Provided by: libbluetooth-dev_5.84-1_amd64 
NAME
sco - SCO transport
SYNOPSIS
#include <sys/socket.h>
#include <bluetooth/bluetooth.h>
#include <bluetooth/sco.h>
sco_socket = socket(PF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_SCO);
DESCRIPTION
The SCO logical transport, is a symmetric, point-to-point transport between the Central and a specific
Peripheral. The SCO logical transport reserves slots and can therefore be considered as a
circuit-switched connection between the Central and the Peripheral.
In addition to the reserved slots, when eSCO is supported, a retransmission window follows immediately
after. Together, the reserved slots and the retransmission window form the complete eSCO window.
SOCKET ADDRESS
struct sockaddr_sco {
sa_family_t sco_family;
bdaddr_t sco_bdaddr;
};
Example:
struct sockaddr_sco addr;
memset(&addr, 0, sizeof(addr));
addr.sco_family = AF_BLUETOOTH;
bacpy(&addr.sco_bdaddr, bdaddr);
SOCKET OPTIONS (SOL_BLUETOOTH)
The socket options listed below can be set by using setsockopt(2) and read with getsockopt(2) with the
socket level set to SOL_BLUETOOTH.
BT_SECURITY (since Linux 2.6.30)
Channel security level, possible values:
┌────────────────────┬──────────────────────┬────────────────────────┬──────────────┐
│ Value │ Security Level │ Link Key Type │ Encryption │
├────────────────────┼──────────────────────┼────────────────────────┼──────────────┤
│ BT_SECURITY_SDP │ 0 (SDP Only) │ None │ Not required │
├────────────────────┼──────────────────────┼────────────────────────┼──────────────┤
│ BT_SECURITY_LOW │ 1 (Low) │ Unauthenticated │ Not required │
├────────────────────┼──────────────────────┼────────────────────────┼──────────────┤
│ BT_SECURITY_MEDIUM │ 2 (Medium - default) │ Unauthenticated │ Desired │
├────────────────────┼──────────────────────┼────────────────────────┼──────────────┤
│ BT_SECURITY_HIGH │ 3 (High) │ Authenticated │ Required │
├────────────────────┼──────────────────────┼────────────────────────┼──────────────┤
│ BT_SECURITY_FIPS │ 4 (Secure Only) │ Authenticated (P-256 │ Required │
│ (since Linux 3.15) │ │ based Secure Simple │ │
│ │ │ Pairing and Secure │ │
│ │ │ Authentication) │ │
└────────────────────┴──────────────────────┴────────────────────────┴──────────────┘
Example:
int level = BT_SECURITY_HIGH;
int err = setsockopt(sco_socket, SOL_BLUETOOTH, BT_SECURITY, &level,
sizeof(level));
if (err == -1) {
perror("setsockopt");
return 1;
}
BT_DEFER_SETUP (since Linux 2.6.30)
Channel defer connection setup, this control if the connection procedure needs to be authorized by
userspace before responding which allows authorization at profile level, possible values:
┌───────┬───────────────────┬───────────────┐
│ Value │ Description │ Authorization │
├───────┼───────────────────┼───────────────┤
│ 0 │ Disable (default) │ Not required │
├───────┼───────────────────┼───────────────┤
│ 1 │ Enable │ Required │
└───────┴───────────────────┴───────────────┘
Example:
int defer_setup = 1;
int err = setsockopt(sco_socket, SOL_BLUETOOTH, BT_DEFER_SETUP,
&defer_setup, sizeof(defer_setup));
if (err == -1) {
perror("setsockopt");
return err;
}
err = listen(sco_socket, 5);
if (err) {
perror("listen");
return err;
}
struct sockaddr_sco remote_addr = {0};
socklen_t addr_len = sizeof(remote_addr);
int new_socket = accept(sco_socket, (struct sockaddr*)&remote_addr,
&addr_len);
if (new_socket < 0) {
perror("accept");
return new_socket;
}
/* To complete the connection setup of new_socket read 1 byte */
char c;
struct pollfd pfd;
memset(&pfd, 0, sizeof(pfd));
pfd.fd = new_socket;
pfd.events = POLLOUT;
err = poll(&pfd, 1, 0);
if (err) {
perror("poll");
return err;
