Gucs Config

gucs-config — custom GUC variables

This is the procedural cookbook for adding custom GUC variables to a PostgreSQL backend patch or extension. For the conceptual model see knowledge/idioms/guc-variables.md.

This skill is one of three siblings that share the _PG_init / postmaster-lifecycle boundary:

• gucs-config (this skill) — custom GUC variables.
• bgworker-and-extensions — RegisterBackgroundWorker, shared-library hooks.
• parallel-query — ParallelContext + parallel-safe markings.

1. Picking the right Define*Variable

Five typed entry points, all in utils/guc.h. [verified-by-code source/src/include/utils/guc.h:358-416]

All five take the same trailing trio of hooks: check_hook, assign_hook, show_hook (any can be NULL).

2. GucContext — when the value can change

[verified-by-code source/src/include/utils/guc.h:71-80]

Rule of thumb: pick the loosest context that's still safe. If changing the value mid-session would require restarting a worker or reallocating shared memory, use PGC_POSTMASTER. If it just changes a runtime threshold, use PGC_SIGHUP or PGC_SUSET.

3. Skeleton — _PG_init for an extension with GUCs

void
_PG_init(void)
{
    DefineCustomIntVariable("my_ext.naptime",
                            "Seconds between scans.",
                            NULL,           /* long_desc */
                            &my_naptime,    /* int *valueAddr */
                            10,             /* boot_val */
                            1, INT_MAX,     /* min, max */
                            PGC_SIGHUP,
                            GUC_UNIT_S,     /* flags — UNIT_S = seconds */
                            NULL, NULL, NULL); /* check/assign/show */

    DefineCustomStringVariable("my_ext.database",
                               "DB to connect to.",
                               NULL,
                               &my_database,
                               "postgres",
                               PGC_POSTMASTER,
                               0,
                               NULL, NULL, NULL);

    /* MUST be called AFTER all DefineCustom* calls. */
    MarkGUCPrefixReserved("my_ext");
}

[verified-by-code source/src/test/modules/worker_spi/worker_spi.c:303-360]

4. MarkGUCPrefixReserved

Call exactly once per extension, after every DefineCustom*Variable for your prefix. It does two things:

1. Removes any placeholder GUCs (GUC_CUSTOM_PLACEHOLDER) under that prefix — these get created when a config file mentions a custom variable before the defining extension loads. Without removal, parallel-worker startup later trips over them.
2. Adds the prefix to a list so future placeholders under it are refused — typos in postgresql.conf (my_ext.napitme) now produce a clear error instead of silently being accepted.

[verified-by-code source/src/backend/utils/misc/guc.c:5185-5228]

The old name EmitWarningsOnPlaceholders is still a #define alias — prefer the new name in new code. [verified-by-code source/src/include/utils/guc.h:421]

5. String GUCs and guc_malloc

For string GUCs, valueAddr is a char **. The pointed-to storage is owned by guc.c and must be allocated with guc_malloc / guc_strdup — never palloc. If a check_hook wants to replace the proposed value it must guc_malloc the new value and guc_free the old one. Use guc_realloc to grow an existing guc_malloc allocation (typical for a derived extra payload). All four functions live in source/src/include/utils/guc.h:473-476. [from-README source/src/backend/utils/misc/README:50-62]

6. The hook trio

check_hook(newval *, void **extra, GucSource source) → bool

Signature varies by GUC type — the first argument's exact pointer type matches the variable's storage:

[from-README source/src/backend/utils/misc/README:25-30]

• Validate the proposed value; return false on reject.
• May modify *newval to canonicalize (round, lowercase, ...).
• May allocate an extra struct with guc_malloc and return it through *extra to pass derived data to the assign hook.
• For error detail: GUC_check_errdetail(...), GUC_check_errhint(...), GUC_check_errcode(...), GUC_check_errmsg(...) — never ereport(ERROR) directly except on OOM. GUC_check_errcode(sqlerrcode) overrides the default rejection SQLSTATE (ERRCODE_INVALID_PARAMETER_VALUE, 22023); only override when you have a more specific code.
• May run outside any transaction (bootstrap, postmaster startup, config reload). Guard catalog lookups with IsTransactionState().
• May also be called just to validate without assigning — must not have side effects.
• For GUC_LIST_INPUT GUCs (see §7), parse *newval inside the check_hook with SplitIdentifierString(rawstring, ',', &elemlist) (identifier-style lists) or SplitGUCList(rawstring, ',', &elemlist) (general case). Both live in source/src/include/utils/varlena.h:33-37; they pstrdup elements and pfree/list_free is the caller's job. Don't roll your own splitter — these match how the parser handles search_path, including quoted elements.

[from-README source/src/backend/utils/misc/README:25-75]

assign_hook(newval, void *extra) → void

• Cannot fail (no return). Do all fallible work in the check hook.
• May be called during transaction rollback → no catalog lookups.

[from-README source/src/backend/utils/misc/README:78-109]

show_hook(void) → const char *

• Customise what SHOW displays. Static buffer is fine; not reentrant.

[from-README source/src/backend/utils/misc/README:112-117]

7. Useful flags bits

[verified-by-code source/src/include/utils/guc.h:214-242]

8. GUC state under workers (cross-cutting)

• Parallel workers — GUC state is serialized by the leader and restored in each worker by ParallelWorkerMain before your code runs. Workers see exactly the leader's GUC values at launch time. If a PGC_USERSET GUC changes mid-flight, in-flight workers do not see the change. See parallel-query for the launch mechanics.

• Background workers (non-parallel) — GUC state is whatever postgresql.conf says at start. To honour SIGHUP, install SignalHandlerForConfigReload and call ProcessConfigFile(PGC_SIGHUP) when ConfigReloadPending is set. See bgworker-and-extensions for the signal-handling skeleton.

9. Checklist

• _PG_init defines every custom GUC.
• MarkGUCPrefixReserved(prefix) is called after all definitions.
• String storage uses guc_malloc / guc_strdup, never palloc.
• check_hook is side-effect-free and uses GUC_check_errdetail (not ereport(ERROR)) for validation failures.
• check_hook guards catalog lookups with IsTransactionState().
• Right GucContext chosen (don't use PGC_USERSET if the change would require restarting workers).
• Unit flag set on int GUCs that represent size or time.
• List GUCs have GUC_LIST_INPUT, and GUC_LIST_QUOTE if elements are identifiers.

10. Useful greps

• All custom GUC definitions: grep -RIn 'DefineCustom\(Bool\|Int\|Real\|String\|Enum\)Variable' source/src
• All check / assign hooks in tree: grep -RIn 'check_hook\|assign_hook' source/src/include/utils/guc.h

Cross-references

• .claude/skills/bgworker-and-extensions/SKILL.md — _PG_init flow for the surrounding extension; SIGHUP signal handler for picking up config reloads.
• .claude/skills/parallel-query/SKILL.md — how GUCs get restored in parallel workers; GUC_ALLOW_IN_PARALLEL flag.
• .claude/skills/extension-development/SKILL.md — _PG_init, .control file, shared_preload_libraries.
• .claude/skills/coding-style/SKILL.md — backend C style for the hook functions.
• .claude/skills/error-handling/SKILL.md — GUC_check_errdetail vs ereport(ERROR); OOM contract.
• knowledge/idioms/guc-variables.md — conceptual model.
• knowledge/files/src/include/utils/guc.h.md — per-file doc for the public API surface.
• knowledge/files/src/backend/utils/misc/guc.c.md — per-file doc for the implementation.