Removing conditional macros when tracing C/C++ code with Vim

Working with source code buried in conditional macros(#if, #ifdef, #ifndef, ...) is sometimes really annoying, especially when we are trying to get the whole picture of what it does instead of every details in it.

Simply expanding the conditionals with gcc -E usually doesn't help in this situation. Expansion of macros as well as the header files even makes the picture muddier. The unifdef(1) utility come in handy for this.

$ man unifdef(1)

UNIFDEF(1)                    Programmer's Manual                   UNIFDEF(1)

NAME
     unifdef, unifdefall — remove preprocessor conditionals from code

SYNOPSIS
     unifdef [-bBcdeKknsStV] [-Ipath] [-Dsym[=val]] [-Usym] [-iDsym[=val]] [-iUsym] ...
             [-o outfile] [infile]
     unifdefall [-Ipath] ... file

Apply it as vim filter command repeatedly and incrementally until you get your picture clear enough.

 vim /tmp/dummy.c

#if (LOG_LEVEL > 3)
  #define log printk
#else
  #define log(...) 
#endif

#ifdef ENABLE_FOO
void foo_xxx()
{
    ...
}

#endif

#ifdef ENABLE_BAR
void bar_yyy()
{
    ...
}
#endif

:%! unifdef -k -UENABLE_FOO

 vim /tmp/dummy.c

#if (LOG_LEVEL > 3)
  #define log printk
#else
  #define log(...) 
#endif

#ifdef ENABLE_BAR
void bar_yyy()
{
    ...
}
#endif

:%! unifdef -k -DENABLE_BAR -DLOG_LEVEL=4

 vim /tmp/dummy.c


  #define log printk

void bar_yyy()
{
    ...
}

Note that:

By default, unifdef ignores #if and #elif lines with constant expressions; it can be told to process them by specifying the -k flag on the command line.

[tip] Transform file or directory names during the extraction of tarball

Suppose we have a tarball containing a directory structure of root filesystem for an embedded device. ex:

  rootfs/
  rootfs/init.rc
  rootfs/dev/...
  rootfs/etc/...
  rootfs/usr/...

We'd like to extract the contained directory structure under the rootfs/ directly into the root directory of a mounted partition, for example, /mnt/sdb1. Otherwise, we will have to deal with the relocation of the device node files(rootfs/dev/...), which is a little bit cumbersome.

To skip the containing folder 'rootfs/', we can specify the --transform option during the extracting process,

tar xvfj ~/rootfs.tar.bz2 -C /mnt/sdb1/ --transform 's/rootfs//'

and yield the following extracted directory structure.

  /mnt/sdb1/
  /mnt/sdb1/init.rc
  /mnt/sdb1/dev/...
  /mnt/sdb1/etc/...
  /mnt/sdb1/usr/...

[setting] Error occurred while mounting usb stick via the device notifier

[reference] https://bbs.archlinux.org/viewtopic.php?pid=909650 draugdel @ 2011-03-26 10:08:27

---

Error occurred while mounting usb stick via the device notifier

org.freedesktop.UDisks.Error.PermissionDenied: Not Authorized

Sol: Add a file named with .pkla suffix in /etc/polkit-1/localauthority/50-local.d/
ex:

/etc/polkit-1/localauthority/50-local.d/mount.pkla

and stuff the following contents into it

[Mount system internal media for group storage]
Identity=unix-group:storage
Action=org.freedesktop.udisks.filesystem-*
ResultAny=yes
ResultInactive=yes
ResultActive=yes

Introduction to Git Basics

Check out this SlideShare Presentation:

Introduction to Git Basics

View more presentations from Roy Lee.

Zygote Initialization Flow

The code flow of zygote initialization is rather complicated, especially when the JNIs are extensively involved. My IDE often fails to cross reference between them. So I log the code flow here and the location they are defined as well.