真的是太欣赏c的速度了，但如果你连c的指针计算都搞不定，那就好比你活生生的错过了绝世美女,如果真的是那样，人生就太遗憾了。

c常见问题

  1. double free:free已经free的内存。

  2. 访问已经释放的内存

gcc

gdb

glibc

/usr/include

https://en.wikibooks.org/wiki/C_Programming/Simple_input_and_output

data types

In Standard C there are four basic data types. They are int, char, float, and double.

output and input

printf

%d  int
%f  float
%c  char
%s   string
%p pointer

scanf

it might read as: “Read in an integer from the user and store it at the address of variable a “.

scanf("%d", &a);


scanf("%s", a);

This is because, whenever you use a format specifier for a string (%s), the variable that you use to store the value will be an array and, the array names (in this case - a) themselves point out to their base address and hence, the address of operator is not required.

oprator

   &         Address-of; value is the location of the operand(取地址)
   
   *         Contents-of; value is what is stored at the location（取内容）

或与非

||
&&
!

Control flow

if

if (conditions)
{
// do something
}
else if (conditions)
{
// do otherthing
}
else
{
// do something
}

for

int i = 0;
for(i=0;i<100;i++)
{
  printf("%d", i);
}

while

int i = 0;
while (i < 100)
{
 //do something
 i++;
}

do…while

do{
}while(condition)

break continue

break 跳出循环

continue 跳出本次循环

goto

Start:
   if (!''cond1'') goto End;
   S;
   if (''cond2'') goto End;
   T;
   goto Start;
 End:
   /* ... */

switch case

 switch (something) {
 case 2:
 case 3:
 case 4:
   /* some statements to execute for 2, 3 or 4 */
   break;
 case 1:
 default:
   /* some statements to execute for 1 or other than 2,3,and 4 */
   break;
 }

Pointers and arrays

指针是地址

指针变量是用来存放指针的变量

空指针的存放的值为0

#include <stdio.h>

int main () {

   int  *ptr = NULL;

   printf("The value of ptr is : %x\n", ptr  );
 
   return 0;
}
When the above code is compiled and executed, it produces the following result −

The value of ptr is 0

指针算数 https://www.tutorialspoint.com/cprogramming/c_pointer_arithmetic.htm 有四种算数操作符可以应用到指针变量上面，++, --, +, -

Incrementing a Pointer

Decrementing a Pointer

Pointer Comparisons

sizeof

glibc一些函数

https://www.tutorialspoint.com/c_standard_library/stdio_h.htm

stdlib.h

void *malloc(size_t size); //使用malloc分配的内存是没有初始化的
void *calloc(size_t nitems, size_t size)

stdio.h

int puts(const char *str)
printf()
scanf()
sprintf()
sprintf_s()
snprintf()
snprintf_s()

string.h

strcpy()
strlen()

strncpy()
strncpy_s()
memset()