struct tm *gmtime(const time_t *timer);
struct tm *gmtime_r(const time_t *restrict timer,
struct tm *restrict result);
For gmtime(): 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 IEEE Std 1003.1-2001 defers to the ISO C standard.
The gmtime() function shall convert the time in seconds since the Epoch pointed to by timer into a broken-down time, expressed as Coordinated Universal Time (UTC).
The relationship between a time in seconds since the Epoch used as an argument to gmtime() and the tm structure (defined in the <time.h> header) is that the result shall be as specified in the expression given in the definition of seconds since the Epoch (see the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.14, Seconds Since the Epoch), where the names in the structure and in the expression correspond.
The same relationship shall apply for gmtime_r().
The gmtime() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.
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 type char. Execution of any of the functions may overwrite the information returned in either of these objects by any of the other functions.
The gmtime_r() function shall convert the time in seconds since the Epoch pointed to by timer into a broken-down time expressed as Coordinated Universal Time (UTC). The broken-down time is stored in the structure referred to by result. The gmtime_r() function shall also return the address of the same structure.
Upon successful completion, the gmtime() function shall return a pointer to a struct tm. If an error is detected, gmtime() shall return a null pointer and set errno to indicate the error.
Upon successful completion, gmtime_r() shall return the address of the structure pointed to by the argument result. If an error is detected, gmtime_r() shall return a null pointer.
The gmtime() function shall fail if:
The result cannot be represented.
The following sections are informative.
The gmtime_r() function is thread-safe and returns values in a user-supplied buffer instead of possibly using a static data area that may be overwritten by each call.
COPYRIGHTPortions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .