涉及指针 进程 运算 结构体 函数 内存


gets 函数

#include "stdio.h" 
int main() { 
    char buff[9]; 
    memset(buff,0,sizeof(buff)); 
    gets(buff); 
    // fgets(buff, sizeof(buff), stdin);
    printf("%s\n",buff); 
    return 0; 
}

char* gets(char* buffer)
char* fgets(char* s, int n, FILE* stream)
fgets  最多只能读入n-1个字符
gets 可能导致缓存溢出

MAC 下运行 提示
warning: this program uses gets(), which is unsafe.



strcpy 函数

char *strcpy(char *dest,char *src)
char * strncpy(char *dest, char *src, size_tn)

#include "stdio.h" 
#include "string.h" 

int main(int argc, char *argv[]) { 
    if(argc!=2){
      printf("please input Password\n"); 
      return 0;   
    }
    int flag = 0; 
    char passwd[9]; 
    memset(passwd,0,sizeof(passwd)); 
    // strcpy(passwd, argv[1]); 
    strncpy(passwd, argv[1],sizeof(passwd)); 
 
    if(0 == strcmp("hello", passwd)) { 
        flag = 1; 
    } 
    if(flag) { 
        printf("\n Password cracked \n"); 
    }else { 
        printf("\n Incorrect passwd \n"); 
    } 
    return 0; 
}


main()的返回类型

#include "stdio.h" 
#include "stdlib.h" 

int main(void){ 
  char *ptr = (char*)malloc(10); 
  if(NULL == ptr){ 
    printf("\n Malloc failed \n"); 
    return; 
  } else{ 
    // Do some processing 
    // no free , memory leak
    // free(ptr); 
  }
  return; 
}

warning: return type of 'main' is not 'int'
main()的返回类型应该是 int 而不是 void
int 返回类型会让程序返回状态值


内存泄露

#include "stdio.h" 
#include "stdlib.h" 

int main(void){ 
  char *ptr = (char*)malloc(10); 
  if(NULL == ptr){ 
    printf("\n Malloc failed \n"); 
    return; 
  } else{ 
    // Do some processing 
    // no free , memory leak
    // free(ptr); 
  }
  return; 
}



使用_exit退出

#include "stdio.h" 
#include "stdlib.h" 
#include "unistd.h" 

void func(){ 
  printf("\n Cleanup function called \n"); 
  return; 
} 

int main(void){ 
  int i = 0;
  atexit(func);
  for(;i<0xffffff;i++);
  _exit(0);
  // return 0;
}

_exit() 不会调用 atexit() 注册的函数


接受任何类型的参数并返回 int 的函数
int func(void *ptr)


*和++操作
#include "stdio.h" 
int main(void){ 
  char* ptr = "Linux";
  printf("%c",*ptr++); 
  printf("%c\n",*ptr); 
  return 0;
}

输出 Li
*ptr++ 相当于 *(ptr++)


修改只读字符串

#include "stdio.h" 
int main(void){ 
  char* ptr = "Linux";
  *ptr = 'A';
  printf("%s\n",ptr); 
  return 0;
}


返回局部变量地址



#include "stdio.h" 

int* func(int value){
  int a = value;
  a++;
  return &a;
}
int* func2(int* value){
  (*value)++;
  return value;
}

int main(void){ 
  int a = 9;
  int* p = func(a);
  printf("%d\n",*p);
  
  p = func2(&a);
  printf("%d\n",*p);
  printf("%d\n",a); 
  return 0;
}



#include "stdio.h" 
int main(void){ 
  int a = 1,b=2,c=3;
  printf("%d %d %d\n",a+b+c,b*=2,c*=2); 
  return 0;
}
MAC 下 编译警告
warning: unsequenced modification and access to 'b'
warning: unsequenced modification and access to 'c'
输出
6 4 6

linux
gcc version 4.8.5 20150623 (Red Hat 4.8.5-11) 
编译无警告
输出 
11 4 6