C Basic Notes

编程习惯

Macro(宏)

括号

尽量添加足够的括号,减少宏定义的二义性

特殊用法

• #: 字符串化
• ##: 强制连接符
• do { ... } while (0): 防止语法错误

头文件

缺少标准库头文件

缺少函数原型

链接成功 - 链接器自动装载库函数，不影响程序执行 只警告，不报错

覆盖标准库函数原型

• 定义过多参数原型，调用时传入过多参数，函数正确执行(无视多余参数)
• 定义缺少参数原型，调用时传入不完整参数，函数错误执行，误把 0xc(%ebp)，0x10(%ebp)，…等更多内存单元当作函数参数

缺少宏定义

链接失败 - 宏定义会被识别为函数，但链接器查找不到相应库函数

防止重复包括头文件

#ifndef _FILENAME_H_
#define _FILENAME_H_

...
...
...

#endif

头文件不申请内存单元

除了全局共享静态变量外, 头文件中的定义不允许申请实际的内存单元

检查

边界检查

• 空/满栈检查
• 参数合法性检查 e.g elemSize > 0 检查

指针检查

• Alloctor 失败，需添加 NULL 检查:
  • assert
  • exit

类型转换

机器码转换

• 有符号类型转换: 进行符号扩展
• 无符号类型转换: 进行零扩展

Pointer Tips and Best Practice

Error Prone Pointers

int i = 37;
float f = *(float *)&i;

float f = 7.0;
short s = *(short *)&f;

• 悬挂指针
• 未初始化
• 改写未知区域
  • 下标越界
  • 内存上溢 e.g gets(string);
• 指针相关运算符优先级与结合性
• 返回局部变量的地址
• 重复释放内存空间
• 内存泄漏 e.g 未释放空间/未释放部分深度空间(多维数组)
• 不能引用 void 指针指向的内存单元

Debugging Malloc

处理 void 指针

Tips: 中途运用强制类型转换,使得 void 指针可以执行指针加减运算

void *target = (char *)void_pointer + ...;

利用 void 指针实现 Generic

通用型 Swap 函数

void swap(void *vp1, void *vp2, int size) {
    char buffer[size];
    memcpy(buffer, vp1, size);
    memcpy(vp1, vp2, size);
    memcpy(vp2, buffer, size);
}

通用型 Search Function

实现

void *lsearch(void *key, void *base, int n, int elemSize,
  int (*cmp_fn)(void *, void *)) {
    for (int i = 0;i < n;i++) {
        void * elemAddr = (char *)base + i * elemSize;
        if (cmp_fn(key, elemAddr) == 0) {
            return elemAddr;
        }
    }

    return NULL;
}

int 实例

int IntCmp(void *elem1, void *elem2) {
    int *ip1 = (int *)elem1;
    int *ip2 = (int *)elem2;
    return *ip1 - *ip2;
}

int array[] = {4, 2, 3, 7, 11, 6},
    size = 6,
    target = 7;

// 应进行强制类型转换
int * found = (int *)lsearch(&target, array, 6, sizeof(int), IntCmp);

if (found == NULL) {
    printf("Not Found");
} else {
    printf("Found");
}

string 实例

int StrCmp(void *vp1, void *vp2) {
    // 必须进行强制类型转换
    char *s1 = *(char **)vp1;
    char *s2 = *(char **)vp2;
    return strcmp(s1, s2);
}

char *notes[] = {"Ab", "F#", "B", "Gb", "D"},
    *target = "Eb";
char ** found = lsearch(&target, notes, 5, sizeof(char *), StrCmp);

泛型数据结构

通用型栈

typedef struct {
    void *elements;
    int elemSize;
    int logLen;
    int allocLen;
} stack;

void StackNew(stack *s, int elemSize);
void StackDispose(stack *s);
void StackPush(stack *s, void *elemAddr);
void StackPop(stack *s, void *elemAddr);

void StackNew(stack *s, int elemSize) {
    // 参数合法性检查
    if (s->elemSize <= 0) {
        perror("ElemSize <= 0");
        return;
    }

    s->elemSize = elemSize;
    s->logLen = 0;
    s->allocLen = 4;
    s->elements = (int *)malloc(s->allocLen * elemSize);

