Program has no errors but still crashes

Question

This code works fine and also produces the required output, but it results to this window popping-up in the end.

As it can be seen, no errors or warnings are mentioned. I wish to rectify this. It is definitely not stuck in a loop, since it properly returns the required value. What could be the reason behind this issue? Please help.

Here's the code:

#include <assert.h>
#include <limits.h>
#include <math.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define size 10000

int max(int a, int b)
{
    if(a>b)
        return a;
    return b;
}

int find_subseq(char *a, char *b)
{
int i, j, la = strlen(a), lb = strlen(b);
int *arr = (int *)malloc(((la+1)*(lb+1)*sizeof(char)));

for(i = 0; i < la; i+=(lb+1))
{
    *(arr+i) = 0;
}

for(j = 0; j < lb+1; j++)
{
    *(arr+j) = 0;
}

for(i = 1; i <= la; i++)
{
    for(j = 1; j <= lb; j++)
    {
        if(*(a+i) == *(b+j))
            *(arr+i*lb+j) = *(arr+i*(lb-1)+(j-1)) + 1;
        else
            *(arr+i*lb+j) = max(*(arr+i*(lb-1)+(j)),*(arr+i*(lb)+(j-1)));
    }
}
return *(arr+la*lb+lb);
}

int main()
{
    char* a = (char *)malloc(size*sizeof(char));
    scanf("%[^\n]%*c",a);

    char* b = (char *)malloc(size*sizeof(char));
    scanf("%[^\n]%*c",b);

    printf("%d",strlen(a) + strlen(b) - (2 * find_subseq(a,b)));

    return 0;
}

later I checked on a different compiler, it gives this error:

solution: malloc.c:2394: sysmalloc: Assertion `(old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)' failed.

GDB trace:
Reading symbols from solution...done.
[New LWP 10243]
Core was generated by `solution'.
Program terminated with signal SIGABRT, Aborted.
#0  0x00007f622e4b2428 in __GI_raise (sig=sig@entry=6)
    at ../sysdeps/unix/sysv/linux/raise.c:54
#0  0x00007f622e4b2428 in __GI_raise (sig=sig@entry=6)
    at ../sysdeps/unix/sysv/linux/raise.c:54
#1  0x00007f622e4b402a in __GI_abort () at abort.c:89
#2  0x00007f622e4fa2e8 in __malloc_assert (
    assertion=assertion@entry=0x7f622e60e190 "(old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)", 
    file=file@entry=0x7f622e60abc5 "malloc.c", line=line@entry=2394, 
    function=function@entry=0x7f622e60e9d8 <__func__.11509> "sysmalloc")
    at malloc.c:301
#3  0x00007f622e4fe426 in sysmalloc (nb=nb@entry=4112, 
    av=av@entry=0x7f622e841b20 <main_arena>) at malloc.c:2391
#4  0x00007f622e4ff743 in _int_malloc (
    av=av@entry=0x7f622e841b20 <main_arena>, bytes=bytes@entry=4096)
    at malloc.c:3827
#5  0x00007f622e501184 in __GI___libc_malloc (bytes=bytes@entry=4096)
    at malloc.c:2913
#6  0x00007f622e4ea1d5 in __GI__IO_file_doallocate (
    fp=0x7f622e842620 <_IO_2_1_stdout_>) at filedoalloc.c:127
#7  0x00007f622e4f8594 in __GI__IO_doallocbuf (
    fp=fp@entry=0x7f622e842620 <_IO_2_1_stdout_>) at genops.c:398
#8  0x00007f622e4f78f8 in _IO_new_file_overflow (
    f=0x7f622e842620 <_IO_2_1_stdout_>, ch=-1) at fileops.c:820
#9  0x00007f622e4f628d in _IO_new_file_xsputn (
    f=0x7f622e842620 <_IO_2_1_stdout_>, data=0x7ffe22f98f77, n=1)
    at fileops.c:1331
#10 0x00007f622e4cae00 in _IO_vfprintf_internal (
    s=0x7f622e842620 <_IO_2_1_stdout_>, format=<optimized out>, 
    format@entry=0x4008dd "%zu", ap=ap@entry=0x7ffe22f98fc8)
    at vfprintf.c:1631
#11 0x00007f622e5939ef in ___printf_chk (flag=flag@entry=1, 
    format=format@entry=0x4008dd "%zu") at printf_chk.c:35
#12 0x000000000040062b in printf (__fmt=0x4008dd "%zu")
    at /usr/include/x86_64-linux-gnu/bits/stdio2.h:104
#13 main () at solution.c:56

What does this mean? Is this the reason for crash?

Answer 1

First thing always free() allocated memory. In your main() function you didn't free array a and array b. This is not an error but it is bad for your program to leave it like that

Second, in your find_subseq function when you are allocating memory for array arr you can not say int* array = malloc(... *sizeof(char)); It must be int *array=malloc(... *sizeof(int))

//this is wrong
int *arr = (int *)malloc(((la+1)*(lb+1)*sizeof(char)));
//this is right
int *arr = (int *)malloc(((la+1)*(lb+1)*sizeof(int)));

Third, in those for loops in find_subseq functions indexes go out of range. This is huge problem because you will try to access array member which is not in range for that particular array. This causes undefined behavior in your program.

for(j = 0; j < lb+1; j++)

when j=lb in last iteration you will try to access array[lb], but the indexes go from 0 to lb-1

Maybe if you explain what are you trying to do in your program i could help you more.