# Interaction with libdpm-core.so {#consumers}

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.

## Compiling and Linking

With the library installed, include the public header and link it:

```
#include <dpm/core.h>
```

```
g++ myprog.cpp -ldpm-core
```

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.

`<dpm/core.h>` names no module and carries no module-specific type. It offers two things: discovery of modules, and interaction with them.

## The Context

All work happens through a context handle:

```
dpm_ctx* ctx = dpm_open(NULL);
...
dpm_close(ctx);
```

`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.

To point a context elsewhere, pass overrides — every field is optional:

```
dpm_open_overrides ov = {
    "/path/to/conf.d",
    "/path/to/modules",
    "/path/to/root",
    -1
};
dpm_ctx* ctx = dpm_open(&ov);
```

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.

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.

## Acquiring and Using Modules

`dpm_require` loads a module by name, on demand:

```
dpm_module* mod = dpm_require(ctx, "mymodule");
```

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.

`dpm_module_info_of` reports what the library saw in the loaded module:

```
dpm_module_info info;
dpm_module_info_of(ctx, mod, &info);
```

`info` carries `name`, `version`, and `description`.

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.

`dpm_execute` invokes the module — a command name and arguments:

```
int rc = dpm_execute(ctx, mod, "command", argc, argv);
```

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.

## Enumerating Modules

```
dpm_cursor* cur = dpm_list_modules(ctx);
dpm_module_info info;
while (dpm_cursor_next(cur, &info) == 0) {
    /* each iteration fills info */
}
dpm_cursor_free(cur);
```

The cursor covers every valid module in the module path; invalid candidates are excluded and logged.

## Services

- `dpm_core_version`()** — the library version; callable without a context.
- `dpm_module_info_of`(ctx, mod, out)** — the name, version, and description read from a loaded module.
- `dpm_config_get`(ctx, module, section, key)** — a value from a module's configuration namespace, or NULL if unset.
- `dpm_log`(ctx, level, message)** — writes to the context's configured log targets; levels are `DPM_LOG_FATAL` through `DPM_LOG_DEBUG`.
- `dpm_module_path`(ctx)** — the resolved module directory.
- `dpm_last_error`(ctx)** — a human-readable description of the most recent failure on the context, or NULL.

## Ownership and Errors

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`.

## Complete Example

```
#include <dpm/core.h>
#include <stdio.h>

int main(void) {
    dpm_ctx* ctx = dpm_open(NULL);
    if (!ctx) {
        fprintf(stderr, "failed to initialize\n");
        return 1;
    }

    dpm_module* mod = dpm_require(ctx, "info");
    if (!mod) {
        fprintf(stderr, "%s\n", dpm_last_error(ctx));
        dpm_close(ctx);
        return 1;
    }

    int rc = dpm_execute(ctx, mod, "version", 0, NULL);

    dpm_close(ctx);
    return rc;
}
```