fwrite portability

Question

Is fwrite portable? I'm not really faced to the problem described below but I'd like to understand the fundamentals of C.

Lest assume we have two machines A8 (byte = 8bits) and B16 (byte = 16 bits).

Will the following code produce the same output on both machines ?

unsigned char[10] chars;
...
fwrite(chars,sizeof(unsigned char),10,mystream);

I guess A8 will produce 80 bits (10 octets) and B16 will produce 160 bits (20 octets). Am I wrong?

This problem won't appear if only uintN_t types were used as their lengths in bits are independent of the size of the byte. But maybe uint8_t won't exist on B16.

What is the solution to this problem?

I guess building an array of uint32_t, putting my bytes in this array (with smart shifts and masks depending on machine's architecture) and writing this array will solve the problem. But this not really satisfactory.There is again an assumption that uint32_t exists on all platforms.The filling of this array will be very dependant on the current machine's architecture.

Thanks for any response.

Answer 1

fwrite() is a standard library function. So it must be portable for each C compiler. That is it must be defined in C standard library of that compiler to support your machine.

So machine of 8bit, 16 bit, 32 bit give you same high level operation.

But if you want to design those library function then you have to consider machine architecture, memory organization of that machine.

As a C compiler user you should not bother about internal behavior.

I think you just want to use those C library function. So no difference in behavior of the function for different machine.