[c] Stack smashing detected

Please look at the following situation:

ab@cd-x:$ cat test_overflow.c 
#include <stdio.h>
#include <string.h>

int check_password(char *password){
    int flag = 0;
    char buffer[20];
    strcpy(buffer, password);

    if(strcmp(buffer, "mypass") == 0){
        flag = 1;
    }
    if(strcmp(buffer, "yourpass") == 0){
        flag = 1;
    }
    return flag;
}

int main(int argc, char *argv[]){
    if(argc >= 2){
        if(check_password(argv[1])){
            printf("%s", "Access granted\n");
        }else{
            printf("%s", "Access denied\n");
        }
    }else{
        printf("%s", "Please enter password!\n");
    }
}
ab@cd-x:$ gcc -g -fno-stack-protector test_overflow.c 
ab@cd-x:$ ./a.out mypass
Access granted
ab@cd-x:$ ./a.out yourpass
Access granted
ab@cd-x:$ ./a.out wepass
Access denied
ab@cd-x:$ ./a.out wepassssssssssssssssss
Access granted

ab@cd-x:$ gcc -g -fstack-protector test_overflow.c 
ab@cd-x:$ ./a.out wepass
Access denied
ab@cd-x:$ ./a.out mypass
Access granted
ab@cd-x:$ ./a.out yourpass
Access granted
ab@cd-x:$ ./a.out wepassssssssssssssssss
*** stack smashing detected ***: ./a.out terminated
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6(__fortify_fail+0x48)[0xce0ed8]
/lib/tls/i686/cmov/libc.so.6(__fortify_fail+0x0)[0xce0e90]
./a.out[0x8048524]
./a.out[0x8048545]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe6)[0xc16b56]
./a.out[0x8048411]
======= Memory map: ========
007d9000-007f5000 r-xp 00000000 08:06 5776       /lib/libgcc_s.so.1
007f5000-007f6000 r--p 0001b000 08:06 5776       /lib/libgcc_s.so.1
007f6000-007f7000 rw-p 0001c000 08:06 5776       /lib/libgcc_s.so.1
0090a000-0090b000 r-xp 00000000 00:00 0          [vdso]
00c00000-00d3e000 r-xp 00000000 08:06 1183       /lib/tls/i686/cmov/libc-2.10.1.so
00d3e000-00d3f000 ---p 0013e000 08:06 1183       /lib/tls/i686/cmov/libc-2.10.1.so
00d3f000-00d41000 r--p 0013e000 08:06 1183       /lib/tls/i686/cmov/libc-2.10.1.so
00d41000-00d42000 rw-p 00140000 08:06 1183       /lib/tls/i686/cmov/libc-2.10.1.so
00d42000-00d45000 rw-p 00000000 00:00 0 
00e0c000-00e27000 r-xp 00000000 08:06 4213       /lib/ld-2.10.1.so
00e27000-00e28000 r--p 0001a000 08:06 4213       /lib/ld-2.10.1.so
00e28000-00e29000 rw-p 0001b000 08:06 4213       /lib/ld-2.10.1.so
08048000-08049000 r-xp 00000000 08:05 1056811    /dos/hacking/test/a.out
08049000-0804a000 r--p 00000000 08:05 1056811    /dos/hacking/test/a.out
0804a000-0804b000 rw-p 00001000 08:05 1056811    /dos/hacking/test/a.out
08675000-08696000 rw-p 00000000 00:00 0          [heap]
b76fe000-b76ff000 rw-p 00000000 00:00 0 
b7717000-b7719000 rw-p 00000000 00:00 0 
bfc1c000-bfc31000 rw-p 00000000 00:00 0          [stack]
Aborted
ab@cd-x:$ 

When I disabled the stack smashing protector no errors were detected, which should have happened when I used "./a.out wepassssssssssssssssss"

So to answer your question above, the message "** stack smashing detected : xxx" was displayed because your stack smashing protector was active and found that there is stack overflow in your program.

Just find out where that occurs, and fix it.