    // NULL检查
    if (s->elements == NULL) {
        perror("No Mem");
        exit(0);
    }
}

void StackPush(stack *s, void *elemAddr) {
    // 满栈检查
    if (s->logLen == s->allocLen) {
        s->allocLen *= 2;
        s->elements = (int *)malloc(s->elements, s->allocLen * s->elemSize);
    }

    void *target = (char *)s->elements + s->logLen * s->elemSize;
    memcpy(target, elemAddr, s->elemSize);
    s->logLen++;
}

void StackPop(stack *s, void *elemAddr) {
    // 空栈检查
    if (s->logLen == 0) {
        perror("Empty Stack");
        return;
    }

    s->logLen--;
    void *source = (char *)s->elements + s->logLen * s->elemSize;
    memcpy(elemAddr, source, s->elemSize);
}

Tools

Valgrind - GitHub Repo

Useful Functions

memset

free

free 函数会回退 4/8 字节，取出 heap 块的长度/信息,根据此信息进行 heap 块的释放.

Strings

strdup

string duplicate - char *strdup(string) 封装 allocator 细节

strchr and strstr

返回字符/串在字符串中出现的位置(地址)

strtok

strcasecmp

getopt

解析命令行参数, 轻松地提取以 - 或 / 开头的参数

I/O

String Scanf

可以用作简易匹配读取函数

// 提取除 http:// 外的字符串
sscanf(buf, "http://%s", url_part);

Exceptions

perror(string) - 用来将上一个函数发生错误的原因输出到标准设备(stderr)

Process

Fork and Execve

• fork(): 创建当前进程的拷贝
• execve(): 用另一程序的代码代替当前进程的代码
  • int execve(char *filename, char *argv[], char *env_p[])

void fork_exec(char *path, char *argv[]) {
    pid_t pid = fork();

    if (0 != pid) {
        printf("Parent: created a child %d\n", pid);
    } else {
        printf("Child: exec-ing new program now\n");
        execv(path, argv);
    }

    printf("This line printed by parent only!\n");
}

Other

• getpid().
• wait(int *child_status)/waitpid(pid).
• exit().

Threads

PThread

typedef unsigned long int pthread_t;

/**
 * create thread
 * @param  {指向线程标识符的指针}   pthread_t *__thread
 * @param  {设置线程属性}         __const pthread_attr_t *__attr
 * @param  {线程运行函数的起始地址} void *(*__start_routine) (void *)
 * @param  {运行函数的参数}        void *__arg
 */
extern int pthread_create __P ((
  pthread_t *__thread,
  __const pthread_attr_t *__attr,
  void *(*__start_routine) (void *)， void *__arg)
);

/**
 * 等待线程
 * @param  {被等待的线程标识符}                              pthread_t __th
 * @param  {一个用户定义的指针，它可以用来存储被等待线程的返回值} void **__thread_return
 */
extern int pthread_join __P ((pthread_t __th, void **__thread_return));

/**
 * 退出线程
 * @param  {函数的返回代码} (void *__retval)) __attribute__ ((__noreturn__)
 */
extern void pthread_exit __P ((void *__retval)) __attribute__ ((__noreturn__));

// 一个线程不能被多个线程等待，否则第一个接收到信号的线程成功返回，其余调用pthread_join的线程则返回错误代码 ESRCH

// 以下为互斥锁相关函数

pthread_mutex_init
pthread_mutexattr_init

/**
 * 设置属性 pshared
 * PTHREAD_PROCESS_PRIVATE
 * PTHREAD_PROCESS_SHARED
 */
pthread_mutexattr_setpshared

