GCC linaro compiler throws error “unknown type name size_t”

Question

I am using GCC Linaro compiler for compiling my code. Its throwing the error unknown type name size_t from libio.h. Its included from stdio.h

Answer 1

For what it's worth, I had this exact same problem with a QT project, where I was using a Linaro compiler to (on both x86 Windows and x86 Linux) build for ARM Linux. Using the exact same code and .pro file, I had no problems building on Windows, but I had a litany of errors building on the Linux box, beginning with the unknown type name 'size_t' in libio.h which traced back to a #include <stdio.h>. I looked in the stdio.h (in the sysroot for the target hardware, not on the host machine), and a few lines down was #include <stddef.h> (much before #include <libio.h>), so stddef.h was definitely getting included. However, upon further inspection, stddef.h was completely empty with a file size of 1 byte. This was true for stddef.h in my sysroot and on my host machine. I have no idea why these files were empty.

Anyway, turns out I had an extraneous INCLUDEPATH += /usr/include/linux in my .pro file. On my Linux build machine, this added -I/usr/include/linux to the Makefile generated by qmake. On my Windows build machine, this added -isystem /usr/include/linux to the Makefile generated by qmake. Once I commented this out, these lines were removed from the Makefiles and it built right up on both build machines. -isystem /usr/include/linux apparently never caused any trouble on the Windows build machine, so there was no harm in removing INCLUDEPATH += /usr/include/linux.

I don't really know why this fixed my problem, but I suspect it was some kind of conflict between header files. Perhaps it was mixing host header files with sysroot header files, or creating a circular dependency somehow. GCC documentation says that anything included with the -I option will take precedence over a system header file. My best advice for this problem is to take a hard look at exactly which header files are being included and where they are coming from.

Answer 2

As per C99, §7.17, size_t is not a builtin type but defined in <stddef.h>.

Including the <stddef.h> header should fix your problem.

Answer 3

Both stdio.h and stdlib.h include the data type size_t. They include this data type because the functions declared in these headers either take size_t as a parameter, or return it as a return type. size_t itself is a typedef to an unsigned integral type and it's also returned by the sizeof operator.

And because the sizeof operator is built into the C Programming Language itself, not included via some library, then how can size_t be an unknown type name?