Christopher M. Punches
d3f134aabf
The page titles were restating the project name and the artifact names they cover. They read as Overview, DPM Design, Interaction with libdpm-core.so, and Building DPM Core and CLI.
130 lines
5.2 KiB
Markdown
130 lines
5.2 KiB
Markdown
# Interaction with libdpm-core.so {#consumers}
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Programs link libdpm-core.so as an ordinary shared library dependency — the same way they link any other library — to reach the package manager in-process: build systems, installers, image builders, system tooling, and foreign-language bindings all use the library the `dpm` binary is built on. A program holding a default context is working against system configuration, the system module path, the system tree, and system locking, the same environment the installed `dpm` binary sees.
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## Compiling and Linking
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With the library installed, include the public header and link it:
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```
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#include <dpm/core.h>
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```
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```
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g++ myprog.cpp -ldpm-core
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```
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The header installs to the standard include path and the library to the standard lib path, so no additional flags are required. The interface is a C ABI: every function is extern "C", every type crossing the boundary is a C type, and state passes through opaque handles — callable from C, C++, or any language with C FFI.
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`<dpm/core.h>` names no module and carries no module-specific type. It offers two things: discovery of modules, and interaction with them.
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## The Context
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All work happens through a context handle:
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```
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dpm_ctx* ctx = dpm_open(NULL);
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...
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dpm_close(ctx);
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```
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`dpm_open`(NULL) reads the system configuration (`/etc/dpm/conf.d/`), resolves the system module path, and targets the root filesystem. `dpm_close` releases every handle the context issued; all pointers obtained through the context are invalid after it.
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To point a context elsewhere, pass overrides — every field is optional:
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```
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dpm_open_overrides ov = {
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"/path/to/conf.d",
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"/path/to/modules",
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"/path/to/root",
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-1
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};
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dpm_ctx* ctx = dpm_open(&ov);
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```
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Leaving `config_dir` NULL selects `/etc/dpm/conf.d/`; leaving `module_path` NULL selects the configured value and then the built-in default; leaving `root` NULL selects `/`; a `log_level` of -1 takes the configured value.
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The root override is what makes chroot builds, image assembly, and sysroot management work: package operations act on the given tree instead of the running system. Multiple simultaneous contexts with different roots are legal.
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## Acquiring and Using Modules
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**`dpm_require`** loads a module by name, on demand:
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```
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dpm_module* mod = dpm_require(ctx, "mymodule");
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```
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libdpm-core.so validates the module completely at load; a handle is returned only for a fully valid module. NULL means the module is absent or invalid — `dpm_last_error`(ctx) carries the precise reason. Modules load at most once per context; repeated calls return the same handle.
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**`dpm_module_info_of`** reports what the library saw in the loaded module:
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```
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dpm_module_info info;
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dpm_module_info_of(ctx, mod, &info);
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```
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`info` carries `name`, `version`, and `description`.
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Deciding whether that version is suitable is yours. libdpm-core.so applies no version criterion of its own — a handle means the module is valid, not that it suits you.
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**`dpm_execute`** invokes the module — a command name and arguments:
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```
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int rc = dpm_execute(ctx, mod, "command", argc, argv);
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```
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This is the only path into module code, and it is the same path the `dpm` binary uses and the same path a module uses to reach a peer. You address a module by name and a capability by command string, so your program compiles against no module header, no struct layout, and no module symbol. What a module accepts as commands and arguments, and what its return codes mean, is documented by that module.
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## Enumerating Modules
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```
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dpm_cursor* cur = dpm_list_modules(ctx);
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dpm_module_info info;
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while (dpm_cursor_next(cur, &info) == 0) {
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/* each iteration fills info */
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}
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dpm_cursor_free(cur);
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```
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The cursor covers every valid module in the module path; invalid candidates are excluded and logged.
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## Services
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- **`dpm_core_version`()** — the library version; callable without a context.
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- **`dpm_module_info_of`(ctx, mod, out)** — the name, version, and description read from a loaded module.
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- **`dpm_config_get`(ctx, module, section, key)** — a value from a module's configuration namespace, or NULL if unset.
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- **`dpm_log`(ctx, level, message)** — writes to the context's configured log targets; levels are `DPM_LOG_FATAL` through `DPM_LOG_DEBUG`.
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- **`dpm_module_path`(ctx)** — the resolved module directory.
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- **`dpm_last_error`(ctx)** — a human-readable description of the most recent failure on the context, or NULL.
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## Ownership and Errors
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Strings returned by the library are owned by the context (or by the module that produced them) and remain valid until `dpm_close`; callers never free them. Functions returning int use 0 for success. Functions returning pointers use NULL for failure, with detail available from `dpm_last_error`.
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## Complete Example
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```
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#include <dpm/core.h>
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#include <stdio.h>
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int main(void) {
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dpm_ctx* ctx = dpm_open(NULL);
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if (!ctx) {
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fprintf(stderr, "failed to initialize\n");
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return 1;
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}
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dpm_module* mod = dpm_require(ctx, "info");
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if (!mod) {
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fprintf(stderr, "%s\n", dpm_last_error(ctx));
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dpm_close(ctx);
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return 1;
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}
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int rc = dpm_execute(ctx, mod, "version", 0, NULL);
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dpm_close(ctx);
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return rc;
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}
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```
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