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NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |NOTES |EXAMPLES |HISTORY |SEE ALSO |COLOPHON | |
SD_BUS_MESSAGE_READ(3) sd_bus_message_readSD_BUS_MESSAGE_READ(3)sd_bus_message_read, sd_bus_message_readv, sd_bus_message_peek_type - Read a sequence of values from a message
#include <systemd/sd-bus.h>int sd_bus_message_read(sd_bus_message *m, const char *types,...);int sd_bus_message_readv(sd_bus_message *m, const char *types,va_listap);int sd_bus_message_peek_type(sd_bus_message *m, char *type,const char **contents);
sd_bus_message_read()reads a sequence of fields from the D-Bus message objectm and advances the read position in the message. The type stringtypes describes the types of items expected in the message and the field arguments that follow. The type string may beNULLor empty, in which case nothing is read. The type string is composed of the elements described insd_bus_message_append(3), i.e. basic and container types. It must contain zero or more single "complete types". The type string isNUL-terminated. For each type specified in the type string, one or more arguments need to be specified after thetypes parameter, in the same order. The arguments must be pointers to appropriate types (a pointer toint8_tfor a "y" in the type string, a pointer toint32_tfor an "i", a pointer toconst char*for an "s", ...) which are set based on the values in the message. As an exception, in case of array and variant types, the first argument is an "input" argument that further specifies how the message should be read. See the table below for a complete list of allowed arguments and their types. Note that, if the basic type is a pointer (e.g.,const char *in the case of a string), the argument is a pointer to a pointer, and also the pointer value that is written is only borrowed and the contents must be copied if they are to be used after the end of the message's lifetime. If the type is "h" (UNIX file descriptor), the descriptor is not duplicated by this call and the returned descriptor remains in possession of the message object, and needs to be duplicated by the caller in order to keep an open reference to it after the message object is freed. Each argument may also beNULL, in which case the value is read and ignored.Table 1. Item type specifiers ┌───────────┬──────────────────────────────┬────────────────┬──────────────┬──────────────┐ │Specifier│Constant│Description│Type of the│Types of the│ │ │ │ │first│subsequent│ │ │ │ │argument│arguments,│ │ │ │ │ │if any│ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "y" │SD_BUS_TYPE_BYTE│ 8-bit │uint8_t *│ │ │ │ │ unsigned │ │ │ │ │ │ integer │ │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "b" │SD_BUS_TYPE_BOOLEAN│ boolean │int *(NB: │ │ │ │ │ │ notbool *) │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "n" │SD_BUS_TYPE_INT16│ 16-bit │int16_t *│ │ │ │ │ signed │ │ │ │ │ │ integer │ │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "q" │SD_BUS_TYPE_UINT16│ 16-bit │uint16_t *│ │ │ │ │ unsigned │ │ │ │ │ │ integer │ │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "i" │SD_BUS_TYPE_INT32│ 32-bit │int32_t *│ │ │ │ │ signed │ │ │ │ │ │ integer │ │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "u" │SD_BUS_TYPE_UINT32│ 32-bit │uint32_t *│ │ │ │ │ unsigned │ │ │ │ │ │ integer │ │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "x" │SD_BUS_TYPE_INT64│ 64-bit │int64_t *│ │ │ │ │ signed │ │ │ │ │ │ integer │ │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "t" │SD_BUS_TYPE_UINT64│ 64-bit │uint64_t *│ │ │ │ │ unsigned │ │ │ │ │ │ integer │ │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "d" │SD_BUS_TYPE_DOUBLE│ IEEE 754 │double *│ │ │ │ │ double │ │ │ │ │ │ precision │ │ │ │ │ │ floating-point │ │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "s" │SD_BUS_TYPE_STRING│ UTF-8 string │const char│ │ │ │ │ │**│ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "o" │SD_BUS_TYPE_OBJECT_PATH│ D-Bus object │const char│ │ │ │ │ path string │**│ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "g" │SD_BUS_TYPE_SIGNATURE│ D-Bus │const char│ │ │ │ │ signature │**│ │ │ │ │ string │ │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "h" │SD_BUS_TYPE_UNIX_FD│ UNIX file │int *│ │ │ │ │ descriptor │ │ │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "a" │SD_BUS_TYPE_ARRAY│ array │int, which │n sets of │ │ │ │ │ specifies │ arguments │ │ │ │ │ the expected │ appropriate │ │ │ │ │ lengthn of │ for the │ │ │ │ │ the array │ array │ │ │ │ │ │ element type │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤ │ "v" │SD_BUS_TYPE_VARIANT│ variant │ signature │ arguments │ │ │ │ │ string │ appropriate │ │ │ │ │ │ for the │ │ │ │ │ │ types │ │ │ │ │ │ specified by │ │ │ │ │ │ the │ │ │ │ │ │ signature │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┴──────────────┤ │ "(" │SD_BUS_TYPE_STRUCT_BEGIN│ array start │ arguments appropriate │ ├───────────┼──────────────────────────────┼────────────────┤ for the structure │ │ ")" │SD_BUS_TYPE_STRUCT_END│ array end │ elements │ ├───────────┼──────────────────────────────┼────────────────┼──────────────┬──────────────┤ │ "{" │SD_BUS_TYPE_DICT_ENTRY_BEGIN│ dictionary │ arguments │ arguments │ │ │ │ entry start │ appropriate │ appropriate │ ├───────────┼──────────────────────────────┼────────────────┤ for the │ for the │ │ "}" │SD_BUS_TYPE_DICT_ENTRY_END│ dictionary │ first type │ second type │ │ │ │ entry end │ in the pair │ in the pair │ └───────────┴──────────────────────────────┴────────────────┴──────────────┴──────────────┘ If objects of the specified types are not present at the current position in the message, an error is returned. Thesd_bus_message_readv()is equivalent to thesd_bus_message_read(), except that it is called with a "va_list" instead of a variable number of arguments. This function does not call theva_end()macro. Because it invokes theva_arg()macro, the value ofap is undefined after the call.sd_bus_message_peek_type()determines the type of the next element inm to be read bysd_bus_message_read()or similar functions. On success, the type is stored intype, if it is notNULL. If the type is a container type, the type of its elements is stored incontents, if it is notNULL. If this function successfully determines the type of the next element inm, it returns a positive integer. If there are no more elements to be read, it returns zero.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 Specified type string is invalid or the message parameter isNULL.-ENXIO The message does not contain the specified type at current position.-EBADMSG The message cannot be parsed.-EBUSY When reading from a container, this error will be returned if unread elements are left in the container. Added in version 251.
Functions described here are available as a shared library, which can be compiled against and linked to with thelibsystemd pkg-config(1) file. The code described here usesgetenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not callsetenv(3) from a parallel thread. It is recommended to only do calls tosetenv()from an early phase of the program when no other threads have been started.
Read a single basic type (a 64-bit integer): sd_bus_message *m; int64_t x; sd_bus_message_read(m, "x", &x); Read a boolean value: sd_bus_message *m; int x; /* Do not use C99 "bool" type here, it is typically smaller in memory and would cause memory corruption */ sd_bus_message_read(m, "b", &x); Read all types of integers: uint8_t y; int16_t n; uint16_t q; int32_t i; uint32_t u; int32_t x; uint32_t t; double d; sd_bus_message_read(m, "ynqiuxtd", &y, &n, &q, &i, &u, &x, &t, &d); Read a structure composed of a string and a D-Bus path: const char *s, *p; sd_bus_message_read(m, "(so)", &s, &p); Read a variant, with the real type "gt" (signature, unsigned integer): const char *s; uint64_t *v; sd_bus_message_read(m, "v", "gt", &s, &v); Read a dictionary containing three pairs of type {integer=>string}: int i, j, k; const char *s, *t, *u; sd_bus_message_read(m, "a{is}", 3, &i, &s, &j, &t, &k, &u); Read a single file descriptor, and duplicate it in order to keep it open after the message is freed. sd_bus_message *m; int fd, fd_copy; sd_bus_message_read(m, "h", &fd); fd_copy = fcntl(fd, FD_DUPFD_CLOEXEC, 3);sd_bus_message_read()andsd_bus_message_readv()were added in version 240.sd_bus_message_peek_type()was added in version 246.
systemd(1),sd-bus(3),sd_bus_message_read_basic(3),sd_bus_message_skip(3),sd_bus_message_append(3),sd_bus_message_enter_container(3)
This page is part of thesystemd (systemd system and service manager) project. Information about the project can be found at ⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug report for this manual page, see ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This page was obtained from the project's upstream Git repository ⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that time, the date of the most recent commit that was found in the repository was 2025-08-11.) If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up-to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgsystemd 258~rc2SD_BUS_MESSAGE_READ(3)Pages that refer to this page:sd-bus(3), sd_bus_message_at_end(3), sd_bus_message_open_container(3), sd_bus_message_read_array(3), sd_bus_message_read_basic(3), sd_bus_message_read_strv(3), sd_bus_message_rewind(3), sd_bus_message_skip(3), systemd.directives(7), systemd.index(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |