库函数学习(1) strdup snprintf setenv atoi assert

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int main(void){
char *source_str = "shanghai";
char *target_str = strdup(source_str);//malloc memory and duplicate a string
target_str[0] = 'z';
puts(source_str);
puts(target_str);
return 0;
}

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int main(){
char target[5];
char *source = "xianer";
//it's safer than sprintf, so just use it!
snprintf(target, sizeof(target), "%s", source);
puts(target);
return 0;
}

#include<stdio.h>
#include<stdlib.h>
int main(){
char *user;
if((user = getenv("USER")) != NULL){
printf("USER = %s\n", user);
}

setenv("USER", "test", 1);
printf("USER = %s\n", getenv("USER"));

unsetenv("USER");
printf("USER = %s\n", getenv("USER"));

putenv("CITY=shanghai");
printf("CITY = %s\n", getenv("CITY"));
return 0;
}

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

int main(void)
{
char *str = "123456";
int n = atoi(str);
printf("The string %s as an integer is = %d\n",str,n);

char *str2 = "314hello";
n = atoi(str2);
printf("The string %s as an integer is = %d\n",str2,n);

return 0;
}

#include<stdio.h>
#include<assert.h>
int main(){
int i = 0x80;
assert(i & 128);
puts("iiiiiiiiiiii");
int j = 0x20;
assert(j & 128);
puts("jjjjjjjjj");
return 0;
}