
Introduction
The gmtime()
function in C++ is a part of the C standard library, which manipulates and formats date and time based on Coordinated Universal Time (UTC). This function converts the local time, provided as a timestamp in the form of a time_t
value, into UTC format, represented as a tm
structure. Understanding how to use this function is essential for applications requiring time coordination across different geographical regions.
In this article, you will learn how to use the gmtime()
function in various C++ programming contexts. Discover the steps for converting local time to UTC and handling the returned structure to extract and format time elements such as year, month, day, and time.
Using gmtime()
in C++
Convert Time to UTC
Include the necessary headers,
ctime
for time manipulations andiostream
for output operations.Obtain the current local time using
time(nullptr)
.Convert the local time to a
tm
structure representing UTC time usinggmtime()
.cpp#include <ctime> #include <iostream> int main() { // Obtain current local time time_t current_time = time(nullptr); // Convert local time to UTC tm* utc_time = gmtime(¤t_time); // Output the UTC time details std::cout << "UTC time: " << 1900 + utc_time->tm_year << "-" << 1 + utc_time->tm_mon << "-" << utc_time->tm_mday << " " << utc_time->tm_hour << ":" << utc_time->tm_min << ":" << utc_time->tm_sec << std::endl; return 0; }
Here, the program first captures the current local time.
gmtime()
then converts this to UTC, storing the result in atm
structure. The elements of this structure are adjusted to human-readable format and displayed.
Handling Potential Issues
Note that
gmtime()
returns a pointer to a statically allocatedtm
structure, which may be overwritten by subsequent calls.Ensure to use this pointer or copy its contents before any subsequent time manipulation function calls that might alter it.
Improving Safeness with
gmtime_r()
For thread-safe implementation, consider using
gmtime_r()
available in POSIX systems, which uses a user-provided buffer for thetm
structure.cpp#include <ctime> #include <iostream> int main() { time_t current_time = time(nullptr); tm utc_time_struct; tm* utc_time = gmtime_r(¤t_time, &utc_time_struct); if (utc_time) { std::cout << "UTC time: " << 1900 + utc_time->tm_year << "-" << 1 + utc_time->tm_mon << "-" << utc_time->tm_mday << " " << utc_time->tm_hour << ":" << utc_time->tm_min << ":" << utc_time->tm_sec << std::endl; } return 0; }
This code uses
gmtime_r()
whereutc_time_struct
is used to store the time data, ensuring that the time data is preserved and not overwritten by other time functions running on different threads.
Conclusion
The gmtime()
function provides a straightforward approach for converting local time into UTC in C++. Given its ease of use and critical role in time manipulation in software, understanding how to properly utilize and handle the tm
structure is fundamental. Whether for logging, synchronization, or any other purpose requiring time stamps in universal time, gmtime()
serves as an essential tool in the C++ programmer's toolkit. Remember to consider thread safety and the static nature of the output when integrating this function into multi-threaded applications.
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