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Null terminating character

Hi all,

I've this strange problem with the \0 character.
I'll show you the code first:

#include <REG52.H>

void test(char *s)
{
        int idx;

        for(idx = 0; *s != '\0'; idx++, s++)
        {
        }

        P1 = idx; // Led's are connected to P1 so i can see the idx value.
}

void main()
{
        test("12345");

        while(1);
}

Now the problem:
When i call the test function with 4 characters or less, the condition

*s != '\0'

does not work. The led's show me a value of 0x63 when i call the test function with 1, 2, 3 or 4 characters. There's no problem when i call the test function with 5 characters or more. The led's show me 0x05 with a call of 5 characters.

I've also tested this code with the SDCC compiler and the call with 4 characters or less did worked like it should.

I'm working with the C51 COMPILER V8.05a

Any ideas? Thanks!

Parents

0 Jon Ward over 18 years ago in reply to Jon Ward

Following is the generated code for a test string of 3 characters:

C51 COMPILER V8.05a, COMPILATION OF MODULE MAIN
OBJECT MODULE PLACED IN main.OBJ
COMPILER INVOKED BY: C:\Keil\C51\BIN\C51.EXE main.c DEBUG OBJECTEXTEND CODE SYMBOLS

   1          #include 
   2
   3          void test(char *s)
   4          {
   5   1              int idx;
   6   1
   7   1              for(idx = 0; *s != '\0'; idx++, s++)
   8   1              {
   9   2              }
  10   1
  11   1              P1 = idx; // Led's are connected to P1 so i can see the idx value.
  12   1      }
  13
  14          void main()
  15          {
  16   1              test("123");
  17   1
  18   1              while(1);
  19   1      }

ASSEMBLY LISTING OF GENERATED OBJECT CODE


             ; FUNCTION _test (BEGIN)
                                           ; SOURCE LINE # 3
;---- Variable 's' assigned to Register 'R1/R2/R3' ----
                                           ; SOURCE LINE # 4
                                           ; SOURCE LINE # 7
;---- Variable 'idx' assigned to Register 'R6/R7' ----
0000 E4                CLR     A
0001 FF                MOV     R7,A
0002 FE                MOV     R6,A
0003         ?C0001:
0003 120000      E     LCALL   ?C?CLDPTR
0006 600E              JZ      ?C0002
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 9
0008 0F                INC     R7
0009 BF0001            CJNE    R7,#00H,?C0008
000C 0E                INC     R6
000D         ?C0008:
000D 7401              MOV     A,#01H
000F 29                ADD     A,R1
0010 F9                MOV     R1,A
0011 E4                CLR     A
0012 3A                ADDC    A,R2
0013 FA                MOV     R2,A
0014 80ED              SJMP    ?C0001
0016         ?C0002:
                                           ; SOURCE LINE # 11
0016 8F90              MOV     P1,R7
                                           ; SOURCE LINE # 12
0018 22                RET
             ; FUNCTION _test (END)

             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 14
                                           ; SOURCE LINE # 15
                                           ; SOURCE LINE # 16
0000 7BFF              MOV     R3,#0FFH
0002 7A00        R     MOV     R2,#HIGH ?SC_0
0004 7900        R     MOV     R1,#LOW ?SC_0
0006 120000      R     LCALL   _test
0009         ?C0005:
                                           ; SOURCE LINE # 18
0009 80FE              SJMP    ?C0005
             ; FUNCTION main (END)

And following is the code for a test string of 5 characters:

C51 COMPILER V8.05a, COMPILATION OF MODULE MAIN
OBJECT MODULE PLACED IN main.OBJ
COMPILER INVOKED BY: C:\Keil\C51\BIN\C51.EXE main.c DEBUG OBJECTEXTEND CODE SYMBOLS

line level    source

   1          #include 
   2
   3          void test(char *s)
   4          {
   5   1              int idx;
   6   1
   7   1              for(idx = 0; *s != '\0'; idx++, s++)
   8   1              {
   9   2              }
  10   1
  11   1              P1 = idx; // Led's are connected to P1 so i can see the idx value.
  12   1      }
  13
  14          void main()
  15          {
  16   1              test("12345");
  17   1
  18   1              while(1);
  19   1      }

ASSEMBLY LISTING OF GENERATED OBJECT CODE


             ; FUNCTION _test (BEGIN)
                                           ; SOURCE LINE # 3
;---- Variable 's' assigned to Register 'R1/R2/R3' ----
                                           ; SOURCE LINE # 4
                                           ; SOURCE LINE # 7
;---- Variable 'idx' assigned to Register 'R6/R7' ----
0000 E4                CLR     A
0001 FF                MOV     R7,A
0002 FE                MOV     R6,A
0003         ?C0001:
0003 120000      E     LCALL   ?C?CLDPTR
0006 600E              JZ      ?C0002
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 9
0008 0F                INC     R7
0009 BF0001            CJNE    R7,#00H,?C0008
000C 0E                INC     R6
000D         ?C0008:
000D 7401              MOV     A,#01H
000F 29                ADD     A,R1
0010 F9                MOV     R1,A
0011 E4                CLR     A
0012 3A                ADDC    A,R2
0013 FA                MOV     R2,A
0014 80ED              SJMP    ?C0001
0016         ?C0002:
                                           ; SOURCE LINE # 11
0016 8F90              MOV     P1,R7
                                           ; SOURCE LINE # 12
0018 22                RET
             ; FUNCTION _test (END)

             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 14
                                           ; SOURCE LINE # 15
                                           ; SOURCE LINE # 16
0000 7BFF              MOV     R3,#0FFH
0002 7A00        R     MOV     R2,#HIGH ?SC_0
0004 7900        R     MOV     R1,#LOW ?SC_0
0006 120000      R     LCALL   _test
0009         ?C0005:
                                           ; SOURCE LINE # 18
0009 80FE              SJMP    ?C0005
             ; FUNCTION main (END)

Careful comparison between the test function will show that they are identical.

