Converting defines to strings in precompiler directives

Often times it is useful to construct a string visible to the pre-compiler and assemble it from one more more other values, defined by means of a #define dierctive. Here’s some example code (this is some custom environemnt config for U-Boot for a STM-based device I work with):

#if defined( DEVELOPER_ALAN )
#define CONFIG_ETHADDR      00:23:b0:00:00:00
#define CONFIG_NETMASK      255.255.255.0
#define CONFIG_IPADDR       192.168.0.12
#define CONFIG_GATEWAYIP    192.168.0.2
#define CONFIG_SERVERIP     192.168.0.170

 #if defined( BOOT_FROM_NFS )
  #define CONFIG_BOOTFILE       "/work/project_A18/target/boot/uImage"
  #define CONFIG_BOOTCOMMAND    "nfs;bootm 80000000"
  #define CONFIG_BOOTARGS "root=/dev/nfs rootdelay=10 \
nfsroot=192.168.0.170:/work/project_A18/target,rsize=4096,wsize=8192,tcp \
nwhwconf=device:eth0,hwaddr:00:23:b0:00:00:00 \
ip=192.168.0.12::192.168.0.2:255.255.255.0:host-nfs::off \
console=ttyAS0,115200 mem=58m"

 #elif defined( BOOT_FROM_USBFAT )
  #define CONFIG_BOOTFILE       "/boot/uImage"
  #define CONFIG_BOOTCOMMAND    "usb reset;fatload usb 0:1 80000000 /uImage;\
bootm 80000000"
  #define CONFIG_BOOTARGS "root=/dev/sda2 rootdelay=10 \
nwhwconf=device:eth0,hwaddr:00:23:b0:00:00:00 \
ip=192.168.0.12::192.168.0.2:255.255.255.0:host-usb::off \
console=ttyAS0,115200 mem=58m bigphysarea=1000"

 #else
  #error "No valid boot option selected."

 #endif

#elif defined( DEVELOPER_BOB )
#define CONFIG_ETHADDR      00:23:b0:00:00:01
#define CONFIG_NETMASK      255.255.255.0
#define CONFIG_IPADDR       192.168.0.13
#define CONFIG_GATEWAYIP    192.168.0.2
#define CONFIG_SERVERIP     192.168.0.174

 #if defined( BOOT_FROM_NFS )
   ...
 #endif
#endif

It’s easily seen that each of the 5 defines CONFIG_ETHADDR, CONFIG_NETMASK, CONFIG_IPADDR, CONFIG_GATEWAYIP and CONFIG_SERVERIP is then used as one of the building blocks for the CONFIG_BOOTARGS define a little further down. So it would be nice to re-use these already defined values to construct the CONFIG_BOOTARGS define.

“No problem” – you say – “I’ll just use the stringizing operator ‘#’…”. Yup, first thing that comes to mind is to use stringification to substitute the defines’ values in the BOOTARGS define:

#define CONFIG_BOOTARGS "root=/dev/nfs rootdelay=5 \
nfsroot=" #CONFIG_SERVERIP ":/work/project_A18/target,rsize=4096,wsize=8192,tcp \
nwhwconf=device:eth0,hwaddr:" #CONFIG_ETHADDR " ip=" #CONFIG_IPADDR "::\
" #CONFIG_GATEWAYIP ":" #CONFIG_NETMASK ":no-nfs::off"

While the key to converting a #define into a string is indeed the stringification operator ‘#’, saddly, the above define as it is writen, breaks compilation.

To make it work we’ll use two macros which will convert the defines values to strings and after that there should be no problem:

#define DEF2STR(x)	#x
#define MK_STR(x)	DEF2STR(x)

#if defined( DEVELOPER_ALAN )
#define CONFIG_ETHADDR 00:23:D0:00:00:00
#define CONFIG_NETMASK 255.255.255.0
#define CONFIG_IPADDR 192.168.0.12
#define CONFIG_GATEWAYIP 192.168.0.2
#define CONFIG_SERVERIP 192.168.0.170

#ifdef BOOT_FROM_NFS
#define CONFIG_BOOTCOMMAND "nfs;bootm 80000000"
#define CONFIG_BOOTFILE "/work/project_A18/target/boot/uImage"
#define CONFIG_BOOTARGS "root=/dev/nfs rootdelay=5 \
nfsroot=" MK_STR(CONFIG_SERVERIP) ":/work/project_A18/target,rsize=4096,wsize=8192,tcp \
nwhwconf=device:eth0,hwaddr:" MK_STR(CONFIG_ETHADDR) " ip=" MK_STR(CONFIG_IPADDR) "::\
" MK_STR(CONFIG_GATEWAYIP) ":" MK_STR(CONFIG_NETMASK) ":ae-nfs::off \
console=ttyAS0,115200 mem=58m bigphysarea=1000"

#elif defined( BOOT_FROM_USBFAT )
    ...
#endif

This code DOES compile and it is easy to re-use it – just configure the 5 defines according to your setup and CONFIG_BOOTARGS will be generated correctly from these values 🙂