How to use ## pre-processor in C

Here is a simple example to quickly understand the usage of ## pre-processing operator. It is used to merge two tokens into one, I mean two strings into one, which is refereed as token pasting or token concatenation. When a macro is expanded, the two tokens on either side of each ## operator are combined into a single token, which then replaces the ## and the two original tokens in the macro expansion.

Example code:

Code:

/*
 * main.c file
 */

#include <stdio.h>

#define MY_DEVICE_BASE_ADDR  0x40004000
#define MY_DEVICE_REG_CTRL_OFS  0x20
#define MY_DEVICE_REG_STATUS_OFS 0x30
#define MY_DEVICE_REG_INTR_OFS  0x40

#define REG(name) \
 ((unsigned int)(MY_DEVICE_BASE_ADDR) + MY_DEVICE_REG_##name##_OFS)

int main()
{
 printf("Example to use ## Pre-Processor!\n");
 
 printf("   *** My-Device Register ***\n");
 printf("   - Control-Register Addr = 0x%x\n", REG(CTRL));
 printf("   - Status-Register Addr = 0x%x\n", REG(STATUS));
 printf("   - Interrupt-Register Addr = 0x%x\n", REG(INTR));
 printf("[END]\n");
 return 0;
}

Output:

Example to use ## Pre-Processor!
   *** My-Device Register ***
   - Control-Register Addr = 0x40004020
   - Status-Register Addr = 0x40004030
   - Interrupt-Register Addr = 0x40004040
[END]

Note: Copying the code directly into your source.c file will also copies the invisible characters and finally you will left out with stray errors, i recommend you to type the code and that even becomes a practice.

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