof(test)) or whatever you need to do
return 0;
}
Recently in some microprocessor code, however, I saw something similar to this:
typedef struct {
int field1;
char field2;
} testStruct;
int main()
{
char buf[5];
testStruct test;
*(testStruct *)buf = test;
// write(buf, sizeof(test)) or whatever you need to do
return 0;
}
To me, the former feels a little more safe. You just have one pointer, and you assign the address of the structure to the pointer.
In the latter case, it seems like if you allocate the wrong size to the array buf by accident, you'll end up with undefined behavior, or a segfault.
With optimizations on, I get a -Wstrict-aliasing warning from gcc. However, again, this code runs on a microprocessor, so is there something I might be missing there?
There's no pointers in the structures, or anything, it's very straight forward.

Answer 1

(testStruct *)buf may generate a mis-aligned address for a testStruct leading to a bus fault. Do not use.
A union is better. It helps cope with anti-aliasing issues as well as alignment ones.
Also see #Steve Summit's good comment.
Consider a master type like testStruct_all.
typedef struct { // OP's structure
int field1;
char field2;
} testStruct1;
typedef struct { // Perhaps another structure to send
double field1;
char field2;
} testStruct2;
// A union of all possible structures used in this app
typedef union {
testStruct1 tS1;
testStruct2 tS2;
char buf[1];
} testStruct_all;
int main(void) {
testStruct_all ux;
foo(&ux.tS1); // populate ux.tSn of choice.
write(ux.buf, sizeof ux.tS1);
read(ux.buf, sizeof ux.tS1);
// the union insures alignment and avoids AA issues
bar(&ux.tS1);
return 0;
}
write() usually accepts a void * #user58697, so code could drop the buf member and use:
write(&ux, sizeof ux.tS1); // or whatever you need to do