dpm-core-ng/docs/CONSUMERS.md

# Consuming libdpm-core

Programs link libdpm-core to operate the package manager directly: build systems, installers, image builders, system tooling, and foreign-language bindings all use the same library the dpm CLI is built on. A program holding a default context is operating the installed package manager itself — system configuration, system module path, system tree, system locking — identically to invoking the installed dpm command.

## Compiling and linking

With libdpm-core installed, include the public header and link the library:

```
#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.

## 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",    /* config_dir:  NULL = /etc/dpm/conf.d/      */
    "/path/to/modules",   /* module_path: NULL = config, then default  */
    "/path/to/root",      /* root: target root for package operations  */
    -1                    /* log_level: -1 = from config               */
};
dpm_ctx* ctx = dpm_open(&ov);
```

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 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 libdpm-core saw in the loaded module:

```
dpm_module_info info;
dpm_module_info_of(ctx, mod, &info);   /* info.name, .version, .description, .core_min */
```

Deciding whether that version is too new or too old is yours. libdpm-core applies no version criterion of its own — a handle means the module is valid, not that it suits you.

**dpm_execute** drives a module the way the CLI does — a command name and arguments:

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

**dpm_get_api** returns a module's typed function table for direct calls:

```
const mymodule_api_v1_s* api = (const mymodule_api_v1_s*)dpm_get_api(ctx, mod, "mymodule", 1);
```

The returned table was validated at load and is usable for the life of the context. NULL means the module does not provide that API at that version.

## Enumerating modules

```
dpm_cursor* cur = dpm_list_modules(ctx);
dpm_module_info info;
while (dpm_cursor_next(cur, &info) == 0) {
    /* info.name, info.version, info.description, info.core_min */
}
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 libdpm-core version; callable without a context.
- **dpm_module_info_of(ctx, mod, out)** — the name, version, description, and minimum-libdpm-core version 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;
}
```