/**
 * 设置互斥锁类型
 * PTHREAD_MUTEX_NORMAL
 * PTHREAD_MUTEX_ERRORCHECK
 * PTHREAD_MUTEX_RECURSIVE
 * PTHREAD_MUTEX_DEFAULT
 */
pthread_mutexattr_settype

pthread_mutex_lock
pthread_mutex_unlock
pthread_delay_np

InitThreadPackage;
ThreadNew;
ThreadSleep;
RunAllThreads;
SemaphoreNew(int > 0);
SemaphoreWait(lock);
SemaphoreSignal(lock);

Semaphore

• 哲学家就餐问题
• 将 Semaphore 变量的值在允许范围内(不至于使得线程锁失效)使得其取最大值，减少线程阻塞
• EmptyBuf 8, FullBuf 0
• 双向通信，互相唤醒 - Writer:sw(empty),ss(full); Reader:sw(full),ss(empty);

void SellTickets(int agent, int *ticketsNum, Semaphore lock) {
  while (true) {
    // 当 lock == 0 时,当前进程阻塞, 等待 lock > 0
    // 当 lock > 0 时, 当前进程继续进行, 并且 lock--
    SemaphoreWait(lock);

    if (*ticketsNum == 0) break;  // 票已售磬

    (*ticketsNum)--;  // 售出一张票
    printf("Sell One Ticket.\n");

    // lock++ 使得 lock > 0
    // 若有其他进程调用了SemaphoreWait, 且因之前 lock == 0 而被阻塞, 则此时其他进程可继续进行
    SemaphoreSignal(lock);
  }

    // break to here
  // 作用同循环内的 Signal 函数
  SemaphoreSignal(lock);
}

联合体

• 机器码 e.g 理解 IEEE 754 标准
• 区分大/小端模式

Naming Conventions

常用缩写词

原词	缩写
addition	add
answer	ans
array	arr
average	avg
buffer	buf 或 buff
capture	cap 或 capt
check	chk
count	cnt
column	col
control	ctrl
decode	dec
define	def
delete	del
destination	dst 或 dest
display	disp
division	div
encode	enc
environment	env
error	err
float	flt
frequency	freq
header	hdr
index	idx
image	img
increment	inc
initialize	init
iteration	itr
length	len
memory	mem
middle	mid
make	mk
message	msg
multiplication	mul
number	num
operand	opnd
optimization	opt
operator	optr
packet	pkt
position	pos
previous	pre/prev
payload	type
pointer	ptr/pt
return	code
record	rcd/rc
receive	recv
result	res
return	ret
source	src
stack	stk
string	str
subtraction	sub
table	tab
temporary	tmp 或 temp
total	tot
time	stamp
value	val

Header File

防止其他文件重复#include 本文件

#ifndef MONGOOSE_HEADER_INCLUDED
#define MONGOOSE_HEADER_INCLUDED

/*.................................
 * do something here
 *.................................
 */

#endif /* MONGOOSE_HEADER_INCLUDED */

C Standard Library

Assert

• 关闭断言

#define NDEBUG
#include <assert.h>

• 开启断言

#undef NDEBUG
#include <assert.h>

Types

• 可检测字符

getc、fgetc、getchar 函数可返回值(EOF 值/unsigned char 类型)

• 不可检测字符

非 EOF 值/非 unsigned char 类型(会引发严重错误)

