ANSI C (3) —— 常用系统函数

ANSI C (3) —— 常用系统函数

字符测试函数

简单例子：

#include #include #include 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 #include #include 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;}/*./timespent 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 #include #include #include 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 ​​​ 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 #include #include int main(){ time_t t; time(&t); printf("%d\n",t); /* 32 bit integer */ printf("%s\n",ctime(&t)); return 0;}/*./time31477848041Mon 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 #include #include 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 TDA[time_t]-->B[struct tm]B-->|asctime|C[GMT time string]A-->|ctime|C

#include #include #include 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 2016GMT time is Mon Oct 31 21:20:44 2016*/

localtime, mktime

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

环境控制函数

getenv

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

putenv

#include #include 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 #include 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 #include #include 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

malloc申请的内存是不会被初始化的。

getpagesize

#include #include int main(){ printf("page size is %d byte\n",getpagesize()); return 0;}/*$ ./pagesizepage size is 4096 byte$ uname -aLinux 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 #include #include #include #include #include #include #include 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;}

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