The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


ctime, ctime_r - convert a time value to a date and time string


[OB] [Option Start] #include <time.h>

char *ctime(const time_t *
[Option End]
[OB CX] [Option Start] char *ctime_r(const time_t *clock, char *buf); [Option End]


For ctime(): [CX] [Option Start]  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. [Option End]

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:


[CX] [Option Start] 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. [Option End]


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

[CX] [Option Start] 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. [Option End]


No errors are defined.

The following sections are informative.




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.


These functions may be removed in a future version.


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

XBD <time.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

Normative text previously in the APPLICATION USAGE section is moved to the DESCRIPTION.

The ctime_r() function is included for alignment with the POSIX Threads Extension.

A note indicating that the ctime() function need not be reentrant is added to the DESCRIPTION.

Issue 6

Extensions beyond the ISO C standard are marked.

In the DESCRIPTION, the note about reentrancy is expanded to cover thread-safety.

The APPLICATION USAGE section is updated to include a note on the thread-safe function and its avoidance of possibly using a static data area.

Issue 7

Austin Group Interpretation 1003.1-2001 #156 is applied.

SD5-XSH-ERN-25 is applied, updating the APPLICATION USAGE.

Austin Group Interpretation 1003.1-2001 #053 is applied, marking these functions obsolescent.

The ctime_r() function is moved from the Thread-Safe Functions option to the Base.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0066 [321,428] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0075 [664] is applied.

End of informative text.


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