Linux C Programming

SEIE, Soochow University

Week 10, 2015-2016-1

Ref: 宋劲杉. Linux C 编程一站式学习

编码风格
构建：编译、汇编与链接
调试
Makefile 基础

Coding Style

Ugly and Suffering

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void)
{
char gesture[3][10]={"scissor","stone","cloth"};
int man,computer,result, ret;
srand(time(NULL));
while(1){
computer=rand()%3;
printf("\nInput your gesture (0-scissor 1-stone 2-cloth):\n");
ret=scanf("%d",&man);
if(ret!=1||man<0||man>2){
printf("Invalid input! Please input 0, 1 or 2.\n");
continue;
}
printf("Your gesture: %s\tComputer's gesture:
%s\n",gesture[man],gesture[computer]);
result=(man-computer+4)%3-1;
if(result>0)printf("You win!\n");
else if(result==0)printf("Draw!\n");
else printf("You lose!\n");
}
return 0;
}

Indentation and Spaces

A space after if, switch, case, for, do, while
```
while (1)
```
one space around (on each side of) most binary
and ternary operators = + - < > * / % | & ^ <= >=
== != ? :
no space after unary operators & * + - ~ !

no space ++ -- . ->
```
i = i + 1;
++i;
!(i < 1);
-x;
&a[1];
```
space after , and ; (one space after punctuation in western writing)
```
for(i = 1; i < 10; i++)
    foo(arg1, arg2);
```

Break long lines longer than 80 columns. Keep aligned.

if (sqrt(x*x + y*y) > 5.0
    && x < 0.0
    && y > 0.0)
foo(sqrt(x*x + y*y),
    a[i-1] + b[i-1] + c[i-1])
    printf("This is such a long sentence that "
           "it cannot be held within a line\n");

indent with <Tab> or spaces. one Tab equals 8 spaces. Use 8 or 4 spaces. Do NOT mix space and Tab indentation.

put the opening brace last on the line, and put the closing brace first for non-function statement blocks.

switch (action) {
case KOBJ_ADD:
    return "add";
case KOBJ_REMOVE:
    return "remove";
case KOBJ_CHANGE:
    return "change";
default:
    return NULL;
}

do {
    body of do-loop
} while (condition);

if (x == y) {
    ..
} else if (x > y) {
    ...
} else {
    ....
}

braces for functions.

int function(int x)
{
    body of function
}

blank line between logical 'paragraphes': functions and statement groups

Commenting

long (multi-line) comments

/*
 * This is the preferred style for multi-line
 * comments in the Linux kernel source code.
 * Please use it consistently.
 *
 * Description:  A column of asterisks on the left side,
 * with beginning and ending almost-blank lines.
 */

comments may be used.
- head of source file
- above a function, no blank line after it.
- above statement groups. no blank line below.
- at the right of statements. at least one space.

/**
 *	radix_tree_shrink    -    shrink height of a radix tree to minimal
 *	@root		radix tree root
 */
static inline void radix_tree_shrink(struct radix_tree_root *root)
{
	/* try to shrink tree height */
	while (root->height > 0) {
		struct radix_tree_node *to_free = root->rnode;
		struct radix_tree_node *slot;

		BUG_ON(!radix_tree_is_indirect_ptr(to_free));
		to_free = indirect_to_ptr(to_free);

		/*
		 * The candidate node has more than one child, or its child
		 * is not at the leftmost slot, we cannot shrink.
		 */
		if (to_free->count != 1)
			break;
        }
}

Naming

names should be meaningful, clear and short.
lowercase letters with _ for variables, functions and types.
constants and macros are all uppercase with _
NO pinyin

Functions

Keep functions simple.
indentations under 4 levels
functions should not be too long. Recommend to fit on 2 screen (24x80)
local variables no more than 10