Errno

• errno 的值在程序启动时为零，但是不会被任何库函数设为零

errno = 0;
y = sqrt(x);
if (errno != 0) {
    printf("invalid x : %e\n", x);
}

#ifndef _I386_ERRNO_H
#define _I386_ERRNO_H
#define EPERM 1 /* Operation not permitted */
#define ENOENT 2 /* No such file or directory */
#define ESRCH 3 /* No such process */
#define EINTR 4 /* Interrupted system call */
#define EIO 5 /* I/O error */
#define ENXIO 6 /* No such device or address */
#define E2BIG 7 /* Arg list too long */
#define ENOEXEC 8 /* Exec format error */
#define EBADF 9 /* Bad file number */
#define ECHILD 10 /* No child processes */
#define EAGAIN 11 /* Try again */
#define ENOMEM 12 /* Out of memory */
#define EACCES 13 /* Permission denied */
#define EFAULT 14 /* Bad address */
#define ENOTBLK 15 /* Block device required */
#define EBUSY 16 /* Device or resource busy */
#define EEXIST 17 /* File exists */
#define EXDEV 18 /* Cross-device link */
#define ENODEV 19 /* No such device */
#define ENOTDIR 20 /* Not a directory */
#define EISDIR 21 /* Is a directory */
#define EINVAL 22 /* Invalid argument */
#define ENFILE 23 /* File table overflow */
#define EMFILE 24 /* Too many open files */
#define ENOTTY 25 /* Not a typewriter */
#define ETXTBSY 26 /* Text file busy */
#define EFBIG 27 /* File too large */
#define ENOSPC 28 /* No space left on device */
#define ESPIPE 29 /* Illegal seek */
#define EROFS 30 /* Read-only file system */
#define EMLINK 31 /* Too many links */
#define EPIPE 32 /* Broken pipe */
#define EDOM 33 /* Math argument out of domain of func */
#define ERANGE 34 /* Math result not representable */
#define EDEADLK 35 /* Resource deadlock would occur */
#define ENAMETOOLONG 36 /* File name too long */
#define ENOLCK 37 /* No record locks available */
#define ENOSYS 38 /* Function not implemented */
#define ENOTEMPTY 39 /* Directory not empty */
#define ELOOP 40 /* Too many symbolic links encountered */
#define EWOULDBLOCK EAGAIN /* Operation would block */
#define ENOMSG 42 /* No message of desired type */
#define EIDRM 43 /* Identifier removed */
#define ECHRNG 44 /* Channel number out of range */
#define EL2NSYNC 45 /* Level 2 not synchronized */
#define EL3HLT 46 /* Level 3 halted */
#define EL3RST 47 /* Level 3 reset */
#define ELNRNG 48 /* Link number out of range */
#define EUNATCH 49 /* Protocol driver not attached */
#define ENOCSI 50 /* No CSI structure available */
#define EL2HLT 51 /* Level 2 halted */
#define EBADE 52 /* Invalid exchange */
#define EBADR 53 /* Invalid request descriptor */
#define EXFULL 54 /* Exchange full */
#define ENOANO 55 /* No anode */
#define EBADRQC 56 /* Invalid request code */
#define EBADSLT 57 /* Invalid slot */
#define EDEADLOCK EDEADLK
#define EBFONT 59 /* Bad font file format */
#define ENOSTR 60 /* Device not a stream */
#define ENODATA 61 /* No data available */
#define ETIME 62 /* Timer expired */
#define ENOSR 63 /* Out of streams resources */
#define ENONET 64 /* Machine is not on the network */
#define ENOPKG 65 /* Package not installed */
#define EREMOTE 66 /* Object is remote */
#define ENOLINK 67 /* Link has been severed */
#define EADV 68 /* Advertise error */
#define ESRMNT 69 /* Srmount error */
#define ECOMM 70 /* Communication error on send */
#define EPROTO 71 /* Protocol error */
#define EMULTIHOP 72 /* Multi hop attempted */
#define EDOTDOT 73 /* RFS specific error */
#define EBADMSG 74 /* Not a data message */
#define EOVERFLOW 75 /* Value too large for defined data type */
#define ENOTUNIQ 76 /* Name not unique on network */
#define EBADFD 77 /* File descriptor in bad state */
#define EREMCHG 78 /* Remote address changed */
#define ELIBACC 79 /* Can not access a needed shared library */
#define ELIBBAD 80 /* Accessing a corrupted shared library */
#define ELIBSCN 81 /* .lib section in a.out corrupted */
#define ELIBMAX 82 /* Attempting to link in too many shared libraries */
#define ELIBEXEC 83 /* Cannot exec a shared library directly */
#define EILSEQ 84 /* Illegal byte sequence */
#define ERESTART 85 /* Interrupted system call should be restarted */
#define ESTRPIPE 86 /* Streams pipe error */
#define EUSERS 87 /* Too many users */
#define ENOTSOCK 88 /* Socket operation on non-socket */
#define EDESTADDRREQ 89 /* Destination address required */
#define EMSGSIZE 90 /* Message too long */
#define EPROTOTYPE 91 /* Protocol wrong type for socket */
#define ENOPROTOOPT 92 /* Protocol not available */
#define EPROTONOSUPPORT 93 /* Protocol not supported */
#define ESOCKTNOSUPPORT 94 /* Socket type not supported */
#define EOPNOTSUPP 95 /* Operation not supported on transport endpoint */
#define EPFNOSUPPORT 96 /* Protocol family not supported */
#define EAFNOSUPPORT 97 /* Address family not supported by protocol */
#define EADDRINUSE 98 /* Address already in use */
#define EADDRNOTAVAIL 99 /* Cannot assign requested address */
#define ENETDOWN 100 /* Network is down */
#define ENETUNREACH 101 /* Network is unreachable */
#define ENETRESET 102 /* Network dropped connection because of reset */
#define ECONNABORTED 103 /* Software caused connection abort */
#define ECONNRESET 104 /* Connection reset by peer */
#define ENOBUFS 105 /* No buffer space available */
#define EISCONN 106 /* Transport endpoint is already connected */
#define ENOTCONN 107 /* Transport endpoint is not connected */
#define ESHUTDOWN 108 /* Cannot send after transport endpoint shutdown */
#define ETOOMANYREFS 109 /* Too many references: cannot splice */
#define ETIMEDOUT 110 /* Connection timed out */
#define ECONNREFUSED 111 /* Connection refused */
#define EHOSTDOWN 112 /* Host is down */
#define EHOSTUNREACH 113 /* No route to host */
#define EALREADY 114 /* Operation already in progress */
#define EINPROGRESS 115 /* Operation now in progress */
#define ESTALE 116 /* Stale NFS file handle */
#define EUCLEAN 117 /* Structure needs cleaning */
#define ENOTNAM 118 /* Not a XENIX named type file */
#define ENAVAIL 119 /* No XENIX semaphores available */
#define EISNAM 120 /* Is a named type file */
#define EREMOTEIO 121 /* Remote I/O error */
#define EDQUOT 122 /* Quota exceeded */
#define ENOMEDIUM 123 /* No medium found */
#define EMEDIUMTYPE 124 /* Wrong medium type */
#endif

