How should I store a time_t timestamp to a file using C?

Question

For a small todo program that I am writing, I have timestamps that are of this form

time_t t = time(NULL);

and are saved every time a task is entered to denote the time that it was entered.

I want to store the tasks to a plain text file, so that the state can be saved and restored. How should I store the timestamps to the text file and how should I get them back in my program after reading the text file?

Answer 1

Convert the time_t to struct tm using gmtime(), then convert the struct tm to plain text (preferably ISO 8601 format) using strftime(). The result will be portable, human readable, and machine readable.

To get back to the time_t, you just parse the string back into a struct tm and use mktime().

For reference:

• gmtime
• strftime
• strptime
• mktime

Code sample:

// Converting from time_t to string
time_t t = time(NULL);
struct tm *ptm = gmtime(&t);
char buf[256];
strftime(buf, sizeof buf, "%F %T", ptm);
// buf is now "2015-05-15 22:55:13"

// Converting from string to time_t
char *buf = "2015-05-15 22:55:13";
struct tm tm;
strptime(buf, "%F %T", &tm);
time_t t = mktime(&tm);