Thread: Writing struct to file

I has read that when writing a whole struct to a file with fwrite() is bad, as it's nonportable.

But does it change anything to write the whole struct with fputc() ?
Something like this:

Code:

char *writePointer = &structVariable;
for(int i=0; i < STRUCT_SIZE; i++) {
  fputc(writePointer[i], outputBuffer);
}

Won't this make just as much portability issues as the fwrite?

Yes.

Originally Posted by MK27

As long as you use "wb", I am sure fwrite() is fine for this on linux and windows and almost certainly Mac, so I'm not sure where "unportable" comes in.

Because compilers always and everywhere use the same padding.

Originally Posted by MK27

Wouldn't that (potential incompatibility) only apply if the file itself was passed from one executable to another, separately (& differently) compiled executable?

It certainly applies to that situation.

Here fputc() will walk along the struct one byte at a time, ignoring the alignment requirements of the various object types leading to portablility problems not only between different executables but also within the same executable.

Originally Posted by itCbitC

Here fputc() will walk along the struct one byte at a time, ignoring the alignment requirements of the various object types leading to portablility problems not only between different executables but also within the same executable.

Isn't there a way to explicitly specify how the padding should be handled in the struct definition?

Originally Posted by Subsonics

Isn't there a way to explicitly specify how the padding should be handled in the struct definition?

You mean by a preprocessor directive like pragma?

Originally Posted by MK27

Wouldn't that (potential incompatibility) only apply if the file itself was passed from one executable to another, separately (& differently) compiled executable?

Well, I suppose. But what else do you think non-portable means in this context? The code is certainly portable, in the sense that it will work on basically any machine. It just won't do the same thing on any of them.

Originally Posted by itCbitC

You mean by a preprocessor directive like pragma?

I saw that GCC had a -fpack-struct flag and that there is a pack(1) alternative as well. I was curious if this could perhaps be combined with making the padding by hand using bitfields. Not sure if that would work or if all compilers support this either.

Originally Posted by Subsonics

I saw that GCC had a -fpack-struct flag and that there is a pack(1) alternative as well. I was curious if this could perhaps be combined with making the padding by hand using bitfields. Not sure if that would work or if all compilers support this either.

Yep! you can certainly change the default alignment of the various object types so that they are packed together side-by-side, but all of this implies non-portable. Other compilers may not support the fpack-struct switch or the fact that some compilers use pragma pack 1 instead of the gcc variant pragma pack(1). Bottom line is that altering the natural alignment of the different object types produces code that is not portable and degrades performance.