The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here

NAME

gmtime, gmtime_r - convert a time value to a broken-down UTC time

SYNOPSIS

#include <time.h>

struct tm *gmtime(const time_t *
timer);

[TSF] [Option Start] struct tm *gmtime_r(const time_t *restrict timer,
       struct tm *restrict
result); [Option End]

DESCRIPTION

For gmtime(): [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 IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

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).

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

[TSF] [Option Start] The same relationship shall apply for gmtime_r(). [Option End]

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

[TSF] [Option Start] 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. [Option End]

RETURN VALUE

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 [CX] [Option Start]  and set errno to indicate the error. [Option End]

[TSF] [Option Start] 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 and set errno to indicate the error. [Option End]

ERRORS

The gmtime() [TSF] [Option Start]  and gmtime_r() [Option End]  functions shall fail if:

[EOVERFLOW]
[CX] [Option Start] The result cannot be represented. [Option End]

The following sections are informative.

EXAMPLES

None.

APPLICATION USAGE

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.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

asctime(), clock(), ctime() , difftime(), localtime(), mktime(), strftime(), strptime(), time(), utime(), the Base Definitions volume of IEEE Std 1003.1-2001, <time.h>

CHANGE HISTORY

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

Issue 5

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

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

Issue 6

The gmtime_r() function is marked as part of the Thread-Safe Functions option.

Extensions beyond the ISO C standard are marked.

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.

The restrict keyword is added to the gmtime_r() prototype for alignment with the ISO/IEC 9899:1999 standard.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/27 is applied, adding the [EOVERFLOW] error.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/48 is applied, updating the error handling for gmtime_r().

End of informative text.

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