The Linux Programming Interface

188 Chapter 10

Figure 10-1: Functions for retrieving and working with calendar time

10.2.1 Converting time_t to Printable Form

The ctime() function provides a simple method of converting a time_t value into

printable form.

Given a pointer to a time_t value in timep, ctime() returns a 26-byte string containing

the date and time in a standard format, as illustrated by the following example:

Wed Jun 8 14:22:34 2011

The string includes a terminating newline character and a terminating null byte. The

ctime() function automatically accounts for local timezone and DST settings when

performing the conversion. (We explain how these settings are determined in

Section 10.3.) The returned string is statically allocated; future calls to ctime() will

overwrite it.

SUSv3 states that calls to any of the functions ctime(), gmtime(), localtime(), or

asctime() may overwrite the statically allocated structure that is returned by any of

the other functions. In other words, these functions may share single copies of the

returned character array and tm structure, and this is done in some versions of

glibc. If we need to maintain the returned information across multiple calls to these

functions, we must save local copies.

asctime()

strftime()*+

strptime()+

gmtime()

localtime()*

mktime()*

time() stime() gettimeofday() settimeofday()

ctime()*

Functions affected as marked:

* by TZ environment variable

+ by locale setting

time_t

(calendar time)

struct tm

(broken-down time)

struct timeval

fixed-format string

Tue Feb 1 21:39:46 2011\n\0

user-formatted,

localized string

Kernel

#include <time.h>

char *ctime(const time_t *timep);

Returns pointer to statically allocated string terminated

by newline and \0 on success, or NULL on error