CS61C Lab2

# Lab 2

# Exercise 0: Makefiles

这个 Exercise 要求我们学会使用 Makefile 来编译 C 语言程序

题目给出了一个 makefile

UNAME_S := $(shell uname -s)
CC=gcc
LD=gcc
CFLAGS=-ggdb -Wall -std=c99
LDFLAGS=

ifeq ($(UNAME_S), Darwin)
    MEMCHECK=valgrind --tool=memcheck --leak-check=full --track-origins=yes --dsymutil=yes
endif

ifeq ($(UNAME_S), Linux)
    MEMCHECK=valgrind --tool=memcheck --leak-check=full --track-origins=yes
endif

BIT_OPS_OBJS = bit_ops.o test_bit_ops.o
BIT_OPS_PROG = bit_ops

LFSR_OBJS = lfsr.o test_lfsr.o
LFSR_PROG = lfsr

VECTOR_OBJS=vector.o vector-test.o
VECTOR_PROG=vector-test

BINARIES=$(VECTOR_PROG) $(BIT_OPS_PROG) $(LFSR_PROG)

all: $(BINARIES)

$(BIT_OPS_PROG): $(BIT_OPS_OBJS)
	$(CC) $(CFLAGS) -g -o $(BIT_OPS_PROG) $(BIT_OPS_OBJS) $(LDFLAGS)

$(LFSR_PROG): $(LFSR_OBJS)
	$(CC) $(CFLAGS) -g -o $(LFSR_PROG) $(LFSR_OBJS) $(LDFLAGS)

lfsr.c: lfsr.h
test_lfsr.c: lfsr.h

bit_ops.c: bit_ops.h
test_bit_ops.c: bit_ops.h

.c.o:
	$(CC) -c $(CFLAGS) $<

vector-memcheck: $(VECTOR_PROG)
	$(MEMCHECK) ./$(VECTOR_PROG)

clean:
	-rm -rf core *.o *~ "#"*"#" Makefile.bak $(BINARIES) *.dSYM

vector.c: vector.h
vector-test.c: vector.h

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然后给出了 10 个问题

Which target is part of a rule that deletes all the compiled programs? 哪个目标是删除所有编译程序的规则的一部分？

clean

Which target is part of a rule that makes all the compiled programs? 哪个目标是创建所有编译程序的规则的一部分？

all

Which compiler is currently being used?

CC = gcc

What C standard are we currently using?

CFLAGS = -ggdb -Wall -std=c99

How would we reference a variable “FOO” in a makefile?

$(FOO)

What operating system does the term “Darwin” represent?

"Darwin"代表的是 macOS（和 iOS 等苹果操作系统）的底层核心操作系统。

What line creates the lfsr program from its object files? (Give its line number.)

$(LFSR_PROG): $(LFSR_OBJS)           # 行号：假设为第 24 行
    $(CC) $(CFLAGS) -g -o $@ $^ $(LDFLAGS)  # 行号：第 25 行（实际编译命令）

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# Exercise 1: Bit Operations

编写程序完成 get_bit，set_bit，flip_bit

#include <stdio.h>
#include "bit_ops.h"

// Return the nth bit of x.
// Assume 0 <= n <= 31
unsigned get_bit(unsigned x,
                 unsigned n) {
    // YOUR CODE HERE
    return (x >> n) & 1;
    // Returning -1 is a placeholder (it makes
    // no sense, because get_bit only returns 
    // 0 or 1)
    return -1;
}
// Set the nth bit of the value of x to v.
// Assume 0 <= n <= 31, and v is 0 or 1
void set_bit(unsigned * x,
             unsigned n,
             unsigned v) {
    // YOUR CODE HERE
    if (v == 1) 
        *x |= (1 << n);
    else
        *x &= ~(1 << n);
}
// Flip the nth bit of the value of x.
// Assume 0 <= n <= 31
void flip_bit(unsigned * x,
              unsigned n) {
    (*x) ^= (1 << n);
    // YOUR CODE HERE
}

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# Exercise 2: Linear Feedback Shift Register

这个 Exercise 要求你实现一个线性反馈移位寄存器，这个图好像挂了

根据描述很容易

void lfsr_calculate(uint16_t *reg) {
    /* YOUR CODE HERE */
    int lfsr = *reg;
    uint16_t bit = ((lfsr >> 0) ^ (lfsr >> 2) ^ (lfsr >> 3) ^ (lfsr >> 5)) & 1;
    lfsr = (lfsr >> 1) | (bit << 15);
    *reg = lfsr;
}

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# Exercise 3: Memory Management

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