getutmp / getutmpx (3) - glibc

Split from the chapter glibc: Users and Groups into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#define _GNU_SOURCE
#include <utmpx.h>

void getutmp(const struct utmpx *ux, struct utmp *u);
void getutmpx(const struct utmp *u, struct utmpx *ux);

Both prototypes sit inside the #ifdef __USE_GNU block of /usr/include/utmpx.h (lines 86-121, getutmp at 111, getutmpx at 120). The gate at lines 42-44 forward-declares struct utmp, because plain <utmpx.h> would not otherwise make the type visible.

Semantics

getutmp copies the fields of a struct utmpx into the corresponding fields of a struct utmp; getutmpx performs the converse. Neither function touches any database file – they are pure struct converters, returning no value. On Linux the two structures have the same fields with the same sizes (manual, XPG User Accounting Database: “identical to struct utmp except that including utmpx.h does not make visible the declaration of struct exit_status”), so both functions compile to a member-wise copy; nm confirming one shared address shows they are interchangeable aliases on this platform.

Gotchas and portability

• The functions exist for systems where struct utmpx is a superset of struct utmp with larger fields (Solaris keeps parallel utmpx/wtmpx files); code that must be portable to those systems converts through these helpers instead of assigning one struct to the other.
• Because the Linux structures agree, dropping the conversion is tempting, but a plain struct assignment would break compilation on SVr4-derived systems where it is required.
• First appeared in glibc 2.1.1 (getutmp(3) VERSIONS).
• musl 1.2.5: absent – include/utmpx.h declares no getutmp/getutmpx and no sources implement them; musl’s utmp family lives in src/legacy/utmpx.c and is stubbed anyway.

References

• glibc manual: users.texi, node XPG Functions (struct equality note)
• man3: getutmp.3 (covers getutmpx)
• headers: /usr/include/utmpx.h
• siblings: utmpx page, utmp page
• symbols: nm -D --with-symbol-versions libc.so.6