Fix remarks on `utmp` byte order and size #34

panhania · 2025-03-17T15:48:05Z

I have verified on a s390x (so, big-endian) Ubuntu 20.04 distribution (using Docker) that the byte order of utmp files is big-endian (at least of entries in /var/log/wtmp and /var/run/utmp). Thus, I think it is reasonable to conclude that these use native endianness.

I have samples of these files if needed for test_data but I am unable to figure out how tests in these repository work to write one myself.

joachimmetz · 2025-03-19T06:55:07Z

Thanks for confirming, will have a closer look when time permits

codecov · 2025-03-19T06:57:29Z

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.38%. Comparing base (d14fb6b) to head (8422bd2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #34   +/-   ##
=======================================
  Coverage   52.38%   52.38%           
=======================================
  Files          41       41           
  Lines        7991     7991           
=======================================
  Hits         4186     4186           
  Misses       3805     3805

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panhania · 2025-05-06T10:41:40Z

While working on RRG builds for aarch64, I noticed that utmp is actually 400-bytes long, not 384 like the current version of the document suggests. Thus, I reworked the PR to also cover this aspect.

Here's the program I used on a aarch64 Debian 12 to gather the data:

#include <stddef.h>
#include <stdio.h>
#include <utmp.h>

int main()
{
	printf("sizeof(struct utmp) = %zu\n", sizeof(struct utmp));

	printf("\n");

	printf("offsetof(struct utmp, ut_session) = %zu\n", offsetof(struct utmp, ut_session));
	printf("offsetof(struct utmp, ut_tv) = %zu\n", offsetof(struct utmp, ut_tv));
	printf("offsetof(struct utmp, ut_addr_v6) = %zu\n", offsetof(struct utmp, ut_addr_v6));
	printf("offsetof(struct utmp, __glibc_reserved) = %zu\n", offsetof(struct utmp, __glibc_reserved));

	printf("\n");

	printf("sizeof(struct utmp.ut_session) = %zu\n", sizeof(((struct utmp){}).ut_session));
	printf("sizeof(struct utmp.ut_tv) = %zu\n", sizeof(((struct utmp){}).ut_tv));
	printf("sizeof(struct utmp.ut_addr_v6) = %zu\n", sizeof(((struct utmp){}).ut_addr_v6));
	printf("sizeof(struct utmp.__glibc_reserved) = %zu\n", sizeof(((struct utmp){}).__glibc_reserved));

	return 0;
}

... and here is its output:

sizeof(struct utmp) = 400

offsetof(struct utmp, ut_session) = 336
offsetof(struct utmp, ut_tv) = 344
offsetof(struct utmp, ut_addr_v6) = 360
offsetof(struct utmp, __glibc_reserved) = 376

sizeof(struct utmp.ut_session) = 8
sizeof(struct utmp.ut_tv) = 16
sizeof(struct utmp.ut_addr_v6) = 16
sizeof(struct utmp.__glibc_reserved) = 20

Fix remark on utmp byte order.

8422bd2

Add information about utmp on 64-bit systems.

0840fb5

panhania changed the title ~~Fix remark on utmp byte order~~ Fix remarks on utmp byte order and size May 6, 2025

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Fix remarks on `utmp` byte order and size #34

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Fix remarks on utmp byte order and size #34

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Fix remarks on utmp byte order and size #34

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Fix remarks on `utmp` byte order and size #34

Fix remarks on `utmp` byte order and size #34