This variable declaration can creates memory issue - c

Question

here i have one function which is call many times in one application this application is running continuously.
Here i am take one character array size of 1024.
here i am declaring char input[1024];
so which is best way
1) char input[1024];
2) char input[1024] = NULL;
this thing will not create any memory issue after so many times if we used this function.char input[1024];
i think may be after using input we have to make NULL ?
or in declaration we have to declare this thing as char input[1024] = NULL; so when it will be called next time so that time first input make null than its taking any memory.
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
void do() {
char input[1024];
strcat(input,"ussp-push 04:18:0F:B1:48:B5#9");
strcat(input,"> dev/null&");
if(system(input) != 0)
{
printf("\nFailed command\n");
}
else
{
printf("\nSuccesss command\n");
}
}

Answer 1

Your program does not initialise input at all. This can lead to buffer overrun and even if that does not happen, your program is likely to have incorrect results. Thus your code invokes undefined behaviour.
Presumably you actually want to initialize input to be an empty string. Like this:
char input[1024] = "";
Once you have done this your calls to strcat will work.
If you don't want to initialise input then you can use strcpy instead of the first strcat:
char input[1024];
strcpy(input, "ussp-push 04:18:0F:B1:48:B5#9");
If ever you need to assign the empty string to input later in the code do it like this:
input[0] = '\0';
The code you suggest, char input[1024] = NULL is not valid. It does not compile.

Answer 2

As you're not changing it inside the function, make it a static const array
void dostuff() {
static const char input[] = "ussp-push 04:18:0F:B1:48:B5#9> dev/null&";
if (system(input) != 0)
{
printf("\nFailed command\n");
}
else
{
printf("\nSuccesss command\n");
}
}

Answer 3

strcat appends a string to another, and to do so it searches in the destination string the terminator char (\0). Since you didn't initialize the destination string, it may or may not find the \0 in that.
just initialize it, for example with:
input[0]='\0';

Answer 4

This will not compile:
char input[1024] = NULL;
$ make nullarray
cc nullarray.c -o nullarray
nullarray.c: In function ‘main’:
nullarray.c:4:2: error: invalid initializer
make: *** [nullarray] Error 1
$
When you declare char input[size], the compiler will cause the function prototype to allocate that much memory on the stack (if in a function) or in the .bss or .data section (if at top-level), and you cannot assign NULL to the array to try to stop it.
Perhaps a take step back and describe what problem you're really trying to solve? Should you be using malloc(3) to allocate memory instead?

Answer 5

This will not cause memory issues. Each time do() is called, an array of 1024 bytes will be allocated on the stack; that memory will be used for input throughout that call of do(). When do() returns, the 1024 bytes will be freed, and a (conceptually) new set of 1024 will be allocated on the next invocation.
In any case char input[1024] = NULL; is not valid code. If you want to initialise input, try something like char input[1024] = ""; instead.