docs: add C integration guide

docs/mkdocs.yml

@@ -50,6 +50,7 @@ nav:
   - Getting Started: getting-started.md
   - SQL Integration: sql.md
   - Performance: benchmark.md
+  - C Integration: c-binding.md
   - Go Integration: go-binding.md
   - Python Integration: python-binding.md
   - Architecture: architecture.md

docs/src/c-binding.md

@@ -0,0 +1,320 @@
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# C Integration
+
+The C integration exposes Apache Paimon Rust through a C ABI. It provides
+catalog and table access, scan planning, predicate push-down, streaming reads,
+writes and commits, and vector search. Record batches cross the ABI through the
+[Arrow C Data Interface](https://arrow.apache.org/docs/format/CDataInterface.html).
+
+The C binding is currently built from source. The repository does not check in
+a generated header or publish pre-built C packages.
+
+## Prerequisites
+
+- A Rust toolchain supported by this repository
+- A C11-compatible compiler
+- [`cbindgen`](https://github.com/mozilla/cbindgen) for generating the C header
+- An Arrow implementation if the application reads or writes record batches
+
+Install `cbindgen` when it is not already available:
+
+```bash
+cargo install cbindgen --locked
+```
+
+## Building the Library and Header
+
+Run the following commands from the repository root:
+
+```bash
+cargo build --release -p paimon-c
+cbindgen bindings/c --lang c --output target/release/paimon.h
+```
+
+The build produces a dynamic library and a static library under
+`target/release/`. Dynamic library names are platform-specific:
+
+| Platform | Dynamic library |
+|----------|-----------------|
+| Linux | `libpaimon_c.so` |
+| macOS | `libpaimon_c.dylib` |
+| Windows | `paimon_c.dll` |
+
+Link the generated header and library into an application:
+
+```bash
+cc -std=c11 example.c \
+  -Itarget/release \
+  -Ltarget/release \
+  -lpaimon_c \
+  -o example
+```
+
+Make the dynamic library visible when running the executable. For example:
+
+```bash
+# Linux
+LD_LIBRARY_PATH=target/release ./example /path/to/warehouse
+
+# macOS
+DYLD_LIBRARY_PATH=target/release ./example /path/to/warehouse
+```
+
+## Opening and Scanning a Table
+
+The following program opens `default.my_table` from a filesystem catalog and
+plans its data splits. Result structs contain either the requested handle or a
+non-null error.
+
+```c
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "paimon.h"
+
+#define CHECK_RESULT(result)                                                \
+    do {                                                                    \
+        if ((result).error != NULL) {                                       \
+            fprintf(stderr, "Paimon error %d: %.*s\n",                     \
+                    (result).error->code,                                   \
+                    (int)(result).error->message.len,                       \
+                    (const char *)(result).error->message.data);            \
+            paimon_error_free((result).error);                              \
+            goto cleanup;                                                   \
+        }                                                                   \
+    } while (0)
+
+int main(int argc, char **argv) {
+    int status = EXIT_FAILURE;
+    paimon_catalog *catalog = NULL;
+    paimon_identifier *identifier = NULL;
+    paimon_table *table = NULL;
+    paimon_read_builder *read_builder = NULL;
+    paimon_table_scan *scan = NULL;
+    paimon_plan *plan = NULL;
+
+    if (argc != 2) {
+        fprintf(stderr, "usage: %s WAREHOUSE\n", argv[0]);
+        return EXIT_FAILURE;
+    }
+
+    paimon_option options[] = {
+        {.key = "warehouse", .value = argv[1]},
+    };
+    paimon_result_catalog_new catalog_result =
+        paimon_catalog_create(options, 1);
+    CHECK_RESULT(catalog_result);
+    catalog = catalog_result.catalog;
+
+    paimon_result_identifier_new identifier_result =
+        paimon_identifier_new("default", "my_table");
+    CHECK_RESULT(identifier_result);
+    identifier = identifier_result.identifier;
+
+    paimon_result_get_table table_result =
+        paimon_catalog_get_table(catalog, identifier);
+    CHECK_RESULT(table_result);
+    table = table_result.table;
+
+    paimon_result_read_builder builder_result =
+        paimon_table_new_read_builder(table);
+    CHECK_RESULT(builder_result);
+    read_builder = builder_result.read_builder;
+
+    paimon_result_table_scan scan_result =
+        paimon_read_builder_new_scan(read_builder);
+    CHECK_RESULT(scan_result);
+    scan = scan_result.scan;
+
+    paimon_result_plan plan_result = paimon_table_scan_plan(scan);
+    CHECK_RESULT(plan_result);
+    plan = plan_result.plan;
+
+    printf("planned splits: %zu\n", paimon_plan_num_splits(plan));
+    status = EXIT_SUCCESS;
+
+cleanup:
+    paimon_plan_free(plan);
+    paimon_table_scan_free(scan);
+    paimon_read_builder_free(read_builder);
+    paimon_table_free(table);
+    paimon_identifier_free(identifier);
+    paimon_catalog_free(catalog);
+    return status;
+}
+```
+
+Catalog options are the same options accepted by the Rust catalog factory. For
+example, a REST catalog can be created with:
+
+```c
+paimon_option options[] = {
+    {.key = "metastore", .value = "rest"},
+    {.key = "uri", .value = "http://localhost:8080"},
+    {.key = "warehouse", .value = "my_warehouse"},
+};
+
+paimon_result_catalog_new result = paimon_catalog_create(options, 3);
+```
