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
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.
Portions 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