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Dec 21st, 2008

pointer assignment

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I am a novice in C. I am trying to assign a value from a file to a pointer as below. However I am gettign a seg fault. Can somebody explain ?

int i1;
char *glblclrtab;
...

for (int i=0; i<20; i++)
{ while ( (i1= fgetc(ipFile))==NULL);
*(glblclrtab+i)= char (i1);
...
}

The pointer assignment in causing segmentation fault.

PLZ HELP
Thanks in Advance

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m24r

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Dec 21st, 2008

Re: pointer assignment

What's glblclrtab pointing to? It'll have a NULL value or a garbage value which is causing segmentation fault. After you make it point to a character array, you can use *(glblclrtab+i)= (char) i1; or glblclrtab[i]= (char) i1; for simplicity.

By the way, use code tags while posting your code.

devnar

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Dec 21st, 2008

Re: pointer assignment

char *glblclrtab; by itself will not allocate any memory to be use.
malloc() must be invoked to set apart as much memory as needed to hold the data you intend to assigned (point) to it.

Last edited by Aia; Dec 21st, 2008 at 4:00 pm.

Aia

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Dec 21st, 2008

Re: pointer assignment

Thanks, I had not pionted "glblclrtab". This is just an array to store the color values from file.

m24r

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Dec 23rd, 2008

Re: pointer assignment

Click to Expand / Collapse Quote originally posted by Aia ...

char *glblclrtab; by itself will not allocate any memory to be use.
malloc() must be invoked to set apart as much memory as needed to hold the data you intend to assigned (point) to it.

I am using malloc now, however when I read the data back it gives garbage data. Following is the part of the code:

...
...
glblclrtab= (char *)malloc(sizeof(3*tmp));

for (int i=0; i<(3*tmp); i++)
{
if (tmp2==3)tmp2=0;
i1=fgetc( ipFile );
if (tmp2==0) {*(glblclrtab + clrcnt) = (char)i1;}
else if (tmp2==1) {*(glblclrtab + tmp +clrcnt) = (char)i1;}
else if (tmp2==2) {*(glblclrtab + 2*tmp + clrcnt)= (char)i1;}

tmp2++;
}
free (glblclrtab);

When I use free as shown above, program dumps backtrace memory range in the end and aborts.

Thanks in advance

m24r

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Dec 23rd, 2008

Re: pointer assignment

Click to Expand / Collapse Quote originally posted by m24r ...

Thanks, I had not pionted "glblclrtab". This is just an array to store the color values from file.

You want 'glblclrtab' to be an array, but C compiler have no idea what your intentions are. So allocate memory before storing.
As arrays have predefined space in the memory, we do not allocate memory for them but in your case its need to be done manually.

You have no idea where 'glblclrtab' is pointing in the memory space. and neither anyone in the world have.

Luckychap

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Dec 23rd, 2008

Re: pointer assignment

That is is reason I used the malloc before storing in the for() statement. However, I cannot use array as the size is not predefined. I have to use it on the fly. I had assigned glblclrab as char pointer as:

char *glblclrtab;

I do not know what is the right way of using the pointer here.

Last edited by m24r; Dec 23rd, 2008 at 2:06 pm.

m24r

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Dec 23rd, 2008

Re: pointer assignment

1. Please, USE code tag for your snippets properly:
[code=c]
your source texts
[/code]
2. You don't understand operator sizeof functionality. See what happens in your (incorrect) memory allocation:

c Syntax (Toggle Plain Text)
glblclrtab= (char *)malloc(sizeof(3*tmp));
glblclrtab= (char *)malloc(sizeof(3*tmp));

The malloc functions allocates [icode]sizeof(3*tmp)[/code] bytes.
The C Standard:

Quote ...

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand.

Suppose the variable tmp is int, integer literal 3 is int by definition. Therefore a type of an expression tmp*3 is int. Now sizeof(tmp*3) == sizeof(int) . For example, you allocate 4 bytes only in 32-bit environment.
If you want allocate tmp*3 bytes, write

c Syntax (Toggle Plain Text)
glblclrtab= malloc(3*tmp); 
/* No need to convert void* to char* in C explicitly */
glblclrtab= malloc(3*tmp); 
/* No need to convert void* to char* in C explicitly */

3. What's a horrible name - glblclrtab! Try to pronounce it

...

ArkM

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Postaholic

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Dec 23rd, 2008

Re: pointer assignment

I made the changes as:

c Syntax (Toggle Plain Text)
 glblclrtab= malloc(3*tmp);
 glblclrtab= malloc(3*tmp);

I could eliminate the error that was coming from free(). The program nolonger aborts.

However, I still do not get the values as stored.

Last edited by m24r; Dec 23rd, 2008 at 3:34 pm.

m24r

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Dec 23rd, 2008

edited: pointer assignment

I made the changes as:

c Syntax (Toggle Plain Text)
 char *glblclrtab;
 glblclrtab= malloc(3*tmp); 
for (int i=0; i<(3*tmp); i++)
{ // effort to sort the data from file, size unknown (tmp), I know when I parse the file
if (tmp2==3)tmp2=0;
i1=fgetc( ipFile );
if (tmp2==0) {*(glblclrtab + clrcnt) = (char)i1;}
else if (tmp2==1) {*(glblclrtab + tmp +clrcnt) = (char)i1;}
else if (tmp2==2) {*(glblclrtab + 2*tmp + clrcnt)= (char)i1;}
tmp2++;
}
 
for (int i=0; i<(tmp); i++)
{ printf("%d::%d, %d, %d \n",i,*(glblclrtab+i),*(glblclrtab + tmp + i),*(glblclrtab+2*tmp+i));
// [U][B]I receive garbage values here[/B][/U]
}
free (glblclrtab);  // program nolonger aborts
 char *glblclrtab;
 glblclrtab= malloc(3*tmp); 
for (int i=0; i<(3*tmp); i++)
{ // effort to sort the data from file, size unknown (tmp), I know when I parse the file
if (tmp2==3)tmp2=0;
i1=fgetc( ipFile );
if (tmp2==0) {*(glblclrtab + clrcnt) = (char)i1;}
else if (tmp2==1) {*(glblclrtab + tmp +clrcnt) = (char)i1;}
else if (tmp2==2) {*(glblclrtab + 2*tmp + clrcnt)= (char)i1;}
tmp2++;
}

for (int i=0; i<(tmp); i++)
{ printf("%d::%d, %d, %d \n",i,*(glblclrtab+i),*(glblclrtab + tmp + i),*(glblclrtab+2*tmp+i));
// [U][B]I receive garbage values here[/B][/U]
}
free (glblclrtab);  // program nolonger aborts

I could eliminate the error that was coming from free(). The program nolonger aborts.

However, I still do not get the values as stored.

m24r

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