--- name: "sd_bus_message_new_method_return" section: "3" platform: "linux" package: "libsystemd-dev" version: "257.13-1~deb13u1" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/sd_bus_message_new_method_call" --- # sd\_bus\_message\_new\_method\_return(3) Create a method call message Part of [sd\_bus\_message\_new\_method\_call(3)](/sd_bus_message_new_method_call/3.md). ## NAME sd\_bus\_message\_new\_method\_call, sd\_bus\_message\_new\_method\_return - Create a method call message ## SYNOPSIS ``` #include ``` **int sd\_bus\_message\_new\_method\_call(sd\_bus \****bus***, sd\_bus\_message \*\****m***, const char \****destination***, const char \****path***, const char \****interface***, const char \****member***);** **int sd\_bus\_message\_new\_method\_return(sd\_bus\_message \****call***, sd\_bus\_message \*\****m***);** ## DESCRIPTION The **sd\_bus\_message\_new\_method\_call()** function creates a new bus message object that encapsulates a D-Bus method call, and returns it in the *m* output parameter. The call will be made on the destination *destination*, path *path*, on the interface *interface*, member *member*. Briefly, the *destination* is a dot-separated name that identifies a service connected to the bus. The *path* is a slash-separated identifier of an object within the destination that resembles a file system path. The meaning of this path is defined by the destination. The *interface* is a dot-separated name that resembles a Java interface name that identifies a group of methods and signals supported by the object identified by path. Methods and signals are collectively called *members* and are identified by a simple name composed of ASCII letters, numbers, and underscores. See the **D-Bus Tutorial**\[1\] for an in-depth explanation. The *destination* parameter may be **NULL**. The *interface* parameter may be **NULL**, if the destination has only a single member with the given name and there is no ambiguity if the interface name is omitted. Note that this is a low level interface. See [**sd\_bus\_call\_method**(3)](/sd_bus_call_method/3.md) for a more convenient way of calling D-Bus methods. The **sd\_bus\_message\_new\_method\_return()** function creates a new bus message object that is a reply to the method call *call* and returns it in the *m* output parameter. The *call* parameter must be a method call message. The sender of *call* is used as the destination. ## RETURN VALUE On success, these functions return a non-negative integer. On failure, they return a negative errno-style error code. ### Errors Returned errors may indicate the following problems: **-EINVAL** The output parameter *m* is **NULL**. The *destination* parameter is non-null and is not a valid D-Bus service name ("org.somewhere.Something"), the *path* parameter is not a valid D-Bus path ("/an/object/path"), the *interface* parameter is non-null and is not a valid D-Bus interface name ("an.interface.name"), or the *member* parameter is not a valid D-Bus member ("Name"). The *call* parameter is not a method call object. **-ENOTCONN** The bus parameter *bus* is **NULL** or the bus is not connected. **-ENOMEM** Memory allocation failed. **-EPERM** The *call* parameter is not sealed. **-EOPNOTSUPP** The *call* message does not have a cookie. ## NOTES Functions described here are available as a shared library, which can be compiled against and linked to with the **libsystemd** [**pkg-config**(1)](/pkg-config/1.md) file. The code described here uses [**getenv**(3)](/getenv/3.md), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not call [**setenv**(3)](/setenv/3.md) from a parallel thread. It is recommended to only do calls to **setenv()** from an early phase of the program when no other threads have been started. ## EXAMPLES **Example 1. Make a call to a D-Bus method that takes a single parameter** ``` /* SPDX-License-Identifier: MIT-0 */ /* This is equivalent to: * busctl call org.freedesktop.systemd1 /org/freedesktop/systemd1 \ * org.freedesktop.systemd1.Manager GetUnitByPID $$ * * Compile with 'cc print-unit-path.c -lsystemd' */ #include #include #include #include #include #define _cleanup_(f) __attribute__((cleanup(f))) #define DESTINATION "org.freedesktop.systemd1" #define PATH "/org/freedesktop/systemd1" #define INTERFACE "org.freedesktop.systemd1.Manager" #define MEMBER "GetUnitByPID" static int log_error(int error, const char *message) { fprintf(stderr, "%s: %s\n", message, strerror(-error)); return error; } int main(int argc, char **argv) { _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL; _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL; _cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL, *m = NULL; int r; r = sd_bus_open_system(&bus); if (r < 0) return log_error(r, "Failed to acquire bus"); r = sd_bus_message_new_method_call(bus, &m, DESTINATION, PATH, INTERFACE, MEMBER); if (r < 0) return log_error(r, "Failed to create bus message"); r = sd_bus_message_append(m, "u", (unsigned) getpid()); if (r < 0) return log_error(r, "Failed to append to bus message"); r = sd_bus_call(bus, m, -1, &error, &reply); if (r < 0) return log_error(r, MEMBER " call failed"); const char *ans; r = sd_bus_message_read(reply, "o", &ans); if (r < 0) return log_error(r, "Failed to read reply"); printf("Unit path is \"%s\".\n", ans); return 0; } ``` This defines a minimally useful program that will open a connection to the bus, create a message object, send it, wait for the reply, and finally extract and print the answer. It does error handling and proper memory management. ## HISTORY **sd\_bus\_message\_new\_method\_call()** and **sd\_bus\_message\_new\_method\_return()** were added in version 246. ## SEE ALSO [**systemd**(1)](/systemd/1.md), [**sd-bus**(3)](/sd-bus/3.md), [**sd\_bus\_call**(3)](/sd_bus_call/3.md), [**sd\_bus\_call\_method**(3)](/sd_bus_call_method/3.md), [**sd\_bus\_path\_encode**(3)](/sd_bus_path_encode/3.md) ## NOTES - 1\. D-Bus Tutorial https://dbus.freedesktop.org/doc/dbus-tutorial.html#concepts