}
if (!(pfd.revents & POLLOUT)) {
err = read(sk, &c, 1);
if (err < 0) {
perror("read");
return err;
}
}
BT_VOICE (since Linux 3.11)
Transport voice settings, possible values:
struct bt_voice {
uint16_t setting;
};
┌────────────────────────────┬────────┬──────────────────────────────┐
│ Define │ Value │ Description │
├────────────────────────────┼────────┼──────────────────────────────┤
│ BT_VOICE_TRANSPARENT │ 0x0003 │ Transparent output │
├────────────────────────────┼────────┼──────────────────────────────┤
│ BT_VOICE_CVSD_16BIT │ 0x0060 │ C-VSD output PCM 16-bit │
│ │ │ input │
├────────────────────────────┼────────┼──────────────────────────────┤
│ BT_VOICE_TRANSPARENT_16BIT │ 0x0063 │ Transparent output PCM │
│ │ │ 16-bit input │
└────────────────────────────┴────────┴──────────────────────────────┘
Example:
struct bt_voice voice;
memset(&voice, 0, sizeof(voice));
voice.setting = BT_VOICE_TRANSPARENT;
int err = setsockopt(sco_socket, SOL_BLUETOOTH, BT_VOICE, &voice,
sizeof(voice));
if (err == -1) {
perror("setsockopt");
return 1;
}
BT_PKT_STATUS (since Linux 5.9)
Enable reporting packet status via BT_SCM_PKT_STATUS CMSG on received packets. Possible values:
┌───────┬───────────────────┐
│ Value │ Description │
├───────┼───────────────────┤
│ 0 │ Disable (default) │
├───────┼───────────────────┤
│ 1 │ Enable │
└───────┴───────────────────┘
BT_SCM_PKT_STATUS
Level SOL_BLUETOOTH CMSG with data:
uint8_t pkt_status;
The values are equal to the "Packet_Status_Flag" defined in Core Specification v6.0 Sec. 5.4.3 pp.
1877:
┌────────────┬─────────────────────────┐
│ pkt_status │ Description │
├────────────┼─────────────────────────┤
│ 0x0 │ Correctly received data │
├────────────┼─────────────────────────┤
│ 0x1 │ Possibly invalid data │
├────────────┼─────────────────────────┤
│ 0x2 │ No data received │
├────────────┼─────────────────────────┤
│ 0x3 │ Data partially lost │
└────────────┴─────────────────────────┘
BT_PHY (since Linux 5.10)
Transport supported PHY(s), read-only (no setsockopt support). Possible values:
┌────────────────────┬───────┬─────────────────┐
│ Define │ Value │ Description │
├────────────────────┼───────┼─────────────────┤
│ BT_PHY_BR_1M_1SLOT │ BIT 0 │ BR 1Mbps 1SLOT │
├────────────────────┼───────┼─────────────────┤
│ BT_PHY_BR_1M_3SLOT │ BIT 1 │ BR 1Mbps 3SLOT │
├────────────────────┼───────┼─────────────────┤
│ BT_PHY_BR_2M_1SLOT │ BIT 3 │ EDR 2Mbps 1SLOT │
├────────────────────┼───────┼─────────────────┤
│ BT_PHY_BR_2M_3SLOT │ BIT 4 │ EDR 2Mbps 3SLOT │
├────────────────────┼───────┼─────────────────┤
│ BT_PHY_BR_3M_1SLOT │ BIT 6 │ EDR 3Mbps 1SLOT │
├────────────────────┼───────┼─────────────────┤
│ BT_PHY_BR_3M_3SLOT │ BIT 7 │ EDR 3Mbps 3SLOT │
└────────────────────┴───────┴─────────────────┘
BT_CODEC (since Linux 5.14)
Transport codec offload, possible values:
struct bt_codec {
uint8_t id;
uint16_t cid;
uint16_t vid;
uint8_t data_path_id;
uint8_t num_caps;
struct codec_caps {
uint8_t len;
uint8_t data[];
} caps[];
} __attribute__((packed));
struct bt_codecs {
uint8_t num_codecs;
struct bt_codec codecs[];
} __attribute__((packed));
Example:
char buffer[sizeof(struct bt_codecs) + sizeof(struct bt_codec)];
struct bt_codec *codecs = (void *)buffer;
memset(codecs, 0, sizeof(codecs));
codec->num_codecs = 1;
codecs->codecs[0].id = 0x05;
codecs->codecs[0].data_path_id = 1;
int err = setsockopt(sco_socket, SOL_BLUETOOTH, BT_CODEC, codecs,
sizeof(buffer));
if (err == -1) {
perror("setsockopt");
return 1;
}
SOCKET OPTIONS (SOL_SOCKET)
SOL_SOCKET level socket options that modify generic socket features (SO_SNDBUF, SO_RCVBUF, etc.) have
their usual meaning, see socket(7).
