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malloc + Null pointer error.

Posted on 2003-12-05
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Last Modified: 2010-04-15
This is one of my first times toying with malloc, so I'm probably overlooking something really simple.  Here are the relevant lines to the problem at hand...

typedef struct Racer{
      char* sName;
      sh_int oddsFor;
      sh_int oddsAgainst;
      
} RCR_MDL;

typedef struct ChariotOrganization{
      int timer;
      int numberOfRacers;
      RCR_MDL* RacerList;
} CHRT_MDL;

int main(){
    ...
    chariot = alloca(sizeof(CHRT_MDL));
    chariot->RacerList = alloca(10 * sizeof(RCR_MDL)); //max 10 racers
    chariot->numberOfRacers = 0;                                 //default it to 0
    addRacer("The King's Centurion", 3, 2);
    addRacer("The Amazon", 2, 1);
    addRacer("The Jackal", 3, 1);
    addRacer("The White Rabbit", 5, 1);
    addRacer("The Village Idiot", 10, 1);
    ...
}

void addRacer(char* racerName, int oddsAgainst, int oddsFor){
       int racerNumber = chariot->numberOfRacers++;
      chariot->RacerList[racerNumber].sName = malloc(strlen(racerName));
       //--------------------------------------------------------------//
      printf(""); //WHY? <-------------------- problem line
       //--------------------------------------------------------------//
        strcpy( chariot->RacerList[racerNumber].sName, racerName); <--- generates NULL pointer error
      chariot->RacerList[racerNumber].oddsAgainst = oddsAgainst;
      chariot->RacerList[racerNumber].oddsFor = oddsFor;
}

My issue is that I must have that printf statement there for the program to operate properly.  It doesn't matter what I print out, so long as I use printf at some point between the allocation and strcpy.  The error occurs while adding the fifth name, and I've traced it to the strcpy line in the addRacer function.  It complains that it can't write to 0x00000000, hence malloc returns a NULL pointer, yet when I added a printf statement to validate this, the program immediately started working again as planned, without the NULL pointer errors.  Any guidance is much appreciated.
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Question by:Gratch06
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by:sunnycoder
ID: 9887186
>  chariot = alloca(sizeof(CHRT_MDL));
>   chariot->RacerList = alloca(10 * sizeof(RCR_MDL)); //max 10 racers
any special reasons for using alloca ?

>chariot->RacerList[racerNumber].sName = malloc(strlen(racerName));
should be
chariot->RacerList[racerNumber].sName = (char *) malloc(strlen(racerName));

>It complains that it can't write to 0x00000000, hence malloc returns a NULL pointer, yet when I added a printf statement to
>validate this, the program immediately started working again as planned, without the NULL pointer errors.
It is always good to check return value from malloc .. It is not very wise to skip such error checks

Why printf solves the problem is really strange ... unless you have multithreaded application with race conditions...
could it be some compiler optimization gone wrong ... thin chance for that though but it is a possibility
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by:Gratch06
ID: 9887277
>any special reasons for using alloca ?
adjusted main:

    chariot->RacerList = (RCR_MDL *)malloc(10 * sizeof(RCR_MDL));
    chariot->numberOfRacers = 0;
      addRacer("The King's Centurion", 3, 2);
      addRacer("The Amazon", 2, 1);
      addRacer("The Jackal", 3, 1);
      addRacer("The White Rabbit", 5, 1);
      addRacer("The Village Idiot", 10, 1);

> chariot->RacerList[racerNumber].sName = (char *) malloc(strlen(racerName));
> It is always good to check return value from malloc .. It is not very wise to skip such error checks

Adjusted addRacer function:

void addRacer(char* racerName, int oddsAgainst, int oddsFor){
       int racerNumber = chariot->numberOfRacers++;
      if(!(chariot->RacerList[racerNumber].sName = (char *) malloc(strlen(racerName)))){
           printf("Invalid allocation in addRacer function");
           return;
       }
        printf(""); //WHY?
        strcpy( chariot->RacerList[racerNumber].sName, racerName);
      chariot->RacerList[racerNumber].oddsAgainst = oddsAgainst;
      chariot->RacerList[racerNumber].oddsFor = oddsFor;
}


The identical error still exists in this code.  It hangs up on the fifth call, with the printf(""); line out, but with it in, runs perfectly.

>Why printf solves the problem is really strange ... unless you have multithreaded application with race conditions...

Nothing near that complex.  It's just a very simple routine at this stage, and this is my starting point.

>could it be some compiler optimization gone wrong ... thin chance for that though but it is a possibility
I'm using the Dev C++ v. 4.8.9.0 compiler currently.  I tried compiling it using gcc on a linux box, and that compiled and ran perfectly both with and without the printf(""); statement.
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by:sunnycoder
ID: 9887296
>I'm using the Dev C++ v. 4.8.9.0 compiler currently.  I tried compiling it using gcc on a linux box, and that compiled and ran
>perfectly both with and without the printf(""); statement.
In that case it is in all probabilities a compiler/library specific issue ... try turning off any optimizations
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Author Comment

by:Gratch06
ID: 9887306
>In that case it is in all probabilities a compiler/library specific issue ... try turning off any optimizations
I've compiled it with both full optimizations and no optimizations (as much as I am able to specify) with no change of behavior.
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Accepted Solution

by:
sunnycoder earned 70 total points
ID: 9887312
>chariot->RacerList[racerNumber].sName = (char *) malloc(strlen(racerName));
to
chariot->RacerList[racerNumber].sName = (char *) malloc(strlen(racerName) + 1);


try compiling this

int main ()
{
     char * a = "abcd";
     char * b = malloc ( strlen (a) + 1 );
     strcpy ( b, a );
     printf ( "%s\n", b );
     free(b);
}
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Author Comment

by:Gratch06
ID: 9887674
>chariot->RacerList[racerNumber].sName = (char *) malloc(strlen(racerName) + 1);

That did the trick.  It makes sense to me now that the strlen would give the number of chars, not including the null terminating char at the end, but in copying, it would do the null-terminating character as well.  I'm curious to know why trying to write past the end of a char array would be trying to access null instead of some other location in memory.

>try compiling this

>int main ()
>{
>    char * a = "abcd";
>     char * b = malloc ( strlen (a) + 1 );
>     strcpy ( b, a );
>     printf ( "%s\n", b );
>     free(b);
>}

Now I'm all confuzzled...this works fine both with and without the + 1...go figure.  Anyway, I understand the flaw in my logic, and I have a solution that satisfies me, so points are now awarded.  Thanks for the help sunnycoder.
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Expert Comment

by:sunnycoder
ID: 9887790
>I'm curious to know why trying to write past the end of a char array would be trying to access null instead of some other
>location in memory.
There has to be some inaccuracy somewhere ... it is illegal access violation

>Now I'm all confuzzled...this works fine both with and without the + 1...go figure.
access is still illegal ... just that it is not offending some other process ... its chance that it is getting through not logic :o)
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