I have a Tiva TM4C ARM processor connected to a GSM modem (SIM900). When the GSM modem receives a new text message it sends a UART string to the MCU, where an interrupt routine increments a new message counter.

Then my program takes over to parse the message. Below is the code. This code currently functions correctly. However, I believe that there may be a better way to do this than using pointers - especially since I don't have a great understanding of how pointers function, and much of those code was adapted from stuff I found online.

I would love insight on ways this code could be improved.

bool testDelete =   0;              // Delete messages during processing
int msgCount = 0;                   // Hold the int value w/count of new messages
                                    // (Set by UART interrupt handler)

// Data from most recent incoming message stored here
char responseLine[10][75];          // Use to store UART inputs
char *msgContent =  NULL;           // Message content holder
char *msgSender =   NULL;           // Message sender
char *msgDate =     NULL;           // Message date
char *msgTime =     NULL;           // Message time

GSMprocessMessage( int msgNum )
    bool msgPresent[4] = {0000};    // Flag to ignore deleted messages
    bool msgVerify = false;         // Flag for message error checking
    char msgErrorCheck[4][225];     // Holder for message error checking
    int lineCount;                  // Hold the number of lines
    int oLoop;                      // Counter for outside error checking loop
    int iLoop;                      // Counter for inside error checking loop

    // Start message retrieval/parsing/error checking (runs simultaneously to
    // reduce calls to the SIM module).
    for ( oLoop=0; oLoop<3; oLoop++ )
        // Request the message and get the lines of the response (includes 
        // envelope, nulls, SIM responses)
        lineCount = GSMgetResponse();

        // Delay for a bit, needed when processing multiple messages (maybe?)

        // Make sure there's message content, process for envelope and content
        msgPresent[oLoop] = GSMparseMessage( lineCount );

        // If there is a message, store it, see if it matches previous retrieval loop
        if (msgPresent[oLoop])
            // Store the message to one big string
            strcpy(msgErrorCheck[oLoop], msgSender);
            strcat(msgErrorCheck[oLoop], msgDate);
            strcat(msgErrorCheck[oLoop], msgTime);
            strcat(msgErrorCheck[oLoop], msgContent);

            // Check that string against previous copies from outer loop
            for ( iLoop = 0; iLoop < oLoop; iLoop++ ) 
                if (strstr(msgErrorCheck[oLoop],msgErrorCheck[iLoop]) != NULL){

                    // Set a flag to use for exiting outer loop
                    msgVerify = true;

                    // Exit inner loop

        // If there's no message, exit retrieval loop
        else { break; }

        // If we verified the message, exit retrieval loop
        if (msgVerify) { break; }

    // Show the user what we found
    UART0printf("\n\r>>> MESSAGE %u:",msgNum);
    if ( msgPresent[oLoop] && msgVerify ) {
        UART0printf("\n\r> FROM: %s ON: %s AT: %s",msgSender,msgDate,msgTime);
        UART0printf("\n\r> TEXT: %s",msgContent);
    else if ( !msgPresent[oLoop] ) { UART0printf("\n\r> NOT PRESENT!"); }
    else { UART0printf("\n\r> COULD NOT VERIFY!"); }

    // Delete the message
    if ( testDelete && msgPresent ){
        UART0printf ( "\n\r>>> MESSAGE %u DELETED",msgNum );

    bool readResponse = true;       // Keeps the loop open while getting message
    int readLine = 1;               // Counts the lines of the message
    char *GSMresponse = NULL;       // Use to grab input
    static char g_cInput[128];      // String input to a UART

    while ( readResponse )
        // Grab a line

        // Stop after newline
        GSMresponse = strtok(g_cInput,"\n");
        strcpy(responseLine[readLine], GSMresponse);

        // If this line says OK we've got the whole message
        if ( strncmp(responseLine[readLine],"OK",2) == 0 ){readResponse = false;}
        else { readLine++; }
    return readLine;

// Stores message envelope and constant to global variables, OR returns true
// for message present, false for no message
GSMparseMessage( int lineCount )
    int activeLine = 1;             // Counter for line being processed
    char *msgEnvelope = NULL;       // Message envelope holder
    const char commaCh[] = ",";     // Comma character

