此代码段中的索引如何达到2154？ （gcc，嵌入式C，ARM Cortex M0）

Question

I'm writing a driver for a GSM modem running on an ARM Cortex M0. The only UART on the system is in use for talking to the modem, so the best I can do for logging the UART conversation with the modem is to build up a string in memory and watch it with GDB.

Here are my UART logging functions.

// Max number of characters user in the UART log, when in use.
#define GSM_MAX_UART_LOG_CHARS (2048)

static char m_gsm_uart_log[GSM_MAX_UART_LOG_CHARS] = "";
static uint16_t m_gsm_uart_log_index = 0;

// Write a character to the in-memory log of all UART messages.
static void gsm_uart_log_char(const char value)
{
    m_gsm_uart_log_index++;

    if (m_gsm_uart_log_index > GSM_MAX_UART_LOG_CHARS)
    {
        // Clear and restart log.
        memset(&m_gsm_uart_log, 0, GSM_MAX_UART_LOG_CHARS); // <-- Breakpoint here
        m_gsm_uart_log_index = 0;
    }

    m_gsm_uart_log[m_gsm_uart_log_index] = value;
}

// Write a string to the in-memory log of all UART messages.
static void gsm_uart_log_string(const char *value)
{
    uint16_t i = 0;
    char ch = value[i++];

    while (ch != '\0')
    {
        gsm_uart_log_char(ch);
        ch = value[i++];
    }
}

If I set a breakpoint on the line shown above, the first time it's reached, m_gsm_uart_log_index is already well over 2048. I've seen 2154 and a bunch of other values between 2048 and 2200 or so.

How is this possible? There's no other code that touches m_gsm_uart_log_index anywhere.

Answer 1

You have a buffer overflow happening which could trample on m_gsm_uart_log_index .

The check for end of buffers should be:

if (m_gsm_uart_log_index >= GSM_MAX_UART_LOG_CHARS) {
...
}

As it stands, m_gsm_uart_log_index can reach 2048, and so writing m_gsm_uart_log_index[2048] is likely to be at the location where m_gsm_uart_log_index is stored.

Answer 2

You are writing to the buffer when m_gsm_uart_log_index == GSM_MAX_UART_LOG_CHARS , which means that you are overrunning the buffer by 1 character. This writes into the first byte of m_gsm_uart_log_index and corrupts it.

Change:

if (m_gsm_uart_log_index > GSM_MAX_UART_LOG_CHARS)

to:

if (m_gsm_uart_log_index >= GSM_MAX_UART_LOG_CHARS)