how to get last modification time of a file with epoch time format, precision millisecond

linux

Solution

If you're on a filesystem that supports sub-second precision (for example ext4 supports it, ext3 does not), then you can get nanosecond precision through the `st_atim.tv_nsec`, `st_ctim.tv_nsec` and `st_mtim.tv_nsec` fields of `struct stat`. The values represent the amount of nanoseconds in addition to the amount of seconds in `st_atime` and friends. For example, you can get millisecond timestamps with:

struct stat info;
stat("/etc", &info);
uint64_t access_ms       = info.st_atime * 1000 + info.st_atim.tv_nsec / 1000000;
uint64_t status_ms       = info.st_ctime * 1000 + info.st_ctim.tv_nsec / 1000000;
uint64_t modification_ms = info.st_mtime * 1000 + info.st_mtim.tv_nsec / 1000000;

A second has 1000ms, so you multiply by that. A millisecond has 1000000ns, so you divide by that.

If the filesystem does not support sub-second precision, then the `tv_nsec` fields will always be 0. This ensures that the same code will also work on filesystems that don't support sub-second precision.

Note that the amount of milliseconds since epoch can't be stored in a 32-bit value. You need at least 64 bits. `uint64_t` is provided by `<stdint.h>`. Alternatively, you can use `unsigned long long`.

Problem

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