I tried to illustrate the time and time in C language

The C language has a time function that I personally find awkward. Even if it is difficult to handle, I will obey quietly because I do not have the skill to make more things, but I will write the connection because it was hard to remember. ..

Maybe we will increase the Windows edition, clock, timer, etc.?

Look at Linux here

tl;dr(too long didn't read) time_correlation_diagram.png

C language standard: time_t type

A type that indicates the number of seconds that have elapsed since 0:00:00 (UTC) on January 1, 1970. In other words, I don't know the date or time as it is </ font>.

Structure definition


#include <time.h>
time_t time(time_t *t);

C language standard time acquisition function. There are two ways to use it.

time()Notation


time_t current_time;
time(&current_time); //If you give a buffer, it will be put in there
or
time_t current_time = time(NULL); //Return value is OK

C language standard: structure struct tm type

Use when you want more detailed date information. It was a big success when I put the date on the log. You can also convert struct timespec and struct tm within the Linux Kernel. ..

Definition


struct tm {
    int tm_sec;        /*Seconds(0-60) */
    int tm_min;        /*Minutes(0-59) */
    int tm_hour;       /*time(0-23) */
    int tm_mday;       /*Date in the month(1-31) */
    int tm_mon;        /*Month(0-11) */
    int tm_year;       /*Year- 1900 */
    int tm_wday;       /*Day of the week(0-6,Sunday= 0) */
    int tm_yday;       /*Total date within the year(0-365,January 1= 0) */
    int tm_isdst;      /*Daylight saving time*/
};

time_t type → character string type

It is used when you want to express the time in characters.

Times of Day->Character notation


#include <time.h>
char *ctime(const time_t *timep);  // Thread unsafe
char *ctime_r(const time_t *timep, char *buf); // Thread safe

-** C language standard : ctime () Since it returns a pointer to the internal memory, it has a thread unsafe problem that the recipient changes when the internal memory is rewritten. - POSIX only **: ctime_r () Since the output buffer is requested from the user side, the value is guaranteed even if it is rewritten internally.

time_t type → struct tm type

It's a hassle to convert from the time_t type, and am I the only one who thinks it's okay to have a function that returns the struct tm of the current time? I'm sure I just don't know.

Conversion function


#include <time.h>
// GMT
struct tm *gmtime(const time_t *timep); // Thread unsafe
struct tm *gmtime_r(const time_t *timep, struct tm *result); // Thread safe
//Local time
struct tm *localtime(const time_t *timep); // Thread unsafe
struct tm *localtime_r(const time_t *timep, struct tm *result);  // Thread safe

struct tm type → string type

Conversion function


#include <stdio.h>
size_t strftime(char *s, size_t max, const char *format, const struct tm *tm);

If you want to output it, you may put it in the printf function separately.

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