So, if it works with 5 characters or more but fails for 4 characters or less, the problem would not appear to come from the generated code.

Jon

Reply

0 Jon Ward over 18 years ago in reply to Jon Ward

Following is the generated code for a test string of 3 characters:

C51 COMPILER V8.05a, COMPILATION OF MODULE MAIN
OBJECT MODULE PLACED IN main.OBJ
COMPILER INVOKED BY: C:\Keil\C51\BIN\C51.EXE main.c DEBUG OBJECTEXTEND CODE SYMBOLS

   1          #include 
   2
   3          void test(char *s)
   4          {
   5   1              int idx;
   6   1
   7   1              for(idx = 0; *s != '\0'; idx++, s++)
   8   1              {
   9   2              }
  10   1
  11   1              P1 = idx; // Led's are connected to P1 so i can see the idx value.
  12   1      }
  13
  14          void main()
  15          {
  16   1              test("123");
  17   1
  18   1              while(1);
  19   1      }

ASSEMBLY LISTING OF GENERATED OBJECT CODE


             ; FUNCTION _test (BEGIN)
                                           ; SOURCE LINE # 3
;---- Variable 's' assigned to Register 'R1/R2/R3' ----
                                           ; SOURCE LINE # 4
                                           ; SOURCE LINE # 7
;---- Variable 'idx' assigned to Register 'R6/R7' ----
0000 E4                CLR     A
0001 FF                MOV     R7,A
0002 FE                MOV     R6,A
0003         ?C0001:
0003 120000      E     LCALL   ?C?CLDPTR
0006 600E              JZ      ?C0002
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 9
0008 0F                INC     R7
0009 BF0001            CJNE    R7,#00H,?C0008
000C 0E                INC     R6
000D         ?C0008:
000D 7401              MOV     A,#01H
000F 29                ADD     A,R1
0010 F9                MOV     R1,A
0011 E4                CLR     A
0012 3A                ADDC    A,R2
0013 FA                MOV     R2,A
0014 80ED              SJMP    ?C0001
0016         ?C0002:
                                           ; SOURCE LINE # 11
0016 8F90              MOV     P1,R7
                                           ; SOURCE LINE # 12
0018 22                RET
             ; FUNCTION _test (END)

             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 14
                                           ; SOURCE LINE # 15
                                           ; SOURCE LINE # 16
0000 7BFF              MOV     R3,#0FFH
0002 7A00        R     MOV     R2,#HIGH ?SC_0
0004 7900        R     MOV     R1,#LOW ?SC_0
0006 120000      R     LCALL   _test
0009         ?C0005:
                                           ; SOURCE LINE # 18
0009 80FE              SJMP    ?C0005
             ; FUNCTION main (END)

And following is the code for a test string of 5 characters:

C51 COMPILER V8.05a, COMPILATION OF MODULE MAIN
OBJECT MODULE PLACED IN main.OBJ
COMPILER INVOKED BY: C:\Keil\C51\BIN\C51.EXE main.c DEBUG OBJECTEXTEND CODE SYMBOLS

line level    source

   1          #include 
   2
   3          void test(char *s)
   4          {
   5   1              int idx;
   6   1
   7   1              for(idx = 0; *s != '\0'; idx++, s++)
   8   1              {
   9   2              }
  10   1
  11   1              P1 = idx; // Led's are connected to P1 so i can see the idx value.
  12   1      }
  13
  14          void main()
  15          {
  16   1              test("12345");
  17   1
  18   1              while(1);
  19   1      }

ASSEMBLY LISTING OF GENERATED OBJECT CODE


             ; FUNCTION _test (BEGIN)
                                           ; SOURCE LINE # 3
;---- Variable 's' assigned to Register 'R1/R2/R3' ----
                                           ; SOURCE LINE # 4
                                           ; SOURCE LINE # 7
;---- Variable 'idx' assigned to Register 'R6/R7' ----
0000 E4                CLR     A
0001 FF                MOV     R7,A
0002 FE                MOV     R6,A
0003         ?C0001:
0003 120000      E     LCALL   ?C?CLDPTR
0006 600E              JZ      ?C0002
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 9
0008 0F                INC     R7
0009 BF0001            CJNE    R7,#00H,?C0008
000C 0E                INC     R6
000D         ?C0008:
000D 7401              MOV     A,#01H
000F 29                ADD     A,R1
0010 F9                MOV     R1,A
0011 E4                CLR     A
0012 3A                ADDC    A,R2
0013 FA                MOV     R2,A
0014 80ED              SJMP    ?C0001
0016         ?C0002:
                                           ; SOURCE LINE # 11
0016 8F90              MOV     P1,R7
                                           ; SOURCE LINE # 12
0018 22                RET
             ; FUNCTION _test (END)

             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 14
                                           ; SOURCE LINE # 15
                                           ; SOURCE LINE # 16
0000 7BFF              MOV     R3,#0FFH
0002 7A00        R     MOV     R2,#HIGH ?SC_0
0004 7900        R     MOV     R1,#LOW ?SC_0
0006 120000      R     LCALL   _test
0009         ?C0005:
                                           ; SOURCE LINE # 18
0009 80FE              SJMP    ?C0005
             ; FUNCTION main (END)

Careful comparison between the test function will show that they are identical.

So, if it works with 5 characters or more but fails for 4 characters or less, the problem would not appear to come from the generated code.

Jon

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