Using indent

indent -kr sourcefile.c

-i8 8 spaces, 8 spaces replaced by a TAB
-nut no tabs

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void)
{
    char gesture[3][10] = { "scissor", "stone", "cloth" };
    int man, computer, result, ret;

    srand(time(NULL));

    while (1) {
        computer = rand() % 3;
        printf("\nInput your gesture (0-scissor 1-stone 2-cloth):\n");
        ret = scanf("%d", &man);
        if (ret != 1 || man < 0 || man > 2) {
            printf("Invalid input! Please input 0, 1 or 2.\n");
            continue;
        }
        printf("Your gesture: %s\tComputer's gesture: %s\n",
               gesture[man], gesture[computer]);
        result = (man - computer + 4) % 3 - 1;
        if (result > 0)
            printf("You win!\n");
        else if (result == 0)
            printf("Draw!\n");
        else
            printf("You lose!\n");
    }

    return 0;
}

Build Process

Overview

Preprocess (预处理)
Compile (编译)
Assemble (汇编)
Link (链接)

C Preprocessor (cpp)

Pass over source
- Insert included files
- Perform macro substitutions
Define macros

$ gcc -E example.c

Compiler (cc1)

Compiler translates one language to another
gcc is a C compiler, cc for unix
gcc compiler translates C to assembly
Compiler consists of
- Parser
- Code generation
gcc -S example.c "saves" example.s

Assembler (as)

Another translator
Assembler to (binary) object
Why not compile straight to binary?
gcc -c example.c to "save" object

Linker (ld)

Combines objects, both user .o files and libraries; makes an executable file
gcc *.o -lm yields a.out
gcc -o mytest *.o -lm

Simple Way

gcc hello.c -o hello

Debug

Techniques

compiler error/warning messages
printf() to get run-time states
using a debugger

gdb

compile with -g option. gcc -g main.c -o main
invoke: gdb main

gdb commands

help
list or l got print source code
quit
start
next or n
step or s

backtrace or bt
info or i
frame or f
print or p
finish
set var
display and undisplay

break or b
continue or c
delete breakpoints
disable breakpoints
run
x
watchpoint

try gdb

#include <stdio.h>

int add_range(int low, int high)
{
    int i, sum;
    for (i = low; i <= high; i++)
        sum = sum + i;
    return sum;
}

int main(void)
{
    int result[100];
    result[0] = add_range(1, 10);
    result[1] = add_range(1, 100);
    printf("result[0]=%d\nresult[1]=%d\n", result[0], result[1]);
    return 0;
}

rectangle.c

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>

/** Program to calculate the area and perimeter of 
 * a rectangle using command line arguments
 */
void print_usage() {
    printf("Usage: rectangle -a|-p -l num -b num\n");
}

int main(int argc, char *argv[]) {
    int option = 0;
    int area = -1, perimeter = -1, breadth = -1, length =-1;

    //Specifying the expected options
    //The two options l and b expect numbers as argument
    while ((option = getopt(argc, argv,"apl:b:")) != -1) {
        switch (option) {
        case 'a' : area = 0;
            break;
        case 'p' : perimeter = 0;
            break;
        case 'l' : length = atoi(optarg); 
            break;
        case 'b' : breadth = atoi(optarg);
            break;
        default: print_usage(); 
            exit(EXIT_FAILURE);
        }
    }
    if (area == -1 && perimeter == -1 || length == -1 || breadth ==-1) {
        print_usage();
        exit(EXIT_FAILURE);
    }

    // Calculate the area
    if (area == 0) {
        area = length * breadth;
        printf("Area: %d\n",area);
    }

    // Calculate the perimeter
    if (perimeter == 0) {
        perimeter = 2 * (length + breadth);
        printf("Perimeter: %d\n",perimeter);
    }
    return 0;
}

Makefile

# LDFLAGS
CFLAGS += -g

all: rectangle

rectangle: rectangle.o

clean:
	$(RM) rectangle *.o

.PHONY: clean