C- static initialization of structures Post: 302744955

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Top Forums Programming C- static initialization of structures Post 302744955 by garysk on Sunday 16th of December 2012 01:58:41 AM

12-16-2012

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Don, a million thanks for your cogent reply.

'Pon our systems, NULL the pointer is defined as (void *)0. NUL the character is (char)0. This is why I have been saying, as you do below, it would be natural to expect a warning about the type coercion implied by my mistake.

But, as I also said at the thread head, zero is zero is zero. And C is enamored with zeros, for breakfast lunch and dinner, where more strongly typed languages would never allow such digital promiscuity.

Our OS is 32-bit kernel running on a 64-bit architecture; we tell the compiler so by using the -march=1.0 which means PA RISC 1 architecture, a 32 bit system. Our int's long's and pointers are all 32-bit. We are stuck in the 32bit timewarp as untangling all the 32-bit words and coding each properly for 64 bits in a 720k-line package where pointer/integer cross-overs are rampant, as are exchanges between longs and ints. It has been judged a monumental rewrite that is not in the current budget. Fortunately, our package compiles links and runs (same gcc version, whatever that means cross-architectures) on RedHat 64-bit (kernel 2.6.18), so we can presumably limp along indefinitely on an antedeluvian setup, as long as gcc supports us.

There is a loss of bts in the compiler's conversion (bits that are always 0). But coercing a 32-bit pointer to fit into an 8-bit char is potentially a little lossy.

Quote:

Originally Posted by Don Cragun

I agree with what Jim McNamara has already said. I hope this may provide a little more that will help explain what you're seeing.

Depending on the programming environment you're using, NULL is likely to be defined to be 0, 0L, or (void *)0. (All three of these are used for one or more of the multiple programming environments provided by OS X and all three of these are legitimate values for NULL according to the C Standard depending on the relative sizes of pointers and integers in your programming environment.)

If NULL expands to 0 in your programming environment, there would be absolutely no difference to the compiler between the initializer:

Code:

NULL, '\0', '\0'

and the initializer:

Code:

'\0', '\0', '\0'

If NULL expands to 0 or 0L, no pointer to integer conversion is needed to turn NULL into an integer constant used to initialize the array of characters in your structure.

I agree that if your compilation environment has NULL defined to be (void *)0 it would have been nice for the compiler to produce a warning, but I don't see anything in the C Standard that would require that to happen since the conversion from a NULL pointer to an integer constant expression is a well defined operation with no loss of data involved.

garysk

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