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USER_FROM_UID(3) Library Functions Manual USER_FROM_UID(3)

user_from_uid, uid_from_user, group_from_gid, gid_from_group
cache password and group entries

#include <pwd.h>
int
uid_from_user(const char *name, uid_t *uid);
const char *
user_from_uid(uid_t uid, int nouser);
#include <grp.h>
int
gid_from_group(const char *name, gid_t *gid);
const char *
group_from_gid(gid_t gid, int nogroup);

The user_from_uid() function returns the user name associated with the argument uid. The user name is cached so that multiple calls with the same uid do not require additional calls to getpwuid(3). If there is no user associated with the uid, a pointer is returned to a string representation of the uid, unless the argument nouser is non-zero, in which case a null pointer is returned.
The uid_from_user() function returns the user ID associated with the argument name. The user ID is cached so that multiple calls with the same name do not require additional calls to getpwnam(3). If there is no user ID associated with the name, the uid_from_user() function returns -1; otherwise it stores the user ID at the location pointed to by uid and returns 0.
The group_from_gid() function returns the group name associated with the argument gid. The group name is cached so that multiple calls with the same gid do not require additional calls to getgrgid(3). If there is no group associated with the gid, a pointer is returned to a string representation of the gid, unless the argument nogroup is non-zero, in which case a null pointer is returned.
The gid_from_group() function returns the group ID associated with the argument name. The group ID is cached so that multiple calls with the same name do not require additional calls to getgrnam(3). If there is no group ID associated with the name, the gid_from_group() function returns -1; otherwise it stores the group ID at the location pointed to by gid and returns 0.

getgrgid(3), getpwuid(3)

The user_from_uid() and group_from_gid() functions first appeared in 4.4BSD.
The uid_from_user() and gid_from_group() functions were ported from NetBSD and first appeared in OpenBSD 6.4.
September 13, 2018 OpenBSD-current