The SOL_SOCKET level SCO socket options that have Bluetooth-specific handling in kernel are listed below.
SO_TIMESTAMPING, SO_TIMESTAMP, SO_TIMESTAMPNS
See <https://docs.kernel.org/networking/timestamping.html>
For SCO sockets, software RX timestamps are supported. Software TX timestamps
(SOF_TIMESTAMPING_TX_SOFTWARE) are supported since Linux 6.15.
The software RX timestamp is the time when the kernel received the packet from the controller driver.
The SCM_TSTAMP_SND timestamp is emitted when packet is sent to the controller driver.
The SCM_TSTAMP_COMPLETION timestamp is emitted when controller reports the packet completed. Completion
timestamps are only supported on controllers that have SCO flow control. Other TX timestamp types are
not supported.
You can use SIOCETHTOOL to query supported flags.
The timestamps are in CLOCK_REALTIME time.
Example (Enable RX timestamping):
int flags = SOF_TIMESTAMPING_SOFTWARE |
SOF_TIMESTAMPING_RX_SOFTWARE;
setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &flags, sizeof(flags));
Example (Read packet and its RX timestamp):
char data_buf[256];
union {
char buf[CMSG_SPACE(sizeof(struct scm_timestamping))];
struct cmsghdr align;
} control;
struct iovec data = {
.iov_base = data_buf,
.iov_len = sizeof(data_buf),
};
struct msghdr msg = {
.msg_iov = &data,
.msg_iovlen = 1,
.msg_control = control.buf,
.msg_controllen = sizeof(control.buf),
};
struct scm_timestamping tss;
res = recvmsg(fd, &msg, MSG_ERRQUEUE | MSG_DONTWAIT);
if (res < 0)
goto error;
for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_TIMESTAMPING)
memcpy(&tss, CMSG_DATA(cmsg), sizeof(tss));
}
tstamp_clock_realtime = tss.ts[0];
Example (Enable TX timestamping):
int flags = SOF_TIMESTAMPING_SOFTWARE |
SOF_TIMESTAMPING_TX_SOFTWARE |
SOF_TIMESTAMPING_OPT_ID;
setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &flags, sizeof(flags));
Example (Read TX timestamps):
union {
char buf[CMSG_SPACE(sizeof(struct scm_timestamping))];
struct cmsghdr align;
} control;
struct iovec data = {
.iov_base = NULL,
.iov_len = 0
};
struct msghdr msg = {
.msg_iov = &data,
.msg_iovlen = 1,
.msg_control = control.buf,
.msg_controllen = sizeof(control.buf),
};
struct cmsghdr *cmsg;
struct scm_timestamping tss;
struct sock_extended_err serr;
int res;
res = recvmsg(fd, &msg, MSG_ERRQUEUE | MSG_DONTWAIT);
if (res < 0)
goto error;
for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_TIMESTAMPING)
memcpy(&tss, CMSG_DATA(cmsg), sizeof(tss));
else if (cmsg->cmsg_level == SOL_BLUETOOTH && cmsg->cmsg_type == BT_SCM_ERROR)
memcpy(&serr, CMSG_DATA(cmsg), sizeof(serr));
}
tstamp_clock_realtime = tss.ts[0];
tstamp_type = serr->ee_info; /* SCM_TSTAMP_SND or SCM_TSTAMP_COMPLETION */
tstamp_seqnum = serr->ee_data;
IOCTLS
The following ioctls with operation specific for SCO sockets are available.
SIOCETHTOOL (since Linux 6.16-rc1)
Supports only command ETHTOOL_GET_TS_INFO, which may be used to query supported SOF_TIMESTAMPING_* flags.
The SOF_TIMESTAMPING_OPT_* flags are always available as applicable.
Example:
#include <linux/ethtool.h>
#include <linux/sockios.h>
#include <net/if.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
...
struct ifreq ifr = {};
struct ethtool_ts_info cmd = {};
int sk;
snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "hci0");
ifr.ifr_data = (void *)&cmd;
cmd.cmd = ETHTOOL_GET_TS_INFO;
sk = socket(PF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_SCO);
if (sk < 0)
goto error;
if (ioctl(sk, SIOCETHTOOL, &ifr))
goto error;
sof_available = cmd.so_timestamping;
RESOURCES
<http://www.bluez.org>
REPORTING BUGS
<linux-bluetooth@vger.kernel.org>
SEE ALSO
socket(7), scotest(1)
COPYRIGHT
Free use of this software is granted under the terms of the GNU Lesser General Public Licenses (LGPL).
BlueZ March 2025 SCO(7)