CODEWITHSUNDEEP
cassembly
Marco
Marco

Reputation: 707

Assembly function terminate but don't get result

I have a C program which calls an assembly function. The assembly function receives two arrays of char and an array to use as output.

The function checks all the characters of the first string and substitutes all characters which are present in the second string with '0' and save the modified string in the 3rd parameter. Finally the function has to check the modified string and return in EAX the address of the first character which differs from '0'.

Here's the C code:

char *subchar(char *str, char *delchar, char *subbedString);

int main()
{
  char a[60];
  char t[60];

  printf("\nInserisci un a stringa (max. 50): ");
  fgets(a, 50, stdin);
  a[strlen(a)-1]=0;      

  printf("\nStringa con i caratteri da eliminare (max. 50): ");
  fgets(t, 50, stdin);
  t[strlen(t)-1]=0;      

  printf("\nHai inserito: \" %s \"\nDa cui eliminare i caratteri: \" %s \"\n",a,t);

  char *sub = (char*)malloc(60*sizeof(char));

  sub = subchar(a,t,sub);
  printf("\nStringa 'a' dopo la sostituzione: %s\n",sub);

  return 0;
}

and here the assembly code:

global subchar

subchar:
    PUSH    EBP
    MOV EBP,ESP

    MOV ESI,[EBP +8]    ;first parameter, string to modify
    MOV ECX,[EBP +12]   ;second parameter, string to get eliminatio char
    MOV EDI,[EBP +16]   ;destination string

    CLD         ;reset direction flag

t_char: 
    PUSH    ESI     ;save ESI register, (first string)
    MOV ESI, ECX
    LODSB
    CMP AL,0        ;if second string is finished I jump to the 
    JE  not_null    ;recover the first not null character
    MOV DL,AL       ;character to delete
    MOV ECX,ESI
    POP ESI     ;recover of the first string
    JMP     sub

sub:    LODSB
    CMP AL,0        ;if I am at the end of string I check for
    JE  fine        ;the next character of the second string
    CMP AL,DL       ;if character matches
    JE  fill0       ;jump to the substitution of the character
    JMP save        ;then jmp to the storing of the character
fill0:  MOV AL,'0'      ; changing the value of AL con '0'
save:   STOSB           ;saving the character
    JMP sub     ;then restar the loop

not_null:   MOV ESI,EDI     ;recover the first string
load_char:  LODSB
        CMP AL,0        ;if the value is not null I put this address as return 
        JNE fine
        LOOP    load_char   ;else restart loop until I found a not null character


fine:   MOV EAX,ESI     ;return the address of the first not null character
    LEAVE
    RET

The problem of mine is that the modified string in the output is always null and I don't know where I've made the error.

Upvotes: 0

Views: 79

Answers (1)

user781847
user781847

Reputation:

You made an error here 

CMP AL,0        ;if I am at the end of string I check for
JE  fine        ;the next character of the second string

Once you finished scanning the input string you have to increment ECX and jump back to t_char.

Instead at the end of the input string your code jumps to the end of the routine, since LODSB post increments ESI, this register points one byte after the string to modify.

Upvotes: 1

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