I'm currently coding my implementation of the malloc() function, so I'm compiling with the following flags: -m64 -fPIC -pedantic -Wall -Wextra -Werror -nostdlib -ggdb3.
For the time being, I'm compiling as an executable and not as a shared library.
For my implementation I'm using the header files stddef.h stdint.h unistd.h, for the moment I need the syscall sbrk(). The problem is that on compilation I get the following error: undefined reference to sbrk.
I assume that sbrk() and more generally unistd.h are disabled if I compile with -nostdlib but how can I compile without the glibc but with the system calls?
Thanks in advance
P.S I really need to use sbrk() I can't use mmap(), don't encourage me to do it.
If you're doing what it sounds like you are trying to do, then you will eventually have to write your own implementation of all of the C library functionality that you need. However, a stepping stone in that direction is to compile your semi-freestanding program like this:
gcc -c -ffreestanding [other options] -o foo.o foo.c
# ... repeat the above for all other object files ...
gcc -ffreestanding -nostdlib -nostartfiles -static -o yourprogram \
foo.o [other objects] -lc -lgcc
This tells the C implementation to supply only the parts of the standard libraries that you actually use yourself. Note that many standard APIs will not work correctly in this mode; don't use anything from stdio.h, nor anything locale-dependent (as the C standard uses that term), nor anything to do with threads. The system call wrappers should be fine, though, as should most of string.h.