SPI_IOC_MESSAGE(N) macro giving me fits

Question

I'm having trouble getting a SPI program I'm working on to behave correctly and it seems to be some issue with the SPI_IOC_MESSAGE(N) macro.

Here's sample code that DOESN'T work (ioctl returns EINVAL (22) ):

std::vector<spi_ioc_transfer> tr;
<code that fills tr with 1+ transfers>
// Hand the transmission(s) off to the SPI driver
if (tr.size() > 0)
{
    int ret = ioctl(fd, SPI_IOC_MESSAGE(tr.size()), tr.data());
    if (ret < 1)
    {
         int err = errno;
    }
}

My test code right now is creating a vector of length 1. If I explicitly change the code to:

int ret = ioctl(fd, SPI_IOC_MESSAGE( 1 ), tr.data());

...then ioctl(...) succeeds and my bits go down the pipe. Looking at the expansion of the SPI_IOC_MESSAGE macro in Eclipse, I don't see why this isn't happy.

Suggestions?

I'm cross-compiling for Linux/ARM (Beaglebone Black) from a 64-bit Linux VM, but that I can't see that affecting the macro.

EDIT: Here are the two macro expansions out of the C pre-processor

int ret = ioctl(fd, (((1U) << (((0 +8)+8)+14)) | ((('k')) << (0 +8)) | (((0)) << 0) | ((((sizeof(char[((((tr.size())*(sizeof (struct spi_ioc_transfer))) < (1 << 14)) ? ((tr.size())*(sizeof (struct spi_ioc_transfer))) : 0)])))) << ((0 +8)+8))), tr.data());

and the literal:

int ret = ioctl(fd, (((1U) << (((0 +8)+8)+14)) | ((('k')) << (0 +8)) | (((0)) << 0) | ((((sizeof(char[((((1)*(sizeof (struct spi_ioc_transfer))) < (1 << 14)) ? ((1)*(sizeof (struct spi_ioc_transfer))) : 0)])))) << ((0 +8)+8))), tr.data());

Absolutely hideous, but I don't see anything surprising in how tr.size() would be getting used there.

Edit to include what seems to be the answer

#ifdef __cplusplus /* If this is a C++ compiler, use C linkage */
extern "C" {
#endif
#include <linux/spi/spidev.h>
#ifdef __cplusplus /* If this is a C++ compiler, use C linkage */
}
#endif

Wrapping the linux SPI include file in an "extern C" instructs the system to treat that section as plain old C, and that seems to let me call SPI_IOC_MESSAGE( tr.size() ) or SPI_IOC_MESSAGE( an_int ) and have the correct thing happen (verified with GDB stepthrough and a signal analyzer).

score 2 · Accepted Answer · answered Jul 24 '15 at 13:18

I suspect the problem might lie in this particular nugget buried in the macro soup:

...sizeof(char[...tr.size()...])...

Noting that Linux code is exclusively C, the C standard (I have C99 draft n1256 here) states for the sizeof operator that an expression operand is unevaluated, and the result is a constant, unless the type of the operand is a variable-length array, in which case it is evaluated and the result is an integer.

The C++ standard (C++11 draft n3242), however, does not seem to give any condition under which the operand is evaluated, and only states the result is a constant.

Thus it looks like this may be one of the corners where C and C++ differ, and compiling one as the other leads to undefined behaviour. In that case, I think the choice is either just hacking up your own version of the macro, or having a separate external C function that wraps it.

Looks like this is somewhere close to the root cause. I changed my code to wrap the Linux SPI file include in an "extern C" directive and that seems to have corrected the behavior. — Phil Mills, Jul 27 '15 at 21:24

SPI_IOC_MESSAGE(N) macro giving me fits

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