# sd_bus_message_read

## NAME



           sd_bus_message_read, sd_bus_message_readv,
           sd_bus_message_peek_type - Read a sequence of values from a
           message
    


## SYNOPSIS



           **#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_list** _ap_**);**
    
           **int sd_bus_message_peek_type(sd_bus_message ***_m_**, char ***_type_**,**
                                        **const char ****_contents_**);**
    


## DESCRIPTION



           **sd_bus_message_read()** reads a sequence of fields from the D-Bus
           message object _m_ and advances the read position in the message.
           The type string _types_ describes the types of items expected in the
           message and the field arguments that follow. The type string may
           be **NULL** or empty, in which case nothing is read.
    
           The type string is composed of the elements described in
           sd_bus_message_append(3), i.e. basic and container types. It must
           contain zero or more single "complete types". The type string is
           **NUL** -terminated.
    
           For each type specified in the type string, one or more arguments
           need to be specified after the _types_ parameter, in the same order.
           The arguments must be pointers to appropriate types (a pointer to
           **int8_t** for a "y" in the type string, a pointer to **int32_t** for an
           "i", a pointer to **const 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 be **NULL** , 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:   │              │
           │           │                              │                │ not **bool ***)  │              │
           ├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤
           │ "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  │
           │           │                              │                │ length _n_ 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.
    
           The **sd_bus_message_readv()** is equivalent to the
           **sd_bus_message_read()** , except that it is called with a "va_list"
           instead of a variable number of arguments. This function does not
           call the **va_end()** macro. Because it invokes the **va_arg()** macro,
           the value of _ap_ is undefined after the call.
    
           **sd_bus_message_peek_type()** determines the type of the next element
           in _m_ to be read by **sd_bus_message_read()** or similar functions. On
           success, the type is stored in _type_ , if it is not **NULL**. If the
           type is a container type, the type of its elements is stored in
           _contents_ , if it is not **NULL**. If this function successfully
           determines the type of the next element in _m_ , it returns a
           positive integer. If there are no more elements to be read, it
           returns zero.
    


## 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**
               Specified type string is invalid or the message parameter is
               **NULL**.
    
           **-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.
    


## NOTES



           Functions described here are available as a shared library, which
           can be compiled against and linked to with the
           **libsystemd pkg-config**(1) file.
    
           The code described here uses getenv(3), which is declared to be
           not multi-thread-safe. This means that the code calling the
           functions described here must not call setenv(3) 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



           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);
    


* * *