1. 回傳類型不建議用void，而用int。
2. 不建議用main()，而用main(void) 或 main(int argc, char *argv[])。
3. 回傳類型寫int，然後不寫return 0;還是可以編譯過，只有main限定，但還是寫出來比較好。

數值和字串的轉換

數值轉字串: snprintf第一個參數是被寫入的地方， 第二個是限制寫多長，第三是數值類型， 第四是數值變數。
code裏有兩個sample, int和float的， 注意float的小數點位數記得設定。

字串轉數值: sscanf第一個參數放字串變數， 第二個放要轉成的類型， 第三個放要放入變數的記憶體位址。
code裏有四個sample， 前兩個爲int和混合string的， 後兩個爲float和混合string。

String

#include <string.h>

size_t strlen ( const char * str );

#include <stdio.h>
#include <string.h>
#define STR "I am fine."

int main(void) {

// strlen()
printf("%zd", strlen(STR));
// output 10

// sizeof
printf("%zd", sizeof STR);
// output 11 (contain '\0' means null character.)

sizeof

printf

 // example
printf("split \
first line.");
//output
split first line. 

strncpy

gcc參數

gcc link dll

gcc sample.c -lthis_dll_sample 

Displaying Octal and Hexadecimal

// print octal and hexadecimal
printf("%#o, %#x\n", octal, hexadecimal);

Integer Type

// "short int" or short
short int a_1;
// or
short a_2;

// "long int" or "long"
long int b_1;
// or 
long b_2

// "long long int" or "long long", at least 64-bit cpu.
long long int c_1;
// or
long long c_2;

integer type

Floating-Point numbers

floating-point

printf("%f\n", -1.32e-11); 

Boolean Type

// example
_Bool b_1;

// C99, stdbool.h
bool b_2;

sizeof

macro

#define MAX(a,b) ((a) > (b) ? (a) : (b)) 

#define SWAP(a, b, tmp) ((tmp) = (a), (a) = (b), (b) = (a))

#ifndef SOME_H
#define SOME_H

/* Declare some data types and public functions. */

#endif

// 測測編譯環境
#if defined(_WIN32)
	#define PLATFORM "windows"
#elif defined(__linux__)
	#define PLATFORM "Linux"
#elif defined(__APPLE__)
	#define PLATFORM "mac"
#else
	#define PLATFORM "other os"
#endif

一個#代表字串

#include <stdio.h>
 
#define pow(a) (#a)

int main(int argc, char *argv[]) {
	printf("%s\n", pow(sdkfjsdkf));
} 

...和__VA_ARGS__

#define test(...) (printf("%d\n", __VA_ARGS__))

...

test(5000);
// output: 5000

##可黏兩個變數

#define test(a,b) (printf("%d\n", a##b))

...

test(50,6);
// output: 506

換行(反斜線)

#define test(5,6) ( \
printf("%d\n", a##b))

function sample

#define cmp(a,b,tmp)({ \
if((a)>(b)) { \
tmp = -1; \
} else if(((a)<(b))) { \
tmp = 1; \
} \
else { \
tmp = 0; \
} \
}) 

pragma

#pragma once
#ifndef UTILS_H 
#define UTILS_H

/* ... */

#endif

assert

#include <assert.h>

/* ...  */

int a;
scanf("%d", &a);
assert(a>10);
printf("a == %d", a);

scanf

int width, percision;
scanf("%d, %d", &width, &percision);
printf("%*.*f", width, percision, 3.2)

處理scanf("%d", var), var為非整數

int status;
int code;
while(status=scanf("%d", code)){
	if(status != 1)
		scanf("%*s"); // dispose non-integer
	printf("Enter an integer, please.\n");
}

static

• static放區域變數前, 可保留變數之值，直到程式結束。
• static放全域變數前, 可防止其他c/cpp利用extern讀取。
• static放函數前, 可防止其他c/cpp利用extern讀取。

while搭配scanf輸入資料

#include <stdio.h>
#define FREEZING 4.0

int main(int argc, char *argv[]) {
	float temperature;
	int all_day = 0;
	int cold_day = 0;
	printf("Input temperature value. (any character to quit)\n");
	while(scanf("%f", &temperature)) {
		all_day++;
		if(temperature < FREEZING)
			cold_day++;
	}
	if(all_day==0)
		printf("No data!\n");
	else
		printf("%d cold day. Total of %d day(s).\n", cold_day, all_day);

	return 0;
}

getchar() and putchar()

#include <stdio.h>

int main(int argc, char *argv[]) {
	char ch;
	/* move characters. */
	/* getchar() be a statement. */
	/* while( (ch=getchar() ) != '\n'), don't need "ch = getchar()" in end. */
	/* The operator '!=' is higher priority than '='. Therefore, It need to bracket on "ch = getchar()". */
	ch = getchar();
	while(ch != '\n') {
		if(ch == ' ') {
			putchar(ch);
		} else {
			putchar(ch+1); // encryption
		}
		ch = getchar();
	}
	putchar(ch); // print the newline.
	return 0;
}

ctype.h

#include <stdio.h>
#include <ctype.h>

int main(int argc, char *argv[]) {
	char ch;
	/* move characters. */
	while( (ch=getchar()) != '\n') {
		if(ch == isalpha()) {
			putchar(ch+1); // encryption
		} else {
			putchar(ch);
		}
	}
	putchar(ch); // print the newline.
	return 0;
}

conio.h

EOF

while( (ch=getchar()) != EOF); 

Pass pointer to function

#include <stdio.h>
#define SIZE 10
int total(int *start, int *end)
{
	int sum = 0;
	while(start < end) {
		sum += *start;
		start++;
	}
	return sum;
}
int main(int argc, char *argv[]) {
	int a[SIZE];
	int sum;
	for(int i=0; i<SIZE; i++)
		a[i] = 10;
	sum = total(a, a+SIZE);
	printf("%d\n", sum);
	return 0;
}

pointer and array

*(array_a + 2) == array_a[2];
*(*(array_b + 4) + 5) == array_b[4][5];

int a[3][2] = {{1,2}, {3,4}, {5,6}};
	int (*p)[2]; // The 'p' is an address that point to two ints' address.
	/* Alternatively, p[][2] */
	p = a+1;
	printf("%d\n", (*p)[1]);
	/* output: 4 */

Variable-Length Arrays (VLAs)

#define COLS 4
int Sum(int ar[][COLS], int rows)
{
	int total = 0;

	for(int r=0; r<rows; r++)
		for(int c=0; c<COLS; c++)
			total += ar[r][c];

	return total;
}

int Sum(int rows, int cols, int ar[rows][cols]); 

int Sum(int, int, int ar[*][*]); 

Compound Literals

int div[2] = {10, 20};
/* equal */
/* nameless */
(int [2]){10,20};

/* initialize */
(int []){50, 20, 100}; // a compound literal with 3 elements.

/* example */
int *pt1;
pt1 = (int [2]) {10, 20};


/* example */
/* pass parameters */
int sum(const int1r[], int n);

...

int total;
total = sum((int []{4, 2, 2, 4, 5}, 6);


/* example */
int (*pt)[4]; // int pt[][4] can not initialize.

pt = (int [2][4]){ {1,2,3,4}, {5,6,7,8} };

Strings

puts(); 

char str[] = "First" " Second" 
	" Third.";
/* Output: First Second Third.*/

putchar with pointer

#include <stdio.h>
void put1(const char *str)
{
	while(*str!='\0') // while(*str)
		putchar(*str++);
}
int main(int argc, char *argv[]) {
	put1("I want to fly!\n");
	return 0;

}

sprintf

#include <stdio.h>
int main(int argc, char *argv[]) {
	char a1[40];
	int count;
	count = sprintf(a1, "Number is %d.", 10);
	puts(a1);
	printf("%d\n", count);
	return 0;

