Thread: Realloc

Im trying to read in from a text file with an unknown amount of memory, its not reading in the entire text file though. Heres my output and snippet of code.

text file

Code:

Bryan has managed to be quite prolific and successful as bears go.
The eighteen known subspecies of Bryan can be found throughout the United States and Canada.
Estimates of the number of black Bryans in North America vary, with 750,000 being the most often suggested.
In the state of Pennsylvania there are believed to be more than 7000 of the animals scattered across the state.

output

Code:

[cat]$ ./cat -A test.txt
mals scattered across the state.$re are believed to be more than 7000 of the ani

code

Code:

        do
        {
                cap+=10;
                buf = realloc(buf,  sizeof(char) * cap);
                fread ( buf, sizeof( char ), cap, fptr );
        }
        while(!feof(fptr));

fread doesn't add to the end of what's already there, but overwrites what had been there (which is why you can see the end of the previous read, since the last read was "short").

so then i should put the buf into a temp, realloc then add the two together?

You could, I suppose. As long as you keep track of where everything is (and how much of everything is there), it'll work.

is there an easier way?

i changed it to this

Code:

        do
        {
                cap+=10;
                buf = realloc(buf,  sizeof(char) * cap);
                fread ( buf+cap-10, sizeof( char ), cap, fptr );
        }
        while(!feof(fptr));

and got this error
*** glibc detected *** ./cat: realloc(): invalid next size: 0x0000000015df1010 ***

The difference between your line and Neon's line is significant. You have added 10 characters to your buffer. You must not try to then read another 20 (or 30, or 40) characters into those new 10 spots, because, well, you've only got 10 new spots.

(And edit to clarify an additional possible piece of confusion: and also, fread doesn't go back to the beginning of the file each time; it will pick up where it left off.)

Code:

        do
        {
                cap+=10;
                buf = realloc(buf,  sizeof(char) * cap);
                fread ( buf+cap-10, sizeof( char ), 10, fptr );
                printf("%d\n", num_items++);
                printf("%s\n", buf);
        }
        while(!feof(fptr));

this is what i have now and when i run it, it goes through my loop 39 times buf each time it prints a blank line for buf....

Code:

[dodoherty@vulcan cat]$ ./cat -A test.txt
0

1

2

3

4

5

6

7

8

9

.
.
/*put period in to skip all the other numbers*/
.
.
.

36

37

38

Do you have a priming/preliminary read outside this loop? (And please tell me you remembered to set cap = 0 before the loop started, if not.)

actually i have cap set to 10, and no the first read is inside the loop

Make sure cap starts as 0, also num_items.
And make sure your buf pointer points to NULL (or points to valid malloc'ed memory) or the realloc will try and work with memory it might *not* be able to.

Also with your do/while you are going to run it one more time even after you hit the end of the file.

Dylan

ok everything seems to work fine now thank you, i have my cap intialized to 10. In a previous program i had written i had done pretty much the same thing but i was reading in from a binary file and all i was reading was and array of numbers. But thanks a lot.