ANSI C (3) —— 常用系统函数

字符测试函数

简单例子：

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

int main(){
    char *s = "Hello world.";
    while(*s){
        if(isupper(*s)) printf("%c,*s);
        else if(islower(*s)) printf("%c,*s);
        else if(isspace(*s)) printf("%c,*s);
        else if(ispunct(*s)) printf("%c,*s);
        s++;
    }
    return 0;
}
/*
H is upper character.
e is lower character.
l is lower character.
l is lower character.
o is lower character.
  is space character.
w is lower character.
o is lower character.
r is lower character.
l is lower character.
d is lower character.
. is punction mark.
*/

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系统时间和日期函数

clock

clock_t是long替换，程序从启动到函数调用占用CPU的时间，在MSDN中称之为挂钟时间（wal-clock）；若挂钟时间不可取，则返回-1。其中clock_t是用来保存时间的数据类型

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

int main(){
    clock_t start = clock();
    sleep(1);  /* 将程序挂起1秒 */
    clock_t end = clock();
    printf("spent time is %.8lf\n",(double)(end-start)/CLOCKS_PER_SEC);
    return 0;
}
/*
./time
spent time is 0.00000000
*/

分析：在程序挂起的时间段内，进程被调离出内存，所以没有占用cpu，那么程序的运行结果也就是0了。
换一个例子：

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

int global;
void stop(int sig){
    printf("ctrl + c change global to 1.\n");
    global=1;
}
int main(){
    signal(SIGINT,stop);
    clock_t start = clock();
    while(global == 0);
    clock_t end = clock();
    printf("spent time is %.8lf\n",(double)(end-start)/CLOCKS_PER_SEC);
    return 0;
}
/*
/time2
^Cctrl + c change global to 1.
spent time is 1.56000000
*/

time, ctime

在/usr/include/time.h中有这样一句话：
​​​typedef __time_t time_t;​​​
并且在它的上面还有一句话： ​​​# include <bits/types.h>​​​
在此文件中，我们搜索到：​​​__STD_TYPE __TIME_T_TYPE __time_t; /* Seconds since the Epoch. */
​​
这说明time_t是一种时间类型，存放自1970年1月1日0点0时0分开始的秒数

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

int main(){
    time_t t;
    time(&t);
    printf("%d\n",t); /* 32 bit integer */
    printf("%s\n",ctime(&t)); 
    return 0;
}
/*
./time3
1477848041
Mon Oct 31 01:20:41 2016
*/

difftime

因为time获取的是系统时间，和CPU占用没有关系，所以这里的sleep()达到了效果（和clock的例子作比较）。

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

int main(){
    time_t t1 = time(NULL); /* get system time */
    sleep(1);
    time_t t2 = time(NULL);
    printf("spent time is %.8lf\n",difftime(t2,t1));
    return 0;
}

gmtime asctime

struct tm的定义是这样的：

/* /usr/include/time.h  */
struct tm
{
  int tm_sec;           /* Seconds. [0-60] (1 leap second) */
  int tm_min;           /* Minutes. [0-59] */
  int tm_hour;          /* Hours.   [0-23] */
  int tm_mday;          /* Day.     [1-31] */
  int tm_mon;           /* Month.   [0-11] */
  int tm_year;          /* Year - 1900.  */
  int tm_wday;          /* Day of week. [0-6] */
  int tm_yday;          /* Days in year.[0-365] */
  int tm_isdst;         /* DST.     [-1/0/1]*/

# ifdef __USE_MISC
  long int tm_gmtoff;       /* Seconds east of UTC.  */
  const char *tm_zone;      /* Timezone abbreviation.  */
# else
  long int __tm_gmtoff;     /* Seconds east of UTC.  */
  const char *__tm_zone;    /* Timezone abbreviation.  */
# endif

下面的例子是：

graph TD
A[time_t]
-->B[struct tm]
B-->|asctime|C[GMT time string]
A-->|ctime|C

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

int main(){
    time_t t = time(NULL);
    struct tm *gmt=gmtime(&t);
    printf("time is %s\nGMT time is %s\n",ctime(&t),asctime(gmt));
    return 0;
}
/*
./gmtime 
time is Mon Oct 31 21:20:44 2016

GMT time is Mon Oct 31 21:20:44 2016

*/

localtime, mktime

graph TD
A[struct tm]-->|mktime|B[time_t]
B-->|localtime|A

环境控制函数

getenv

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

int main(){
    char *p = getenv("USER");
    if(p){
        printf("USER = %s\n",p);
    }
    return 0;
}
/*
$ ./getenv 
USER = edemon
*/

putenv

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

int main(){
    char *p = "pet=dog";
    putenv(p);
    printf("pet is %s\n",getenv("pet"));
    return 0;
}

putenv不会对环境变量文件产生影响

/etc/environment

setenv

#include <stdio.h>
#include <stdlib.h>
int main(){
    putenv("pet=dog");
    int ret=0;
    ret = setenv("pet","cat",1);
    if(ret!=-1){
        printf("change success, pet is %s\n",getenv("pet"));
    }
    return 0;
}

内存分配函数

calloc

calloc申请内存空间后，会自动初始化内存空间为0：

#include <iostream>
#include <stdlib.h>
#include <stdio.h>
using namespace std;

int main() {
    // void *calloc(size_t n, size_t size)；
    int NUM =10;
    int *p1 = (int *)calloc(NUM,sizeof(int));
    int i;
    for(i=0;i<NUM;i++){
        printf("%4d",p1[i]);
    }
    puts("");
    if(p1) free(p1);
    return 0;
}
//   0   0   0   0   0   0   0   0   0   0

malloc申请的内存是不会被初始化的。

getpagesize

#include <stdio.h>
#include <unistd.h>
int main(){
    printf("page size is %d byte\n",getpagesize());
    return 0;
}
/*
$ ./pagesize
page size is 4096 byte
$ uname -a
Linux ubuntu 4.2.0-42-generic #49-Ubuntu SMP Tue Jun 28 21:26:26 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
*/

mmap, munmap

#include <stdio.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <errno.h>

int main(){
    int fd=open("/etc/passwd",O_RDONLY);
    if(fd == -1){
        perror("open ");
        exit(1);
    }
    struct stat info;
    fstat(fd,&info);
    /* start set NULL, means system set it. */
    /* PROT_READ means memory can be read. */
    /* MAP_PRIVATE means create a copy file, not influce origin file. */
    void *start=mmap(NULL,info.st_size,PROT_READ,MAP_PRIVATE,fd,0);
    if(start == MAP_FAILED) {
        perror("mmap ");
        exit(1);
    }
    printf("%s",(char *)start);
    munmap(start,info.st_size);
    close(fd);
    return 0;
}