Float and Limits

• 宏定义：CHAR/UCHAR/SCHAR/SHRT/USHRT/INT/UINT/LONG/ULONG/FLT/DBL/LDBL有关的MIN/MAX/EPSILON

Locale

• 实现时间/单位/货币等一系列的国际化
• 常用函数

_CRTIMP char * __cdecl setlocale(int, const char *);
_CRTIMP struct lconv * __cdecl localeconv(void);

• int 值

#define LC_ALL          0
#define LC_COLLATE      1
#define LC_CTYPE        2
#define LC_MONETARY     3
#define LC_NUMERIC      4
#define LC_TIME         5

Math

数学函数库(包括后缀 f(float)/l(long double))

SetJMP

• 常用函数

int setjmp(jmp_buf env);
void longjmp(jmp_buf env, int val);

• 使用：用于 if/else、loop、switch 语句
  1. 直接调用 setjmp 函数时，返回值为 0;
  2. 调用 longjmp 函数时，若 val 值不为 0,则跳转至上次 setjmp 返回值为 0 处，继续向后执行语句
• 功能
  1. 实现非本地(局部)跳转(跨越多层函数调用栈进行跳转)
  2. 实现类 Java 异常机制(异常抛出及捕获)

Signal

信号处理程序中所有数据应为 volatile 类型

_CRTIMP int __cdecl raise(int);
_CRTIMP void (__cdecl * __cdecl signal(int, void (__cdecl *)(int)))(int);

