FAQ

Kernel Monitor FAQ

I've added #include<stdlib.h> in the source file, but why the compiler still gives me warning when using malloc?

That is the typical first "culture shock" doing kernel programming—Welcome to The Matrix!

Basically, you cannot use functions like gets from what you would use in your regular C program. Pintos make it less shocking (and your life easier) by providing a set of routines that mimics common standard C library functions like printf. But their implementations are not really exactly the same because standard C library works in the user mode and usually leverages syscalls to implement the functions while the lib in pintos is at kernel-level. Also, this is a Pintos-only choice. In other kernels or real OSes, you wouldn't even be able to see function names like printf. Instead, they are named differently to avoid confusion. For example, in Linux kernel, to print something, you will need to use printk because printf is not available in kernel.

For malloc, it is the same story. Pintos does provide the malloc routine. But its implementation is different from the malloc in standard C library. It's just named so to make you feel more comfortable. The malloc kernel routine is defined in threads/malloc.h. So you will have to include it to use this header file instead.

Wait, then how does the compiler know it should include the stdlib.h in the Pintos codebase (pintos/src/lib), instead of the standard one at /usr/include/stdlib.h?

Good question. A #include<XXX.h> directive will indeed search for system headers instead of a local header file. However, as explained earlier: (1) we should not use the /usr/include/stdlib.h because later the kernel and standard C library are not operating in the same level and linking them together later won't work; (2) we must use the pintos/src/lib/stdlib.h. How to achieve this? The magic happens in the GCC flags.

For (1), we need to use a flag called -nostdinc to tell GCC to not use the standard C headers in its header search path. For (2), we need to use the -I/path/to/my/headers flag to tell GCC to treat /path/to/my/headers in its header file search path. Once both are done, you can use #include<stdlib.h> in a pintos source file.

You can find this trick is done for compiling all pintos source files in the CPPFLAGS in Make.config. Again, this system header include format is just the pintos author's attempt to make programming pintos as familiar to regular C programming as possible for you. You will need to be conscious that there is some good-will smoke and mirror here :)

I tried to use scanf but got a undefined reference to `scanf' error.

This is for the same reason explained in the malloc question. Pintos only implements a subset of C standard library functions. You can check the header files in src/lib to see what are these available functions. If a standard function is not listed there, you cannot use it (and will have to implement it yourself if you really need it).