Question

The code:

int main(int argc, char *argv[])
{
    typedef struct lb_data_US
    {
        char uname[255];
        char Eid[4];
        char myrole[4];
        char Login_t[30];
        char Logout_t[30];
        char ClientIP[20];
        char ZoneName[256];
    };

    int x = atoi(argv[1]);
    lb_data_US  lb_local[x];
    printf("stands for %d value\n", x);
    exit(0);
}

When I run this code using ./structure_testop 20995, it runs completely but when i run this code with a larger argument (like 20996 or more), it fails occasionally…
When i tried to debug it by gdb it says

Program received signal SIGSEGV, Segmentation fault.
0x003d6773 in _IO_vfprintf_internal (s=<value optimized out>, format=<value optimized out>, ap=<value optimized out>)
at vfprintf.c:233
233       int save_errno = errno;
Current language:  auto; currently c"

Can anyone explain this?

Answer 1

Assuming you are actually using C++ which supports dynamic array sizing like that, consider what an argument of 20995 does: it dynamically allocates 20995 times sizeof lb_data_US (which is about 600) for a total allocation of 12.5+ megabytes. Few environments support such a large stack size. Instead, use the heap via malloc() supported in many environments for process limit sorts of sizes.