如果C中的語句評估不正確？

Question

我目前正在用C語言對TI MSP430進行編程...調試器中出現一個奇怪的問題，該問題將if語句顯示為滿足向前擴展的條件，但在運行時不這樣做。 有任何想法嗎？ 在函數中未評估的if語句位於星號之間。

void gps_parse(void){
  gps_write();
  lcd_out("                ", LCD_LINE_1);
  lcd_out("                ", LCD_LINE_2);
  lcd_out(GPSlat, LCD_LINE_1);
  lcd_out(GPSlong, LCD_LINE_2);
  SW2Press = NOT_PRESSED;
  while(!SW2Press){
    if (GPS_rx_ring_rd != GPS_rx_ring_wr) {
      **if (GPS_Char_Rx[GPS_rx_ring_rd] == '$')**{
        if (++GPS_rx_ring_rd >= (64)) {
          GPS_rx_ring_rd = BEGINNING;
        }
        char GPS_data[64];
        for (int i = 0; i < 64; i++) {
          while(GPS_rx_ring_rd == GPS_rx_ring_wr);
          GPS_data[i] = GPS_Char_Rx[GPS_rx_ring_rd];
          if (++GPS_rx_ring_rd >= (64)) {
            GPS_rx_ring_rd = 0; // Circular buffer back to beginning
          }
        }
    if(GPS_data[0] == 'G' && GPS_data[1] == 'P' && GPS_data[2] == 'R' && GPS_data[3] == 'M' && GPS_data[4] == 'C'){
      if(GPS_data[17] == 'A'){
        //Fill in lats
        GPSlat[4]  = GPS_data[19]; 
        GPSlat[5]  = GPS_data[20]; 
        GPSlat[6]  = GPS_data[21]; 
        GPSlat[7]  = GPS_data[22]; 
        GPSlat[8]  = GPS_data[23]; 
        GPSlat[9]  = GPS_data[24]; 
        GPSlat[10] = GPS_data[25]; 
        GPSlat[11] = GPS_data[26];   
        GPSlat[12] = GPS_data[27];
        GPSlat[15] = GPS_data[29];
        //Fill in longs
        GPSlong[4]  = GPS_data[31]; 
        GPSlong[5]  = GPS_data[32]; 
        GPSlong[6]  = GPS_data[33]; 
        GPSlong[7]  = GPS_data[34]; 
        GPSlong[8]  = GPS_data[35]; 
        GPSlong[9]  = GPS_data[36]; 
        GPSlong[10] = GPS_data[37]; 
        GPSlong[11] = GPS_data[38];   
        GPSlong[12] = GPS_data[39];
        GPSlong[13] = GPS_data[40];
        GPSlong[15] = GPS_data[42];
      } else{
        GPSlat[15]  = '?';
        GPSlong[15] = '?';
      }
      lcd_out(GPSlat, LCD_LINE_1);
      lcd_out(GPSlong, LCD_LINE_2);
    }
    else {
        if (++GPS_rx_ring_rd >= (64)) {
          GPS_rx_ring_rd = 0;
          }    
        }
      }
    }  
 }
}

Answer 1

嘗試禁用編譯器優化。 有時，由於優化了代碼，代碼路徑在調試器中看起來很奇怪。

Answer 2

// the following compiles, but has not been run.
// it corrects the obvious logic errors
// it corrects certain coding errors
// and corrects certain erroneous assumptions
// I added several data definitions so it would compile

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

extern int SW2Press;
extern int GPS_rx_ring_rd; // contains next char index to extract from circular buffer
extern int GPS_rx_ring_wr; // contains next char index to write to circular buffer
extern char *GPS_Char_Rx;

void gps_parse(void);
void gps_write(void);
void lcd_out( char*, int);

#define LCD_LINE_1 (1)
#define LCD_LINE_2 (2)
#define MASK64     (0x40)
#define NOT_PRESSED (0)


void gps_parse()
{
    // GPS_rx_ring_rd is a counter into a circular buffer
    // GPS_rx_ring_wr is a counter into a circular buffer
    // ??perhaps as head/tail indexes??

