double free or corruption(fasttop) error/segmentation fault in C

Question

I'm trying to dynamically allocate an array to read user input from the command line. It works 99/100 times, but if I type in a bunch of characters repeatedly I will sometimes get a segmentation fault error OR a double free or corruption(fasttop) error. This error is relatively hard to reproduce.

I'm pretty sure the error occurs because of the way I'm reallocating the array.

while(1){
        char *buf_in;               // Holds user keyboard input
        int cnt = 0, length = 0;    // cnt stores current read buffer size, length allows input into buf_in
        char ch;
        int buf_max = 64;           // Current buffer size. Dynamically allocated

        buf_in = malloc(buf_max * sizeof(char));
        if (buf_in==NULL){
            fprintf(stderr,"Error allocating memory!\n");
            exit(EXIT_FAILURE);
        }

        do{
            if (cnt > (buf_max/2)){
                cnt = 0;
                buf_max *= 2; // Double size of buffer
                printf("Doubling buffer: %d\n",buf_max);
                buf_in = realloc(buf_in,buf_max);
                if (buf_in == NULL){
                    fprintf(stderr,"Error re-allocating memory!\n");
                    exit(EXIT_FAILURE);
                }
            }
            /* Store line-by-line into buffer */
            ch = getc(stdin);
            buf_in[length] = ch;
            length++;
            cnt++;
        }while(ch != '\n');

        /* Handles different option arguments */
        processOptions(buf_in,&opt_n_inc);

        // stdout
        fprintf(stdout,"%s",buf_in);
        fflush(stdout);

        free(buf_in);
        buf_in=NULL;
    }

You are `realloc`ating the array in a bad way, it's not wrong though! can you post `processOptions()`? — Iharob Al Asimi, Jan 24 '15 at 23:30
Thanks for the help. What's a clean way to reallocate it? I think I fixed it by saying buf_in[length] = '\0' I also run into this error if I make the buf_max bound smaller: realloc(): invalid next size — Jamie Yang, Jan 25 '15 at 01:46
just don't overwrite the previous pointer unless `realloc` returns a valid pointer. — Iharob Al Asimi, Jan 25 '15 at 01:53
Thanks for the tip. I added a temp_alloc pointer, and then wrote its contents to buf_in and freed temp_alloc on success. — Jamie Yang, Jan 25 '15 at 02:34
No, don't `free` it... Do this `void *tmp; tmp = realloc(buf_in, buf_max); if (tmp == NULL) abortExecutionHere(); buf_in = tmp;` — Iharob Al Asimi, Jan 25 '15 at 02:36
Right because its an realloc, so we don't have to free it right? Why should I do void? Is it OK if I do char *tmp? — Jamie Yang, Jan 25 '15 at 02:50
@Jamie Yang Either `void *tmp` or `char *tmp` work equally well here. — chux - Reinstate Monica, Jan 25 '15 at 03:23

chux - Reinstate Monica · Accepted Answer · 2015-01-24T23:59:11.280

1

Code appears to be attempting to print using "%s" an array of char and not a string. The null character '\0' termination is missing.

Also the problem may be manifesting itself in processOptions() as that function call does not pass the length of valid data.

buf_in[length] = ch;

// Add    
buf_in[length+1] = '\0';

...
processOptions(buf_in,&opt_n_inc);
fprintf(stdout,"%s",buf_in);

Note: infinite loop should getc(stdin) return EOF. Better to use

int ch = getc(stdin);
if (ch == EOF) break;
buf_in[length] = ch;

edited Jan 24 '15 at 23:59

answered Jan 24 '15 at 23:53

chux - Reinstate Monica

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Thanks for the response. In this case, I'm trying to mimic the behavior of cat in bash. The infinite loop is meant to never terminate (only terminating on an interrupt signal from the keyboard). Do I have to free buffer on a termination signal? I'm worried about memory leaks. – Jamie Yang Jan 25 '15 at 01:34
@Jamie Yang 1) Code has 2 infinite loops: `while(1){` which is certainly the one in your comment. This answer points out a 2nd infinite loop. 2) Code does not need to free buffers on a "termination signal". When this program is done, the OS will get all its memory back. – chux - Reinstate Monica Jan 25 '15 at 02:22

double free or corruption(fasttop) error/segmentation fault in C

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