Stdarg

用于编写可变参数函数

void printargs(int arg1, ...) /* 输出所有int类型的参数，直到-1结束 */
{
    va_list ap;
    int i;
    va_start(ap, arg1);
    for (i = arg1; i != -1; i = va_arg(ap, int))
    printf("%d ", i);
    va_end(ap);
    putchar('\n');

}

Stddef

• 宏
  • NULL Null 指针常量
  • offsetof(type, member-designator) 获得字段在结构体中的偏移量
• 类型
  • ptrdiff_t 带符号的整数类型, 用来表示指针相减的结果类型
  • wchar_t 宽字符类型
  • size_t 无符号整数类型, 用来表示 sizeof 操作符的结果类型

String

• men 系函数：操作任意字符序列
• strn 系函数：操作非空字符序列
• str 系函数：操作字符串序列('\0')

GDB Tutorial

Basic Command

• r(run)
• l(list)
• b(break)
  • b line_num
  • b filename:line_num
• display/format address

e.g display $pc/i $pc.

• t 按二进制格式显示变量.
• d 按十进制格式显示变量.
• o 按八进制格式显示变量.
• u 按十六进制格式显示无符号整型.
• x 按十六进制格式显示变量.
• a 按十六进制格式显示变量.
• f 按浮点数格式显示变量.
• c 按字符格式显示变量.
• s 按字符串格式显示变量.
• disas 显示汇编代码.
• x /num-size-format $pc/rsp/rbp.

e.g size:w(2 字节) format:x/d/s(十六进制/十进制/字符串) 2wx

• examine - 查看内存 n、f、u 是可选的参数
  • n 是一个正整数，表示显示内存的长度，也就是说从当前地址向后显示几个地址的内容。
  • f 表示显示的格式，参见上面。如果地址所指的是字符串，那么格式可以是 s，如果地十是指令地址，那么格式可以是 i。
  • u 表示从当前地址往后请求的字节数，如果不指定的话，GDB 默认是 4 个 bytes. u 参数可以用下面的字符来代替，b 表示单字节，h 表示双字节，w 表示四字节，g 表示八字节. 当我们指定了字节长度后，GDB 会从指内存定的内存地址开始，读写指定字节，并把其当作一个值取出来. 表示一个内存地址.

命令：x/3uh 0x54320 表示，从内存地址 0x54320 读取内容，h 表示以双字节为一个单位，3 表示三个单位，u 表示按十六进制显示。

• s step into
  • step1 下一条汇编指令
• n next line
• p print
• q quit
• up last stack
• bt(back trace) function stack 显示堆栈回溯信息
• info breakpoints/register

GDB Set Command

• set disassembly
• set variable

GDB Shell

shell command

GDB Assemble

GDB Core Dump

ulimit -c unlimited
gdb -c core_file_path target_exe_path

GDB Disasm

• CS Segment

(gdb) disass
(gdb) x/i
(gdb) x/5i $pc
(gdb) ni/si

• Registers

(gdb) i r
(gdb) i r a
(gdb) i r ds

• DS Segment
• SS Segment

GDB Stack Frame

(gdb) bt
(gdb) frame n
(gdb) info locals

GCC

• -E: cpp(c preprocessor) 预处理 => .i
• -S: cll 编译 => .s
• -c: as(assemble) 汇编 => .o
• -time Time the execution of each subprocess
• -std=<standard> Assume that the input sources are for <standard>
• -B <directory> Add <directory> to the compiler's search paths
• -v Display the programs invoked by the compiler
• -o <file> Place the output into <file>
• -shared Create a shared library
• -Wall
• -v --help

Awesome Tools

• 用于创建代码文档资料的 NDoc
• 用于生成解决方案的 NAnt
• 用于生成代码的 CodeSmith
• 用于监视代码的 FxCop
• 用于编译少量代码的 Snippet Compiler
• 两种不同的转换器工具：ASP.NET 版本转换器和 Visual Studio .NET 项目转换器
• 用于生成正则表达式的 Regulator
• 用于分析程序集的 .NET Reflector
• 用于单元测试的 NUnit