[SOLVED] puts(rubbish)...

jay73 · 07-21-2010, 07:26 PM

Yes, that's what it does for some reason. The code below prints garbage if I use puts() but it is fine if I use printf() instead (see sample of output at the bottom of this post).

Any idea what could be causing this? The odd thing is that the mess always starts after exactly eight properly printed characters. That sort of regularity can't be a coincidence, can it? It almost looks like an encoding issue (I'm using UTF-8) but if it really is, then I don't understand why the printf() is unaffected.

Code:

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

const char *nChars (const char *, int);

int main (int argc, char * argv[]){
    
    char * name ="Count Dracula++";    
    int len = strlen(name);

    if (argv[1] == 0)
        argv[1]="printf";
    printf("--->%d characters\n", len);        
    
    if (argc >= 2 && !strcmp(argv[1], "puts"))        
        for (int i = 1; i <= len; i++)
            puts(nChars(name, i));
    else
        for (int i = 1; i <= len; i++)    
            printf("%s\n", nChars(name, i));

    return 0;

}

const char * nChars (const char * strptr, int n){ //returns the next n characters of input

    char * ptr;
    char nChars[n+1];
    int slen = strlen(strptr);

    if (slen < n){                 // if the string is shorter than n chars, return it unmodified
        printf ("String too short. Returning %d.\n", slen);
        return strptr;
    } else {
        ptr = nChars;        
        for (int i = 0; i < n; i++)
            ptr[i]=strptr[i];
        ptr[n]='\0';             //append null character            
        return ptr;
    }
}

output of "nChars":

Quote:

C
Co
Cou
Coun
Count
Count
Count D
Count Dr
Count Dra
Count Drac
Count Dracu
Count Dracul
Count Dracula
Count Dracula+
Count Dracula++

output of "nChars puts":

Code:

Co
Cou
Coun
Count
Count 
Count D
Count Dr
Count Dr�
Count Dr�
Count Dr��
Count Dr���1
Count Dr���1
Count Dr���1�
Count Dr���1�

Sergei Steshenko · 07-21-2010, 07:35 PM

Quote:

Originally Posted by jay73

... Any idea what could be causing this? ...

What command line did you use to compile ?

YetAnotherDave · 07-21-2010, 07:41 PM

It looks to me like nChars is returning a pointer ( ptr ) to stack memory ( the nChars[n+1] char array) . This might not actually be the cause of the symptom you see but it's an "all bets are off" kind of problem. Your program will behave unpredictably until you fix it.

Sergei Steshenko · 07-21-2010, 07:45 PM

Quote:

Originally Posted by jay73

...

Code:

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

const char *nChars (const char *, int);

int main (int argc, char * argv[]){
    
    char * name ="Count Dracula++";    
    int len = strlen(name);

    if (argv[1] == 0)
        argv[1]="printf";
    printf("--->%d characters\n", len);        
    
    if (argc >= 2 && !strcmp(argv[1], "puts"))        
        for (int i = 1; i <= len; i++)
            puts(nChars(name, i));
    else
        for (int i = 1; i <= len; i++)    
            printf("%s\n", nChars(name, i));

    return 0;

}

const char * nChars (const char * strptr, int n){ //returns the next n characters of input

    char * ptr;
    char nChars[n+1];
    int slen = strlen(strptr);

    if (slen < n){                 // if the string is shorter than n chars, return it unmodified
        printf ("String too short. Returning %d.\n", slen);
        return strptr;
    } else {
        ptr = nChars;        
        for (int i = 0; i < n; i++)
            ptr[i]=strptr[i];
        ptr[n]='\0';             //append null character            
        return ptr;
    }
}

....

Why/what for do you have the line in red ?

jay73 · 07-21-2010, 07:45 PM

colorgcc -Wall -std=c99 nChars.c

I've tried plain "gcc nChars.c" but that doesn't make any difference.

Sergei Steshenko · 07-21-2010, 07:48 PM

Quote:

Originally Posted by YetAnotherDave

It looks to me like nChars is returning a pointer ( ptr ) to stack memory ( the nChars[n+1] char array) . This might not actually be the cause of the symptom you see but it's an "all bets are off" kind of problem. Your program will behave unpredictably until you fix it.

Yep. A pointer to heap allocated entity should have been used instead.

jay73 · 07-21-2010, 07:53 PM

Quote:

Why/what for do you have the line in red ?

I do not want to modify the original array so I create a new array (nChars) to copy the parts that I need. The ptr is a pointer to that new array. I guess I could return the array instead of the pointer that doesn't help.

jay73 · 07-21-2010, 07:58 PM

So you mean I'll need to use malloc? I'm vaguely aware of the concept since I dabbled some in C++ a year or two ago but I have a chapter or two to go in my C manual before I reach that part. Well, I'll put it aside for now and give it another try in another few days.

Sergei Steshenko · 07-21-2010, 08:07 PM

Quote:

Originally Posted by jay73

So you mean I'll need to use malloc? I'm vaguely aware of the concept since I dabbled some in C++ a year or two ago but I have a chapter or two to go in my C manual before I reach that part. Well, I'll put it aside for now and give it another try in another few days.

Stack variable seize existence when the scope housing them is left. Period.

If you do not want to use 'malloc', allocate the buffer pointer to which you want to return in the calling scope, pass the buffer as argument and return nothing, i.e. implement a void function.

jay73 · 07-21-2010, 08:27 PM

Quote:

If you do not want to use 'malloc', allocate the buffer pointer to which you want to return in thew calling scope, pass the buffer as argument and return nothing, i.e. implement a void function.

Good tips. Thanks!