The gmtime()
function in C is a standard library function that converts a time_t
value to a tm
structure representing Coordinated Universal Time (UTC). It is part of the C standard library (time.h
). This function is useful for converting calendar time to a broken-down time representation in UTC.
Table of Contents
- Introduction
gmtime()
Function Syntax- Examples
- Converting
time_t
totm
as UTC - Using
gmtime()
with Current Time
- Converting
- Real-World Use Case
- Conclusion
Introduction
The gmtime()
function converts a time_t
value, representing calendar time, to a tm
structure that represents the corresponding UTC time. This is useful for applications that need to work with time in a standardized, time zone-independent format.
gmtime() Function Syntax
The syntax for the gmtime()
function is as follows:
#include <time.h>
struct tm *gmtime(const time_t *timer);
Parameters:
timer
: A pointer to atime_t
value that represents the time to be converted.
Returns:
- The function returns a pointer to a statically allocated
tm
structure representing the UTC time. If an error occurs, it returnsNULL
.
Examples
Converting time_t
to tm
as UTC
To demonstrate how to use gmtime()
to convert a time_t
value to a tm
structure in UTC, we will write a simple program that retrieves the current time and converts it to UTC.
Example
#include <stdio.h>
#include <time.h>
int main() {
time_t current_time;
struct tm *utc_time;
// Get the current time
current_time = time(NULL);
// Convert the time to UTC
utc_time = gmtime(¤t_time);
// Check if the conversion was successful
if (utc_time != NULL) {
// Print the result
printf("UTC time: %d-%02d-%02d %02d:%02d:%02d\n",
utc_time->tm_year + 1900, utc_time->tm_mon + 1, utc_time->tm_mday,
utc_time->tm_hour, utc_time->tm_min, utc_time->tm_sec);
} else {
printf("Failed to convert time to UTC.\n");
}
return 0;
}
Output:
UTC time: 2023-07-04 12:34:56
Using gmtime()
with User Input
This example shows how to use gmtime()
to convert a user-provided time_t
value to a tm
structure in UTC.
Example
#include <stdio.h>
#include <time.h>
int main() {
struct tm time_struct = {0};
time_t time_value;
struct tm *utc_time;
// Get user input for the date and time
printf("Enter year: ");
scanf("%d", &time_struct.tm_year);
time_struct.tm_year -= 1900; // Adjust year
printf("Enter month (1-12): ");
scanf("%d", &time_struct.tm_mon);
time_struct.tm_mon -= 1; // Adjust month
printf("Enter day of the month (1-31): ");
scanf("%d", &time_struct.tm_mday);
printf("Enter hour (0-23): ");
scanf("%d", &time_struct.tm_hour);
printf("Enter minute (0-59): ");
scanf("%d", &time_struct.tm_min);
printf("Enter second (0-59): ");
scanf("%d", &time_struct.tm_sec);
// Convert tm structure to time_t
time_value = mktime(&time_struct);
// Convert the time to UTC
utc_time = gmtime(&time_value);
// Print the result
if (utc_time != NULL) {
printf("UTC time: %d-%02d-%02d %02d:%02d:%02d\n",
utc_time->tm_year + 1900, utc_time->tm_mon + 1, utc_time->tm_mday,
utc_time->tm_hour, utc_time->tm_min, utc_time->tm_sec);
} else {
printf("Failed to convert time to UTC.\n");
}
return 0;
}
Output (example user input year "2023", month "7", day "4", hour "12", minute "30", second "0"):
Enter year: 2023
Enter month (1-12): 7
Enter day of the month (1-31): 4
Enter hour (0-23): 12
Enter minute (0-59): 30
Enter second (0-59): 0
UTC time: 2023-07-04 12:30:00
Real-World Use Case
Converting Local Time to UTC for Standardized Storage
In real-world applications, the gmtime()
function can be used to convert local times to UTC for standardized storage in databases or logs, ensuring consistency across different time zones.
Example: Storing Timestamps in UTC
#include <stdio.h>
#include <time.h>
// Function to get user input for date and time
void get_user_input(struct tm *time_struct) {
printf("Enter year: ");
scanf("%d", &time_struct->tm_year);
time_struct->tm_year -= 1900; // Adjust year
printf("Enter month (1-12): ");
scanf("%d", &time_struct->tm_mon);
time_struct->tm_mon -= 1; // Adjust month
printf("Enter day of the month (1-31): ");
scanf("%d", &time_struct->tm_mday);
printf("Enter hour (0-23): ");
scanf("%d", &time_struct->tm_hour);
printf("Enter minute (0-59): ");
scanf("%d", &time_struct->tm_min);
printf("Enter second (0-59): ");
scanf("%d", &time_struct->tm_sec);
}
int main() {
struct tm local_time = {0};
time_t time_value;
struct tm *utc_time;
// Get user input for the local date and time
get_user_input(&local_time);
// Convert tm structure to time_t
time_value = mktime(&local_time);
// Convert the time to UTC
utc_time = gmtime(&time_value);
// Print the result
if (utc_time != NULL) {
printf("UTC time: %d-%02d-%02d %02d:%02d:%02d\n",
utc_time->tm_year + 1900, utc_time->tm_mon + 1, utc_time->tm_mday,
utc_time->tm_hour, utc_time->tm_min, utc_time->tm_sec);
} else {
printf("Failed to convert time to UTC.\n");
}
return 0;
}
Output (example user input year "2023", month "7", day "4", hour "12", minute "30", second "0"):
Enter year: 2023
Enter month (1-12): 7
Enter day of the month (1-31): 4
Enter hour (0-23): 12
Enter minute (0-59): 30
Enter second (0-59): 0
UTC time: 2023-07-04 12:30:00
Conclusion
The gmtime()
function is essential for converting a time_t
value to a tm
structure representing UTC time in C. It is useful in various applications, particularly in date and time manipulation, where it is necessary to convert calendar time to a broken-down time representation in UTC for calculations, storage, and display.
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