ustat(2)

💖 ustat(2) — get filesystem statistics 💖

okay bestie, so this is ustat(2), a real one from the System Calls Manual, and we're gonna talk about it. fair warning: she's deprecated. she's giving legacy. but we still gotta know her, iykyk. 🦋

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✨ LIBRARY ✨

Standard C library (libc, -lc). the classic. 💅

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🌸 SYNOPSIS 🌸

#include <sys/types.h>
#include <unistd.h>    /* libc[45] */
#include <ustat.h>     /* glibc2 */

[[deprecated]] int ustat(dev_t dev, struct ustat *ubuf);

okay notice the [[deprecated]] right there in the signature?? the compiler is literally side-eyeing you. 🙈

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🎀 DESCRIPTION 🎀

so ustat() hands you info about a mounted filesystem. slay, but old.

• dev is a device number that identifies a device containing a mounted filesystem (like, the device holding the fs, not just any rando device).
• ubuf is a pointer to a ustat structure, and that struct has these members:

daddr_t f_tfree;      /* Total free blocks */
ino_t   f_tinode;     /* Number of free inodes */
char    f_fname[6];   /* Filsys name */
char    f_fpack[6];   /* Filsys pack name */

and here's the tea about the last two fields, f_fname and f_fpack: they are not implemented, like at all. they will always be filled with null bytes ('\0'). so if you were hoping for a cute filesystem name in there... no babe. it's null bytes. it's giving empty. 💀

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💜 RETURN VALUE 💜

• on success, zero is returned, and the ustat structure that ubuf points to gets filled in. periodt.
• on error, -1 is returned, and errno is set to tell you what went wrong.

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🔥 ERRORS 🔥

• EFAULT — ubuf points outside of your accessible address space. bestie that pointer is lost. 😭
• EINVAL — dev does not refer to a device containing a mounted filesystem.
• ENOSYS — the mounted filesystem referenced by dev does not support this operation, or any version of Linux before Linux 1.3.16. yeah, 1.3.16. ancient history. 👑

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🌈 STANDARDS 🌈

None. that's it. no standard claims her. she's free-range. 🧚

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👑 HISTORY 👑

SVr4. and then... Removed in glibc 2.28. gone. she left the group chat. 💔

ustat() is deprecated and has been provided only for compatibility. so real talk: all new programs should use statfs(2) instead. no cap, not a drill, use statfs(2). 🙏

🫶 HP-UX notes 🫶

okay so the HP-UX version of the ustat structure has an additional field, f_blksize, that is unknown elsewhere. HP-UX really said "i'll add my own field, it's my world". 💅

and HP-UX warns: for some filesystems, the number of free inodes does not change. such filesystems will return -1 in the field f_tinode. also, for some filesystems, inodes are dynamically allocated — and such filesystems will return the current number of free inodes. so like, f_tinode is lowkey unpredictable depending on the fs. keep that in mind bestie. ⭐

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⭐ SEE ALSO ⭐

• stat(2)
• statfs(2) ← the main character, fr fr. go use this one.

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okay that's the whole page, you ate it, you understood it. 🎀 and for the credits moment: this one came from Linux man-pages 6.20, dated 2026-02-08, so shoutout to the man-pages girlies keeping the lore alive. bye bestie, go call statfs(2) instead. 🦋✨💖