Cross compilation: GCC ignores --sysroot

Question

I'm trying to cross compile programs (currently avconv from libav) for a Nokia N9 phone using arm-linux-gnueabi-gcc from Linux Mint's 64-bit repository. The compiler's libc version is 2.15 and the phone has libc-2.10.1. They have an incompatibility in the math library, which gives me a segfault when I compile and run the avconv program from libav.

I'd need to compile and link against the older libc version, but I haven't managed to get the --sysroot option to work.

I made a small test program to avoid repeatedly configuring and compiling libav.

arm-linux-gnueabi-gcc --sysroot=/opt/CrossCompilation/NokiaN9/ -o output.sysroot hello.c
arm-linux-gnueabi-gcc -o output.nosysroot hello.c

Both commands create an identical output file. This is what hello.c looks like:

#include <stdio.h>
#include <math.h>

int main() {
    printf("Hello, World! Sin = %f\n", sin(0.6451));
}

The strangest part is that gcc completely ignores the --sysroot option. If I pass a nonexisting directory to sysroot, it still produces exactly the same output binary:

arm-linux-gnueabi-gcc --sysroot=/foo/bar -o output.foobar hello.c

It doesn't even complain about any errors. What's the problem?

Skyros

Run the compiler with arm-linux-gnueabi-gcc -v and look at the value of --with-sysroot; this is the directory the compiler was built with. If you have this directory present on your machine (maybe with a different compiler), then the --sysroot may not work. What is the gcc version? – Father

There is no "--with-sysroot" defined. gcc is version 4.6.3 – Skyros

But instead 'grep /usr/arm' finds "--with-gxx-include-dir=/usr/arm-linux-gnueabi/include/c++/4.6.3", "--includedir=/usr/arm-linux-gnueabi/include", "--with-headers=/usr/arm-linux-gnueabi/include", and "--with-libs=/usr/arm-linux-gnueabi/lib". – Skyros

--with-libs means your gcc is compiled without --sysroot support. – Father

Answer 1

since I wasted a few days messing with this before reading the comments, I'm going to post artless noise's comments as an answer:

"Run the compiler with arm-linux-gnueabi-gcc -v and look at the value of --with-sysroot; this is the directory the compiler was built with. If you have this directory present on your machine (maybe with a different compiler), then the --sysroot may not work[; and if you do not see --with-sysroot and instead see --with-libs, it] means your gcc is compiled without --sysroot support."

Protest

Is there any solution? – Suazo

it's been a while since I messed with this, but if you have --with-libs instead of --with-sysroot, use that maybe? or download another toolchain, or build your own. – Protest

"If you have this directory present on your machine (maybe with a different compiler), then the --sysroot may not work" I just checked it with gcc-13.1 built --with-sysroot=/ and this is not the case. --sysroot=whatever works just fine exactly as it should (and / is definitely present on my machine). – Fictitious

Answer 2

Another thing to check for - some build projects sneak in -nostdinc option to compiler arguments. It also makes the build to ignore sysroot includes.

Connors