Your company here — click to reach over 10,000 unique daily visitors

ctime - Man Page

convert a time value to a date and time string


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <time.h>

char *ctime(const time_t *clock);
char *ctime_r(const time_t *clock, char *buf);


For ctime(): The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

The ctime() function shall convert the time pointed to by clock, representing time in seconds since the Epoch, to local time in the form of a string. It shall be equivalent to:


The asctime(), ctime(), gmtime(), and localtime() functions shall return values in one of two static objects: a broken-down time structure and an array of char. Execution of any of the functions may overwrite the information returned in either of these objects by any of the other functions.

The ctime() function need not be thread-safe.

The ctime_r() function shall convert the calendar time pointed to by clock to local time in exactly the same form as ctime() and put the string into the array pointed to by buf (which shall be at least 26 bytes in size) and return buf.

Unlike ctime(), the ctime_r() function is not required to set tzname. If ctime_r() sets tzname, it shall also set daylight and timezone. If ctime_r() does not set tzname, it shall not set daylight and shall not set timezone.

Return Value

The ctime() function shall return the pointer returned by asctime() with that broken-down time as an argument.

Upon successful completion, ctime_r() shall return a pointer to the string pointed to by buf. When an error is encountered, a null pointer shall be returned.


No errors are defined.

The following sections are informative.



Application Usage

These functions are included only for compatibility with older implementations. They have undefined behavior if the resulting string would be too long, so the use of these functions should be discouraged. On implementations that do not detect output string length overflow, it is possible to overflow the output buffers in such a way as to cause applications to fail, or possible system security violations. Also, these functions do not support localized date and time formats. To avoid these problems, applications should use strftime() to generate strings from broken-down times.

Values for the broken-down time structure can be obtained by calling gmtime() or localtime().

The ctime_r() function is thread-safe and shall return values in a user-supplied buffer instead of possibly using a static data area that may be overwritten by each call.

Attempts to use ctime() or ctime_r() for times before the Epoch or for times beyond the year 9999 produce undefined results. Refer to asctime().


The standard developers decided to mark the ctime() and ctime_r() functions obsolescent even though they are in the ISO C standard due to the possibility of buffer overflow. The ISO C standard also provides the strftime() function which can be used to avoid these problems.

Future Directions

These functions may be removed in a future version.

See Also

asctime(), clock(), difftime(), gmtime(), localtime(), mktime(), strftime(), strptime(), time(), utime()

The Base Definitions volume of POSIX.1-2017, <time.h>

Referenced By

asctime(3p), clock(3p), clock_getres(3p), difftime(3p), getdate(3p), gettimeofday(3p), gmtime(3p), localtime(3p), mktime(3p), strftime(3p), time(3p), time.h(0p), tzset(3p).

2017 IEEE/The Open Group POSIX Programmer's Manual