}

malloc

double *pt
pt = (double *) malloc(30 * sizeof(double));

...

free(pt);

Volatile

restrict

int *restruct pt = (int*) malloc(10 sizeof(int));

_Atomic

Files I/O

fopen

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

int main(int argc, char *argv[]) {
	int ch;
	FILE *fp;
	fp = fopen("SomeTextFile.txt", "r");
	while((ch=getc(fp)) != EOF)
		putc(ch, stdout); // putchar
	fclose(fp);

	return 0;
}

getc()

getc(fp); 

putc()

putc(ch, fp); 

rewind()

rewind(fp); 

fscanf()

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

int main(int argc, char *argv[]) {
	int ch;
	char word[50];
	FILE *fp;
	fp = fopen("SomeTextFile.txt", "r");
	while(fscanf(fp, "%s", word)==1);
	fclose(fp);
	printf("%s", word);



	return 0;
}

fgets()

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

int main(int argc, char *argv[]) {
	int ch;
	char word[50];
	FILE *fp;
	fp = fopen("SomeTextFile.txt", "r");
	fgets(word, 7, fp);
	fclose(fp);
	printf("%s", word);

	return 0;
}

fputs()

fputs(word, fp); 

Pointers to Strutures

#include <stdio.h>
 
struct book {
	char linear_algebra[50];
	char Discrete[50];
};

int main(int argc, char *argv[]) {
	struct book bookcase[2] = {
		{"book_a", "book_b"},
		{"book_c", "book_d"}
	};
	struct book *pt;

	pt = bookcase;

	puts(pt->linear_algebra);
	// output: book_a
	
	pt++; // point to next index.
	puts(pt->linear_algebra);
	// output: book_c

	return 0;
}

fwrite()

fwrite(&primer, sizeof(struct book), 1, fp); 

enum

enum DAY {
	MON=1,
	TUE,
	WED,
	THU,
	FRI,
	SAT,
	SUN
};
int main(void)
{
	enum DAY day_var;
	day_var = WED;

	printf("%d\n", day_var);
	// output: 3


	return 0;
}

calloc

realloc

int *returnArr = (int *)malloc(100*sizeof(int));
int *size = 10;
returnArr = realloc(returnArr, (*size)*sizeof(int));

Hash Table

int t['z'+1] = {
	['a'] = 10,
	['c'] = 30,
	['d'] = 40,
};
printf("%d\n", t['f']); // t['f'] == 0
printf("%d\n", t['d']); // t['d'] == 40

Print binary bit

int a= 10
for(int n=31; n>=0; n--) {
	printf("%d", a>>n & 1);
}

Bit Mask

int n = 10;
// set the 5th bit from right to 1
int mask = 1<<4;
n |= mask;

// set the 3rd bit from right to 0
int mask = ~(1<<2);
n &= mask;

// toggle the 4rd bit from right
int mask = 1<<3;
n ^= mask;

conio.h

getch() 

getche() 

    // COORD pos = { 10,5 };
    COORD pos;
    pos.X = (short)5;
    pos.Y = (short)3;
    HANDLE hOut = GetStdHandle(STD_OUTPUT_HANDLE);
 
    SetConsoleCursorPosition(hOut, pos);
 
 	/* set text color */
    SetConsoleTextAttribute(hOut, 0x01 | 0x05);
 
    printf("Something Something");

typedef

function pointer

Pointer

讀取的資料是由多個register內容組合出來

UINT32 EXA;
UINT32 EXB;
UINT32 EXC;
UINT32 EXD;

CHAR8 ID[(32/8)*4+1] = {0};

*(UINT32 *)ID = EXA;
*(UINT32 *)ID = EXB;
*(UINT32 *)ID = EXC;
*(UINT32 *)ID = EXD;

printf("%s\n", ID); // 前提是整個字串要是滿的