https://gcc.gnu.org/onlinedocs/cpp/Initial-processing.html

preprocessor_step1.c

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

char *read_entire_file_to_memory_and_nul_terminate(const char *filename, size_t *file_size)
{
	FILE* file = fopen(filename, "rb");
	if (file == NULL) {
		perror("Failed to open file");
		return NULL;
	}

	fseek(file, 0, SEEK_END);
	size_t sz = ftell(file);
	rewind(file);

	char *buf = (char *)malloc(sz + 1);
	if (buf == NULL) {
		perror("Failed to allocate memory");
		fclose(file);
		return NULL;
	}

	size_t bytes_read = fread(buf, sizeof(char), sz, file);
	if (bytes_read != sz) {
		perror("Failed to read file");
		free(buf);
		fclose(file);
		return NULL;
	}
	buf[bytes_read] = '\0';

	if (file_size) { *file_size = sz; }
	return buf;
}

void normalize_line_endings(char *buffer)
{
	char *src = buffer;
	char *dst = buffer;
	while (*src) {
		if (*src == '\r') {
			if (*(src + 1) == '\n') { src++; }
			*dst++ = '\n';
		} else {
			*dst++ = *src;
		}
		src++;
	}
	*dst = '\0';
}

void merge_lines_ending_in_backslash(char *buffer)
{
	char *src = buffer;
	char *dst = buffer;
	while (*src) {
		if (*src == '\\' && (*(src + 1) == '\n')) {
			src += 2; // Move past the \ and LF
		} else {
			*dst++ = *src++;
		}
	}
	*dst = '\0';
}

void remove_comments(char *buffer)
{
	char *src = buffer;
	char *dst = buffer;
	while (*src) {
		// Multi-line comments
		if (*src == '/' && *(src + 1) == '*') {
			src += 2; // Move past "/*"
			while (*src && !(*src == '*' && *(src + 1) == '/')) {
				src++;
			}
			if (*src) {
				src += 2; // Move past "*/"
			}
			*dst++ = ' ';
		}
		// Single-line comments
		else if (*src == '/' && *(src + 1) == '/') {
			src += 2; // Move past "//"
			while (*src && *src != '\n') {
				src++;
			}
			*dst++ = ' ';
		}
		else {
			*dst++ = *src++;
		}
	}
	*dst = '\0';
}

int main(void) {
	char *test = "/\\\n"
				 "*\n"
				 "*/ # /*\n"
				 "*/ defi\\\n"
				 "ne FO\\\n"
				 "O 10\\\n"
				 "20";
	// read_entire_file_to_memory_and_nul_terminate("test.h", NULL);
	normalize_line_endings(test);
	merge_lines_ending_in_backslash(test);
	remove_comments(test);

	printf(test); // Prints "  #   define FOO 1020"
	return 0;
}