reference guide - Page 6

What is a reference variable in C++?

- by sirius79m

Can someone give me a brief definition of a reference variable in C++?

undefined reference to function, despite giving reference in c

I'm following a tutorial, but when it comes to compiling and linking the code I get the following error: /tmp/cc8gRrVZ.o: In function `main': main.c:(.text+0xa): undefined reference to `monitor_clear' main.c:(.text+0x16): undefined reference to `monitor_write' collect2: ld returned 1 exit status make: *** [obj/main.o] Error 1 What that is telling me is that I haven't defined both 'monitor_clear' and 'monitor_write'. But I have, in both the header and source files. They are as follows: monitor.c: // monitor.c -- Defines functions for writing to the monitor. // heavily based on Bran's kernel development tutorials, // but rewritten for JamesM's kernel tutorials. #include "monitor.h" // The VGA framebuffer starts at 0xB8000. u16int *video_memory = (u16int *)0xB8000; // Stores the cursor position. u8int cursor_x = 0; u8int cursor_y = 0; // Updates the hardware cursor. static void move_cursor() { // The screen is 80 characters wide... u16int cursorLocation = cursor_y * 80 + cursor_x; outb(0x3D4, 14); // Tell the VGA board we are setting the high cursor byte. outb(0x3D5, cursorLocation >> 8); // Send the high cursor byte. outb(0x3D4, 15); // Tell the VGA board we are setting the low cursor byte. outb(0x3D5, cursorLocation); // Send the low cursor byte. } // Scrolls the text on the screen up by one line. static void scroll() { // Get a space character with the default colour attributes. u8int attributeByte = (0 /*black*/ << 4) | (15 /*white*/ & 0x0F); u16int blank = 0x20 /* space */ | (attributeByte << 8); // Row 25 is the end, this means we need to scroll up if(cursor_y >= 25) { // Move the current text chunk that makes up the screen // back in the buffer by a line int i; for (i = 0*80; i < 24*80; i++) { video_memory[i] = video_memory[i+80]; } // The last line should now be blank. Do this by writing // 80 spaces to it. for (i = 24*80; i < 25*80; i++) { video_memory[i] = blank; } // The cursor should now be on the last line. cursor_y = 24; } } // Writes a single character out to the screen. void monitor_put(char c) { // The background colour is black (0), the foreground is white (15). u8int backColour = 0; u8int foreColour = 15; // The attribute byte is made up of two nibbles - the lower being the // foreground colour, and the upper the background colour. u8int attributeByte = (backColour << 4) | (foreColour & 0x0F); // The attribute byte is the top 8 bits of the word we have to send to the // VGA board. u16int attribute = attributeByte << 8; u16int *location; // Handle a backspace, by moving the cursor back one space if (c == 0x08 && cursor_x) { cursor_x--; } // Handle a tab by increasing the cursor's X, but only to a point // where it is divisible by 8. else if (c == 0x09) { cursor_x = (cursor_x+8) & ~(8-1); } // Handle carriage return else if (c == '\r') { cursor_x = 0; } // Handle newline by moving cursor back to left and increasing the row else if (c == '\n') { cursor_x = 0; cursor_y++; } // Handle any other printable character. else if(c >= ' ') { location = video_memory + (cursor_y*80 + cursor_x); *location = c | attribute; cursor_x++; } // Check if we need to insert a new line because we have reached the end // of the screen. if (cursor_x >= 80) { cursor_x = 0; cursor_y ++; } // Scroll the screen if needed. scroll(); // Move the hardware cursor. move_cursor(); } // Clears the screen, by copying lots of spaces to the framebuffer. void monitor_clear() { // Make an attribute byte for the default colours u8int attributeByte = (0 /*black*/ << 4) | (15 /*white*/ & 0x0F); u16int blank = 0x20 /* space */ | (attributeByte << 8); int i; for (i = 0; i < 80*25; i++) { video_memory[i] = blank; } // Move the hardware cursor back to the start. cursor_x = 0; cursor_y = 0; move_cursor(); } // Outputs a null-terminated ASCII string to the monitor. void monitor_write(char *c) { int i = 0; while (c[i]) { monitor_put(c[i++]); } } void monitor_write_hex(u32int n) { s32int tmp; monitor_write("0x"); char noZeroes = 