    // Clear out the old message
    msgContent = NULL;

    // Parse the new message
    while ( activeLine < lineCount+1 )
        // CASE FOR ENVELOPE (which will look like:)
        // +CMGR: "REC READ","+13158078555","","15/10/08,13:18:40-20"
        if ( strstr(responseLine[activeLine],"+CMGR:") != '\0' )
            // Start parsing
            msgEnvelope = responseLine[activeLine];

            // Go to first comma, skipping status
            msgSender = strtok(msgEnvelope,",");

            // Grab the number
            msgSender = strtok(NULL,commaCh);

            // Go to next comma, skipping phonebook entry
            msgDate = strtok(NULL,commaCh);

            // Grab the date
            msgDate = strtok(NULL,commaCh);

            // Grab the time
            msgTime = strtok(NULL,commaCh);

            // Store the number (with null terminator)
            msgSender[11] = '\0';

            // Store the date (with null terminator)
            msgDate[8] = '\0';

            // Store the time (with null terminator)
            msgTime[8] = '\0';

        // If we already found the envelope, and the line's not blank...
        else if ( msgEnvelope != NULL && responseLine[activeLine] != NULL )
            // ... and we haven't found any content, this is the first line.
            if (msgContent == NULL) { msgContent = responseLine[activeLine]; }

            // ... otherwise, add a space and append this line.
            else if ( activeLine + 2 <= lineCount ) {
                strcat(msgContent, " ");
                strcat(msgContent, responseLine[activeLine]);

        // Proceed to next line
    // If we didn't find an envelope, there's no message
    if (msgEnvelope == NULL) { return false; }

    // Otherwise, return true.
    else { return true; }

// Waits for message notification from UART interrupt, and initiates processing
main (void)
    int msgOpen = 0;                    // Message being processed

        // Process new messages.
        while (msgCount > 0)
            // Start working on the oldest message
            msgOpen = msgCount;

            // Process message for envelope and content

Avoid overlapping strings with strncpy()

I can't speak to the correctness of your algorithm because I don't know why you need to read the same message up to 4 times. But I did spot something non-standard in your code. You are trying to strip a prefix from a string like this:

        msgSender[11] = '\0';

Here you are trying to erase the first two characters of msgSender. However, strcpy() and strncpy() technically do not allow the source and destination strings to be overlapping. From the Linux man page for strcpy():

The strcpy() function copies the string pointed to by src, including the terminating null byte ('\0'), to the buffer pointed to by dest. The strings may not overlap, and the destination string dest must be large enough to receive the copy.

No need to copy

Actually, you shouldn't be using strncpy() in the first place. To "remove" the prefix characters, you can simply move your pointer forward. For example, the above code could be replaced with:

     msgSender += 2;          // Skip the "+1" prefix from the phone number.

The part to terminate the string with a null character shouldn't be necessary because strtok() already did that. You would only need to terminate with a null character if you think that there may be a suffix you want to cut off:

    msgSender[10] = '\0';    // Limit the phone # to 10 digits.

Multiple calls to strcat() not efficient

It's not efficient to keep calling strcat() on the same string because strcat() needs to measure the length of the current string before appending the new part. If you append \$n\$ strings using strcat(), it is an \$O(n)\$ operation. So this code:

        strcpy(msgErrorCheck[oLoop], msgSender);
        strcat(msgErrorCheck[oLoop], msgDate);
        strcat(msgErrorCheck[oLoop], msgTime);
        strcat(msgErrorCheck[oLoop], msgContent);

would be better off like this:

        sprintf(msgErrorCheck[oLoop], "%s%s%s%s", msgSender, msgDate,
                msgTime, msgContent);

In this case, since the strings are so short and there are only 4 of them, this change will not even be noticeable.

  • \$\begingroup\$ Had to shift to another project for a while, back to this now... I am reading the message four times because com between the GSM module and my MCU is infrequently interrupted, but having the right message is critical. I applied your advice on No need to copy... check... but I did need the null terminator. And, switching to snprintf was a good call... Thanks! \$\endgroup\$ – LShaver Dec 15 '15 at 1:14

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