    // the variables GPS_Lat[] and GPS_Long[]
    // are set/used locally, so probably should be local variables, on the stack
    char GPSlat[17] = {'\0'};
    char GPSlong[17] = {'\0'};
    strcat(GPSlat, "lat ");
    strcat(GPSlong, "Long");

    // start GPS sending NMEA messages
    gps_write();

    // initialize the LCD display
    lcd_out("                ", LCD_LINE_1);
    lcd_out("                ", LCD_LINE_2);
    lcd_out(GPSlat, LCD_LINE_1);
    lcd_out(GPSlong, LCD_LINE_2);

    SW2Press = NOT_PRESSED; // global variable

    char GPS_data[64];
    int  i = 0;

    while(!SW2Press)
    { // while waiting for button press

        // following assumes that GPS_rx_ring_rd
        // points to next char to consume
        // and should be initialized to same value as GPS_rx_ring_wr


        if ( GPS_rx_ring_rd != GPS_rx_ring_wr )
        { // then, next char(s) has been input

            // code needs to increment/wrap GPS_rs_ring_rd
            // even when current char is NOT a '$'
            // note: always put literal on left so compiler can catch
            // '=' rather than '=='


            // only when current char is a '$'
            // should following code be executed

            if ( '$' == GPS_Char_Rx[GPS_rx_ring_rd] )
            {   // then, current checked char is '$'
                // I.E. start of a new GPS message

                GPS_rx_ring_rd++; // step past '$'
                GPS_rx_ring_rd = (GPS_rx_ring_rd>= MASK64)? GPS_rx_ring_rd : 0;


                // clear old trash from extracted buffer
                memset( GPS_data, 0x00, sizeof(GPS_data) );

                // for next 64 bytes input, extract bytes from
                // circular buffer to array
                // the assumption that each GPS message actually 64 bytes long
                // is not true
                // far better to stop extracting when 'next' char
                // to extract is '$'
                for ( i = 0; i < sizeof(GPS_data); i++)
                {
                    // wait for next available char from GPS??
                    while(GPS_rx_ring_rd == GPS_rx_ring_wr);

                    if( '$' == GPS_Char_Rx[GPS_rx_ring_rd])
                    { // then, all of current GPS message is extracted
                        break;
                    }

                    // extract next GPS char from circular buffer
                    GPS_data[i] = GPS_Char_Rx[GPS_rx_ring_rd];


                    // make special case not special
                    // if at end of circular buffer, wrap to beginning.
                    GPS_rx_ring_rd++;
                    GPS_rx_ring_rd = (GPS_rx_ring_rd>= MASK64)? GPS_rx_ring_rd : 0;
                } // end for


                if(    GPS_data[0] == 'G'
                    && GPS_data[1] == 'P'
                    && GPS_data[2] == 'R'
                    && GPS_data[3] == 'M'
                    && GPS_data[4] == 'C')
                { // then, GPS message type GPRMC - minimum recommended data

                    // for SIRF chip set
                    // $GPRMC,120557.916,A,5058.7456,N,00647.0515,E,0.00,82.33,220503,,*39

                    if(GPS_data[17] == 'A')
                    { // then, full data set available

                        //Fill in latitude
                        GPSlat[4]  = GPS_data[19]; //degree - range 0...90
                        GPSlat[5]  = GPS_data[20]; //degree
                        GPSlat[6]  = GPS_data[21]; //minute - range 0...59
                        GPSlat[7]  = GPS_data[22]; //minute
                        GPSlat[8]  = GPS_data[23]; //decimal point
                        GPSlat[9]  = GPS_data[24]; //fraction of minute
                        GPSlat[10] = GPS_data[25]; //fraction of minute
                        GPSlat[11] = GPS_data[26]; //fraction of minute
                        GPSlat[12] = GPS_data[27]; //fraction of minute
                        GPSlat[15] = GPS_data[29]; // N or S or blank

                        //Fill in longitude
                        GPSlong[4]  = GPS_data[31]; //degree - range 0...360
                        GPSlong[5]  = GPS_data[32]; //degree
                        GPSlong[6]  = GPS_data[33]; //degree
                        GPSlong[7]  = GPS_data[34]; //minute - range 0...59
                        GPSlong[8]  = GPS_data[35]; //minute
                        GPSlong[9]  = GPS_data[36]; //decimal point
                        GPSlong[10] = GPS_data[37]; //fraction of minute
                        GPSlong[11] = GPS_data[38]; //fraction of minute
                        GPSlong[12] = GPS_data[39]; //fraction of minute
                        GPSlong[13] = GPS_data[40]; //fraction of minute
                        GPSlong[15] = GPS_data[42]; // E or W or blank
                    }
                    else
                    { // full data set not available

                        GPSlat[15]  = '?'; // use old data and N/S byte indicates old data
                        GPSlong[15] = '?'; // use old data and E/W byte indicates old data
                    }

                    // update the LCD display
                    lcd_out(GPSlat, LCD_LINE_1);
                    lcd_out(GPSlong, LCD_LINE_2);
                }
                else
                { // else not a $GPRMC message

                    ; // do nothing
                } // end if
            }
            else
            { // else, step to next char

                ; do nothing
            } // end if
        } // end if

        // make special case to be not special
        // if at end of circular buffer, wrap to beginning
        GPS_rx_ring_rd++;
        GPS_rx_ring_rd = (GPS_rx_ring_rd>= MASK64)? GPS_rx_ring_rd : 0;
    } // end while
} // end gps_parse