undefined reference to `le16toh' error in Makefile - c

Question

I'm trying to compile a C program with the following Makefile:
msh: libFAT32.so
gcc -Wall -fPIC -I. -o msh newTest.c -L. -lFAT32
libFAT32.so:
gcc -std=c99 -shared -o libFAT32.so -fPIC fat32.c
clean:
rm *.so msh
However, every time I try to compile the program with make I get the following error:
user#user-VirtualBox:~/fat1$ make
gcc -Wall -fPIC -I. -o msh newTest.c -L. -lFAT32
./libFAT32.so: undefined reference to `le32toh'
./libFAT32.so: undefined reference to `le16toh'
collect2: error: ld returned 1 exit status
Makefile:19: recipe for target 'msh' failed
make: *** [msh] Error 1
Can some one tell how to fix this?

Answer 1

So, here's what's going on (making the safe assumption that you're using a linux distribution in your VM).
With this test program:
#include <stdio.h>
#include <endian.h>
int main(void) {
printf("%d\n", le32toh(1234));
return 0;
}
compiling and running it works:
$ gcc -Wall -Wextra test.c
$ ./a.out
1234
However, you're compiling using -std=c99. So let's try that:
$ gcc -std=c99 -Wall -Wextra test.c
test.c: In function ‘main’:
test.c:5:18: warning: implicit declaration of function ‘le32toh’ [-Wimplicit-function-declaration]
printf("%d\n", le32toh(1234));
^~~~~~~
/tmp/cc7p3cO8.o: In function `main':
test.c:(.text+0xf): undefined reference to `le32toh'
collect2: error: ld returned 1 exit status
Compiling in c99 mode disables a bunch of functions and macros and such that aren't in the 1999 version of the C standard unless they're explicitly requested, hence the implicit declaration warning. le32toh() is a macro, not a function with a symbol in libc, hence the linker error.
If you read the man page for le32toh(), you'll see that it needs the _DEFAULT_SOURCE feature test macro, which must be defined before any headers are included.
So, your options are:
Compile in gnu99 mode instead, since that automatically defines a bunch of the feature test macros.
Continue to use c99 mode and add a #define _DEFAULT_SOURCE at the very start of your fat32.c source file.
Continue to use c99 mode and add -D_DEFAULT_SOURCE to your compiler arguments.