1; int i; for (i = 28; i > 0; i -= 4) { tmp = (n >> i) & 0xF; if (tmp == 0 && noZeroes != 0) { continue; } if (tmp >= 0xA) { noZeroes = 0; monitor_put (tmp-0xA+'a' ); } else { noZeroes = 0; monitor_put( tmp+'0' ); } } tmp = n & 0xF; if (tmp >= 0xA) { monitor_put (tmp-0xA+'a'); } else { monitor_put (tmp+'0'); } } void monitor_write_dec(u32int n) { if (n == 0) { monitor_put('0'); return; } s32int acc = n; char c[32]; int i = 0; while (acc > 0) { c[i] = '0' + acc%10; acc /= 10; i++; } c[i] = 0; char c2[32]; c2[i--] = 0; int j = 0; while(i >= 0) { c2[i--] = c[j++]; } monitor_write(c2); } monitor.h: // monitor.h -- Defines the interface for monitor.h // From JamesM's kernel development tutorials. #ifndef MONITOR_H #define MONITOR_H #include "common.h" // Write a single character out to the screen. void monitor_put(char c); // Clear the screen to all black. void monitor_clear(); // Output a null-terminated ASCII string to the monitor. void monitor_write(char *c); #endif // MONITOR_H common.c: // common.c -- Defines some global functions. // From JamesM's kernel development tutorials. #include "common.h" // Write a byte out to the specified port. void outb ( u16int port, u8int value ) { asm volatile ( "outb %1, %0" : : "dN" ( port ), "a" ( value ) ); } u8int inb ( u16int port ) { u8int ret; asm volatile ( "inb %1, %0" : "=a" ( ret ) : "dN" ( port ) ); return ret; } u16int inw ( u16int port ) { u16int ret; asm volatile ( "inw %1, %0" : "=a" ( ret ) : "dN" ( port ) ); return ret; } // Copy len bytes from src to dest. void memcpy(u8int *dest, const u8int *src, u32int len) { const u8int *sp = ( const u8int * ) src; u8int *dp = ( u8int * ) dest; for ( ; len != 0; len-- ) *dp++ =*sp++; } // Write len copies of val into dest. void memset(u8int *dest, u8int val, u32int len) { u8int *temp = ( u8int * ) dest; for ( ; len != 0; len-- ) *temp++ = val; } // Compare two strings. Should return -1 if // str1 < str2, 0 if they are equal or 1 otherwise. int strcmp(char *str1, char *str2) { int i = 0; int failed = 0; while ( str1[i] != '\0' && str2[i] != '\0' ) { if ( str1[i] != str2[i] ) { failed = 1; break; } i++; } // Why did the loop exit? if ( ( str1[i] == '\0' && str2[i] != '\0' || (str1[i] != '\0' && str2[i] =='\0' ) ) failed =1; return failed; } // Copy the NULL-terminated string src into dest, and // return dest. char *strcpy(char *dest, const char *src) { do { *dest++ = *src++; } while ( *src != 0 ); } // Concatenate the NULL-terminated string src onto // the end of dest, and return dest. char *strcat(char *dest, const char *src) { while ( *dest != 0 ) { *dest = *dest++; } do { *dest++ = *src++; } while ( *src != 0 ); return dest; } common.h: // common.h -- Defines typedefs and some global functions. // From JamesM's kernel development tutorials. #ifndef COMMON_H #define COMMON_H // Some nice typedefs, to standardise sizes across platforms. // These typedefs are written for 32-bit x86. typedef unsigned int u32int; typedef int s32int; typedef unsigned short u16int; typedef short s16int; typedef unsigned char u8int; typedef char s8int; void outb ( u16int port, u8int value ); u8int inb ( u16int port ); u16int inw ( u16int port ); #endif //COMMON_H main.c: // main.c -- Defines the C-code kernel entry point, calls initialisation routines. // Made for JamesM's tutorials <www.jamesmolloy.co.uk> #include "monitor.h" int main(struct multiboot *mboot_ptr) { monitor_clear(); monitor_write ( "hello, world!" ); return 0; } here is my makefile: C_SOURCES= main.c monitor.c common.c S_SOURCES= boot.s C_OBJECTS=$(patsubst %.c, obj/%.o, $(C_SOURCES)) S_OBJECTS=$(patsubst %.s, obj/%.o, $(S_SOURCES)) CFLAGS=-nostdlib -nostdinc -fno-builtin -fno-stack-protector -m32 -Iheaders LDFLAGS=-Tlink.ld -melf_i386 --oformat=elf32-i386 ASFLAGS=-felf all: kern/kernel .PHONY: clean clean: -rm -f kern/kernel kern/kernel: $(S_OBJECTS) $(C_OBJECTS) ld $(LDFLAGS) -o $@ $^ $(C_OBJECTS): obj/%.o : %.c gcc $(CFLAGS) $< -o $@ vpath %.c source $(S_OBJECTS): obj/%.o : %.s nasm $(ASFLAGS) $< -o $@ vpath %.s asem Hopefully this will help you understand what is going wrong and how to fix it :L Thanks in advance. Jamie.

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