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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
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
What's
By the way, use code tags while posting your code.
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.
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.
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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.
...
...
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
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Originally Posted by m24r
Thanks, I had not pionted "glblclrtab". This is just an array to store the color values from file.
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.
When you think you have done a lot, then be ready for YOUR downfall.
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.
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.
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:
The malloc functions allocates [icode]sizeof(3*tmp)[/code] bytes.
The C Standard:
Suppose the variable tmp is int, integer literal 3 is int by definition. Therefore a type of an expression
If you want allocate tmp*3 bytes, write
3. What's a horrible name - glblclrtab! Try to pronounce it 
...
[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)
The C Standard:
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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.
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)
... I made the changes as:
I could eliminate the error that was coming from free(). The program nolonger aborts.
However, I still do not get the values as stored.
c Syntax (Toggle Plain Text)
However, I still do not get the values as stored.
Last edited by m24r; Dec 23rd, 2008 at 3:34 pm.
I made the changes as:
I could eliminate the error that was coming from free(). The program nolonger aborts.
However, I still do not get the values as stored.
c Syntax (Toggle Plain Text)
However, I still do not get the values as stored.
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