mpool(3)

💖 mpool(3) — shared memory buffer pool 💖

okay bestie, gather round, because we're about to talk about mpool — the shared memory buffer pool situation from section 3 of your Library Functions Manual. she's giving page-oriented buffer management, she's giving process sharing, she's lowkey an icon of the 4.4 Berkeley Distribution era. let's get into it 🫶

---

📚 LIBRARY

Standard C library (libc, -lc) — that's the vibe, that's the home base.

---

✨ SYNOPSIS ✨

#include <db.h>
#include <mpool.h>

MPOOL *mpool_open(DBT *key, int fd, pgno_t pagesize, pgno_t maxcache);

void mpool_filter(MPOOL *mp,
                  typeof(void (void *, pgno_t, void *)) *pgin,
                  typeof(void (void *, pgno_t, void *)) *pgout,
                  void *pgcookie);

void *mpool_new(MPOOL *mp, pgno_t *pgnoaddr);
void *mpool_get(MPOOL *mp, pgno_t pgno, unsigned int flags);
int mpool_put(MPOOL *mp, void *pgaddr, unsigned int flags);

int mpool_sync(MPOOL *mp);
int mpool_close(MPOOL *mp);

---

🦋 DESCRIPTION 🦋

Note well, bestie: this page is documenting interfaces that were provided up until glibc 2.1. Since glibc 2.2, glibc no longer provides these interfaces. So probably you're actually looking for the APIs provided by the libdb library instead. not me delivering ancient lore fr 😭

Mpool is the library interface intended to provide page oriented buffer management of files. The buffers may be shared between processes. the way that's literally her whole personality 💅

The function mpool_open() initializes a memory pool. The key argument is the byte string used to negotiate between multiple processes wishing to share buffers. If the file buffers are mapped in shared memory, all processes using the same key will share the buffers. If key is NULL, the buffers are mapped into private memory. The fd argument is a file descriptor for the underlying file, which must be seekable. If key is non-NULL and matches a file already being mapped, the fd argument is ignored.

The pagesize argument is the size, in bytes, of the pages into which the file is broken up. The maxcache argument is the maximum number of pages from the underlying file to cache at any one time. This value is not relative to the number of processes which share a file's buffers, but will be the largest value specified by any of the processes sharing the file. okay so she takes the max, that's just the rules 🎀

The mpool_filter() function is intended to make transparent input and output processing of the pages possible. If the pgin function is specified, it is called each time a buffer is read into the memory pool from the backing file. If the pgout function is specified, it is called each time a buffer is written into the backing file. Both functions are called with the pgcookie pointer, the page number and a pointer to the page to being read or written. (yes, "to being read or written" — that's the original's exact wording and I refuse to touch it, it's iconic 🙈)

The function mpool_new() takes an MPOOL pointer and an address as arguments. If a new page can be allocated, a pointer to the page is returned and the page number is stored into the pgnoaddr address. Otherwise, NULL is returned and errno is set.

The function mpool_get() takes an MPOOL pointer and a page number as arguments. If the page exists, a pointer to the page is returned. Otherwise, NULL is returned and errno is set. The flags argument is not currently used. (she's there for decoration, bestie, she serves no purpose, we respect her anyway ✨)

The function mpool_put() unpins the page referenced by pgaddr. pgaddr must be an address previously returned by mpool_get() or mpool_new(). The flag value is specified by ORing any of the following values:

• MPOOL_DIRTY — the page has been modified and needs to be written to the backing file.

mpool_put() returns 0 on success and -1 if an error occurs.

The function mpool_sync() writes all modified pages associated with the MPOOL pointer to the backing file. mpool_sync() returns 0 on success and -1 if an error occurs.

The mpool_close() function free's up any allocated memory associated with the memory pool cookie. Modified pages are not written to the backing file. 🚨 (that's the "free's" with an apostrophe, verbatim, I am simply the messenger 🫡) mpool_close() returns 0 on success and -1 if an error occurs.

---

💀 ERRORS 💀

The mpool_open() function may fail and set errno for any of the errors specified for the library routine malloc(3).

The mpool_get() function may fail and set errno for the following:

• EINVAL — the requested record doesn't exist.

The mpool_new() and mpool_get() functions may fail and set errno for any of the errors specified for the library routines read(2), write(2), and malloc(3).

The mpool_sync() function may fail and set errno for any of the errors specified for the library routine write(2).

The mpool_close() function may fail and set errno for any of the errors specified for the library routine free(3).

---

👑 STANDARDS

BSD. periodt.

---

🌈 SEE ALSO 🌈

btree(3), dbopen(3), hash(3), recno(3) — go read the whole friend group, they're all slaying in their own lane 💜

---

💖 and that's the whole page, bestie! this one came to you straight from the 4.4 Berkeley Distribution, dated 2025-05-17, and it's an mpool(3) moment forever. go forth and share those buffers responsibly. no cap. 🧚✨