+
+## Reading Arrow Record Batches
+
+Paimon uses a **scan-then-read** flow. A scan creates a plan, and a table read
+consumes a range of that plan's splits through a streaming Arrow reader:
+
+```c
+paimon_result_new_read read_result =
+    paimon_read_builder_new_read(read_builder);
+CHECK_RESULT(read_result);
+paimon_table_read *read = read_result.read;
+
+size_t split_count = paimon_plan_num_splits(plan);
+paimon_result_record_batch_reader reader_result =
+    paimon_table_read_to_arrow(read, plan, 0, split_count);
+CHECK_RESULT(reader_result);
+paimon_record_batch_reader *reader = reader_result.reader;
+
+for (;;) {
+    paimon_result_next_batch next = paimon_record_batch_reader_next(reader);
+    CHECK_RESULT(next);
+
+    if (next.batch.array == NULL && next.batch.schema == NULL) {
+        break; /* End of stream. */
+    }
+
+    /* Import next.batch.array and next.batch.schema with an Arrow C Data
+       Interface consumer before freeing their container structs. */
+    paimon_arrow_batch_free(next.batch);
+}
+
+paimon_record_batch_reader_free(reader);
+paimon_table_read_free(read);
+```
+
+`paimon_table_read_to_arrow` accepts an `offset` and `length`, so separate
+workers can process disjoint contiguous ranges of the same plan. The requested
+range is clamped to the number of available splits.
+
+!!! note "Arrow ownership"
+    After importing a returned batch with the Arrow C Data Interface, call
+    `paimon_arrow_batch_free` to release the heap-allocated `ArrowArray` and
+    `ArrowSchema` container structs. When writing, the ownership direction is
+    reversed: `paimon_table_write_write_arrow_batch` consumes the exported
+    Arrow structures, so the caller must not release them again.
+
+## Projection and Predicates
+
+Projection uses a null-terminated array of column names:
+
+```c
+const char *columns[] = {"id", "name", NULL};
+paimon_error *error =
+    paimon_read_builder_with_projection(read_builder, columns);
+if (error != NULL) {
+    /* Inspect error->code and error->message, then free the error. */
+    paimon_error_free(error);
+}
+```
+
+Predicate literals are passed as a tagged `paimon_datum`. This example builds
+`id = 42` for an `INT` column and transfers the predicate to the read builder:
+
+```c
+paimon_datum value = {0};
+value.tag = 3;       /* INT */
+value.int_val = 42;
+
+paimon_result_predicate predicate_result =
+    paimon_predicate_equal(table, "id", value);
+CHECK_RESULT(predicate_result);
+
+paimon_error *error = paimon_read_builder_with_filter(
+    read_builder, predicate_result.predicate);
+if (error != NULL) {
+    paimon_error_free(error);
+}
+```
+
+The supported datum tags are:
+
+| Tag | Paimon type | Value fields |
+|-----|-------------|--------------|
+| 0 | `BOOL` | `int_val` (`0` or non-zero) |
+| 1–4 | `TINYINT`, `SMALLINT`, `INT`, `BIGINT` | `int_val` |
+| 5–6 | `FLOAT`, `DOUBLE` | `double_val` |
+| 7 | `STRING` | `str_data`, `str_len` |
+| 8–9 | `DATE`, `TIME` | `int_val` |
+| 10–11 | `TIMESTAMP`, `TIMESTAMP WITH LOCAL TIME ZONE` | `int_val`, `int_val2` |
+| 12 | `DECIMAL` | `int_val`, `int_val2`, `uint_val`, `uint_val2` |
+| 13 | `BYTES` | `str_data`, `str_len` |
+
+Leaf constructors support comparisons, null checks, `IN`, string operations,
+and ranges. Combine predicates with `paimon_predicate_and`,
+`paimon_predicate_or`, and `paimon_predicate_not`.
+
+!!! warning "Predicate ownership"
+    `paimon_read_builder_with_filter`, the compound predicate functions, and
+    `paimon_vector_search_builder_with_filter` consume their predicate inputs.
+    Do not reuse or free a predicate after passing it to one of these functions.
+    A predicate that has not been consumed must be released with
+    `paimon_predicate_free`.
+
+## Writing and Committing
+
+Writing uses a **write-then-commit** flow:
+
+1. Create one `paimon_write_builder` from the table.
+2. Create a `paimon_table_write` and `paimon_table_commit` from that same
+   builder.
+3. Export each record batch through the Arrow C Data Interface and pass it to
+   `paimon_table_write_write_arrow_batch`.
+4. Call `paimon_table_write_prepare_commit` to obtain commit messages.
+5. Pass the messages to `paimon_table_commit_commit`.
+6. Free the messages, writer, committer, and builder.
+
+The input Arrow schema must match the table schema exactly, including field
+count, order, names, and types. A non-nullable table field must not contain null
+values.
+
+!!! warning "Write builder consistency"
+    The writer and committer must be created from the same write builder because
+    they share a commit identity. Commit messages must be freed with
+    `paimon_commit_messages_free` even after a successful commit. The caller
+    retains message ownership and may retry a failed commit.
+
+## Error Handling and Resource Ownership
+
+Functions that can fail use one of two conventions:
+
+- Constructors and terminal operations return a result struct with an `error`
+  field. On success, `error` is null.
+- Mutating builder functions return `paimon_error *` directly. A null pointer
+  means success.
+
+Error messages are byte buffers and are not null-terminated. Read exactly
+`error->message.len` bytes, and release the entire error with
+`paimon_error_free`. Do not separately call `paimon_bytes_free` on an error
+message.
+
+Every opaque handle returned by the C API has a matching free function. Free
+resources in reverse construction order. Pointer-based free functions accept
+null, which makes cleanup paths straightforward.