the size of array must be greater than zero??

Robert Stoddard16925

I just ran into the following error:

"main.c", line 10: error: the size of an array must be greater than zero

Since when do arrays have to have a size greater than zero? I've always used zero-length arrays to hold variable amounts of data at the end of a 'header' structure... For example,

struct Packet

{

int version;

...other header values...

int crc;

char data[0];

};

Then I can create my data packet by :

struct Packet *myPacket = (struct Packet*) malloc(sizeof(struct Packet) + datalength);

... I've done this for years, and have seen it done in a lot of very common code (like the Linux kernel). Why does the TI DSP C compiler (TI/C6000CGT7.0.3/bin/cl6x) not support it?

Thanks

Rob

over 15 years ago

0 Archaeologist over 15 years ago

TI__Guru* 84285 points

What you're trying to do is called the "struct hack." This is traditionally done by declaring the final array to have one element. Using a size of zero was never legal C code per the C89 standard. Some compilers, particularly GCC, have implemented it as an extension. You can use the "--gcc" command-line switch to get the compiler to accept this zero-length array. C99 (which the TI compiler does not support) adds the feature "flexible array members," which allow you to do the "struct hack" by declaring the final array to have an incomplete type, specifically no length at all:

char data[];

0 Daniel Allred over 15 years ago

TI__Genius 17355 points

See here for more details.

0 Archaeologist over 15 years ago in reply to Daniel Allred

TI__Guru* 84285 points

Thanks for the nice GCC link, Daniel. The comp.lang.c FAQ page discussing the "struct hack" may also be of interest. It discusses the quasi-legality of the "struct hack," including an amusing quote from Dennis Ritchie.

0 Galen Seitz over 15 years ago in reply to Archaeologist

Intellectual 485 points

A quick check of the calendar confirms the year is 2010. What's preventing us from seeing a C99 compiler?

galen

0 George Mock over 15 years ago in reply to Galen Seitz

TI__Guru**** 249380 points

Our strategy is to adopt the parts of C99 that give us the best bang for the buck. These parts usually meet 2 conditions: they are not difficult to implement, and they provide a clear improvement in usability or performance. This strategy has served us well, and is likely to continue.

As far as I know, a similar strategy is used by most other C compiler vendors. Relatively few C compilers support all of C99. The ones that do tout that claim, of course. But I have not seen them gain much benefit by it.

More background on standards supported by TI compilers is here http://processors.wiki.ti.com/index.php/TI_Compilers_and_Industry_Standards .

Thanks and regards,

-George

0 Ahmet Batur over 12 years ago in reply to Archaeologist

Prodigy 20 points

Hi, I could not understand the "--gcc" command-line switch is it possible to use gcc compiler in code composer studio?

Thanks and regards,

Ahmet

0 George Mock over 12 years ago in reply to Ahmet Batur

TI__Guru**** 249380 points

Ahmet Batur said:
is it possible to use gcc compiler in code composer studio?

Yes. See the screen shot below. If you are not using MSP430, things might look a bit different. But it will be something close to this.

Thanks and regards,

-George

0 Douglas Gwyn over 11 years ago in reply to George Mock

Expert 2210 points

The quickest "fix" is to change the [0] to [1]. (If you want, you can also adjust the constructor code to avoid wasting one byte per struct.)

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the size of array must be greater than zero??