24. Deleting SMS
Messages from Message Storage (AT+CMGD)
The
AT command +CMGD (command name in text: Delete Message) is used to
delete SMS message(s) from message storage. The message storage area
from which SMS messages are deleted is specified by the +CPMS AT
command (command name in text: Preferred Message Storage). Details
about the +CPMS AT command can be found in the earlier section
"Preferred Message Storage
(AT+CPMS)" of this SMS tutorial.
24.1. Syntax of the
+CMGD AT Command
The
syntax of the +CMGD AT command is: (Optional parameters are enclosed
in square brackets.)
+CMGD=index[,flag]
In
the above line, index is an integer specifying the location of
the SMS message to be deleted from the message storage area by the
+CMGD AT command, and flag is an integer specifying whether to
delete SMS messages according to their message status. The SMS
specification has defined these flag
values: 0, 1, 2, 3 and 4.
0.
Meaning: Delete only the SMS message stored at the location index
from the message storage area. This is the default value.
1.
Meaning: Ignore the value of index
and delete all SMS messages whose status is "received
read" from the message storage area.
2.
Meaning: Ignore the value of index
and delete all SMS messages whose status is "received
read" or "stored sent" from the message storage area.
3.
Meaning: Ignore the value of index
and delete all SMS messages whose status is "received
read", "stored unsent" or "stored sent"
from the message storage area.
4.
Meaning: Ignore the value of index
and delete all SMS messages from the message storage area.
To
find the flag values
supported by your GSM/GPRS modem or mobile phone, you can perform a
test operation using the +CMGD AT command. See the next
section for the details.
24.2. Finding the
Supported Indexes and Flag Values
Performing
a test operation with the +CMGD AT command allows you to find all
indexes that are supported by the index
parameter. The test operation may also return all supported values of
the flag parameter.
Here is the format of the information response returned after the
execution of the test command "+CMGD=?": (Optional fields
are enclosed in square brackets.)
+CMGD:
(list_of_indexes)[,(list_of_flag_values)]
The
values inside the first parentheses are the values that are
supported by the index
parameter of the +CMGD AT command.
The
values inside the second parentheses are the values that are
supported by the flag
parameter of the +CMGD AT command. Note: A GSM/GPRS
modem or mobile phone might not provide these values since the SMS
specification defines them as optional.
As
an example, here is the response returned from my Nokia 6021 mobile
phone to Windows' HyperTerminal program after the execution of the
test command "+CMGD=?":
AT+CMGD=?
+CMGD:
(1-10),(0-4)
OK
From
above, we know that the Nokia 6021 mobile phone allows any integer
between 1 and 10 to be assigned to the index
parameter of the +CMGD AT command. Also, the Nokia 6021 mobile
phone supports all of the five flag
values defined in the SMS specification.
Do
you remember that we described the format of values inside
parentheses in the section "Test
Command -- Checks Whether a Certain AT Command is Supported"?
If not, you may want to go back and have a look. Here are a few
examples showing some of the possibilities: "(0-4)",
"(0,1,2,3,4)", "(0,2-4)".
24.3. Example
Demonstrating How to Use the +CMGD AT Command to Delete SMS Text
Messages
Now
let's see an example that demonstrates how to use the +CMGD AT
command to delete an SMS text message from the message storage area.
Suppose you have written an SMS text message to the message storage
area by the +CMGW AT command (details about the +CMGW AT command can
be found in the earlier section "Writing
SMS Messages to Memory / Message Storage (AT+CMGW)" of this
SMS tutorial), like this:
AT+CMGW="+85291234567"
>
A simple demo of SMS text messaging.
+CMGW: 1
OK
The
information response of the +CMGW AT command tells us that the SMS
text message is stored in the memory location at index 1. To delete
the SMS text message, specify 1 to the index
parameter of the +CMGD AT command, like this:
(Important
note: Make sure you are writing text messages to and deleting
text messages from the same message storage area. If not, you may
delete the wrong text message. Details about message storage areas
can be found in the earlier section "Preferred
Message Storage (AT+CPMS)"
of this SMS tutorial.)
AT+CMGD=1
OK
The
final result code OK indicates the SMS text message "A simple
demo of SMS text messaging." was deleted successfully. If the
operation fails, the final result code returned will either be ERROR
or +CMS ERROR.
Here
is another example. Suppose you want to delete all SMS messages
stored in the message storage area. To do this, assign 4 to the flags
parameter of the +CMGD AT command. The value assigned to the index
parameter does not matter since it will be ignored by the GSM/GPRS
modem or mobile phone.
AT+CMGD=1,4
OK
|
Feedback Form (ExpandCollapse)
|
|
