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Strdup function is eating a letter

This is a discussion on Strdup function is eating a letter within the C Programming forums, part of the General Programming Boards category; Hey guys, I have this piece of code : Code: printf("%s\n",cmds1); ficheiro = strdup(cmds1); aux = insereFicheiro(ficheiro,aux); printf("\n *** Ficheiro ...

  1. #1
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    Strdup function is eating a letter

    Hey guys,
    I have this piece of code :

    Code:
     printf("%s\n",cmds1);
     		ficheiro = strdup(cmds1);
      		aux = insereFicheiro(ficheiro,aux);
      		printf("\n *** Ficheiro %s adicionado com sucesso! ***\n\n",ficheiro);
    And the output is the following :

    Code:
    cao
    
    
     *** Ficheiro 
    ao adicionado com sucesso! ***
    Any idea why strdup is eating a letter from the word "cao" ?

    Thanks!

  2. #2
    C++ Witch laserlight's Avatar
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    Post the smallest and simplest compilable program that demonstrates this problem.
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  3. #3
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    To be able to compile this you would need both my server and client code.
    I'll show you the full function code :

    Code:
    void filemon(char *cmds, char *cmds1)
    {       NFicheiros aux;
    	aux=cria();
            char *ficheiro = NULL;
    	if(strncmp(cmds,"addfile",7)== 0)
     	{       printf("%s\n",cmds1);
     		ficheiro = strdup(cmds1);
      		aux = insereFicheiro(ficheiro,aux);
      		printf("\n *** Ficheiro %s adicionado com sucesso! ***\n\n",ficheiro);
                    showLib(aux);
    		//daemon1(cmds1);
    	}
    It prints well cmds1 but when printing ficheiro it only prints the two last letters.

    Any idea what it might be ?

  4. #4
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by DeanWinchester
    To be able to compile this you would need both my server and client code.
    Hence you should come up with the smallest and simplest compilable program that demonstrates this problem.

    Quote Originally Posted by DeanWinchester
    Any idea what it might be ?
    You probably have a buffer overflow problem somewhere.
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  5. #5
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    Thing is,
    if I instead write this piece of code :
    Code:
    void filemon(char *cmds, char *cmds1)
    {       NFicheiros aux;
    	aux=cria();
            char *ficheiro = NULL;
    	if(strncmp(cmds,"addfile",7)== 0)
     	{       printf("%s\n",cmds1);
     		ficheiro = strdup(cmds1);
    		printf("%s\n",ficheiro);
      		aux = insereFicheiro(ficheiro,aux);
    		printf("%s\n",ficheiro);
      		printf("\n *** Ficheiro %s adicionado com sucesso! ***\n\n",ficheiro);
                    showLib(aux);
    		//daemon1(cmds1);
    	}
    }

    It works fine.
    Look at the output :
    Code:
    cao
    cao
    cao
    
    
     *** Ficheiro cao adicionado com sucesso! ***
    
    
    Os ficheiros a serem monitorizados neste momento são : 
    cao
    Isn't it strange ?

  6. #6
    Master Apprentice phantomotap's Avatar
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    O_o

    You have a bug somewhere in the ether when adding unrelated code to a function where a bug is first visible seems to change the behavior.

    My guess would be `insereFicheiro'.

    Soma

  7. #7
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    Hmm, thanks for your help phantom.
    I was just looking at my insereFicheiro code and there shouldn't be a problem because these functions (insereFicheiro,removeFicheiro) are all coming from an include of "array.c" which I tested myself adding files, making prints and they all worked fine.

    Take a look at the function insereFicheiro :

    Code:
    NFicheiros insereFicheiro(char *ficheiro, NFicheiros aux) {
            int d;
    	Ficheiro novo = (Ficheiro)malloc(sizeof (Node));
    	novo->ficheiro=(char*)malloc(sizeof(ficheiro)+1);
    	novo->nbits= read(d,ficheiro,strlen(ficheiro));
    	novo->proximo=NULL;
    	novo->ficheiro=strdup(ficheiro);
    	if(aux.lista==NULL)
    	{
    	aux.lista=novo;
    	aux.numFicheiros=1;
    	}
    	else
    	{	
    		novo->proximo=aux.lista;
    		aux.lista=novo;
    		aux.numFicheiros++;
    	}
    	return aux;
    }
    Thanks!

  8. #8
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    strdup() does not eat a character, something else you wrote does. Try adding this:

    Code:
            ficheiro = strdup(cmds1);
            printf("cmds1: %s ficheiro: %s\n", cmds1, ficheiro);

    Btw, line 4 and 7 are wrong, you are allocating sizeof(char*)+1 (probably 5 or 9 bytes), then you overwrite that in line 7 with new memory created by strdup.
    Last edited by Subsonics; 05-31-2012 at 04:57 AM.

  9. #9
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    Quote Originally Posted by Subsonics View Post
    strdup() does not eat a character, something else you wrote does. Try adding this:

    Code:
            ficheiro = strdup(cmds1);
            printf("cmds1: %s ficheiro: %s\n", cmds1, ficheiro);
    Thanks Subsonics, your advice worked just well.

    I got the following output :
    Code:
    cmds1: cao ficheiro: cao
    
    
    cmds1: cao ficheiro: 
    ao
    which means you were all correct.
    insereFicheiro ain't doing the job properly but which should that be, if I run it separated from the rest it works fine

  10. #10
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    The problem is most likely here :

    Code:
    Ficheiro novo = (Ficheiro)malloc(sizeof (Node));
    	novo->ficheiro=(char*)malloc(sizeof(ficheiro)+1);
    Since it's the first one to be added and most likely it's a problem of memory allocation

  11. #11
    Master Apprentice phantomotap's Avatar
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    O_o

    No. That's not really the right way of looking at it.

    Unless you are overwriting memory you don't own, allocating more chunks of memory doesn't invalidate an existing chunk.

    Just allocating more memory isn't sufficient to cause `strdup' to appear to eat part of the input.

    Soma

  12. #12
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    Indeed, makes sense Soma.
    What do you believe the problem is then ?

  13. #13
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    Managed to solve my problem.
    Instead of printing ficheiro, I started printing aux.lista->ficheiro

    Works fine, thanks for the help everyone!

  14. #14
    Master Apprentice phantomotap's Avatar
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    What do you believe the problem is then ?
    I have a very good idea, but without having all the code I can only speculate. O_o

    I don't have all that code, but even with all the code I'd still just tell you to do it yourself. ^_^

    Also, I don't speak whatever language in which the source and comments are written. That tends to make things difficult.

    You are using `sizeof' on a character pointer which is always going to be the same as (`sizeof') that of a void pointer. That's probably wrong.

    Listen though, you are getting caught up in details that aren't necessarily relevant. If you are lucky the problem is directly related to functions ultimately called from `filemon'; that's isn't necessarily the case. Don't trust what you think you know; you need to examine what you have. This is a great time to learn to use a debugger. Setup a "watch" watching the relevant memory and see what is really happening.

    Soma

  15. #15
    a_capitalist_story
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    Still refusing to learn to debug, instead expecting us to do the work for you. That's sad.

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