C modify global char array [closed]

Let us see what gcc has to say about your code:

gcc -Wall test.c -o test

test.c: In function ‘function1’:
test.c:8:25: warning: comparison between pointer and integer
     while (newstring[i] != NULL) {
                         ^~
test.c:9:7: warning: implicit declaration of function ‘strcpy’ [-Wimplicit-function-declaration]
       strcpy(dest, newstring[i]);
       ^~~~~~
test.c:9:7: warning: incompatible implicit declaration of built-in function ‘strcpy’
test.c:9:7: note: include ‘<string.h>’ or provide a declaration of ‘strcpy’
test.c:9:20: warning: passing argument 2 of ‘strcpy’ makes pointer from integer without a cast [-Wint-conversion]
       strcpy(dest, newstring[i]);
                    ^~~~~~~~~
test.c:9:20: note: expected ‘const char *’ but argument is of type ‘char’
test.c: In function ‘main’:
test.c:19:20: warning: comparison between pointer and integer
     while (dest[i] != NULL) {
                    ^~
test.c:20:22: warning: format ‘%s’ expects argument of type ‘char *’, but argument 3 has type ‘int’ [-Wformat=]
       printf ("[%d] %s\n", i, dest[i]);
                      ^
test.c: At top level:
test.c:3:6: warning: array ‘dest’ assumed to have one element
 char dest[];
      ^~~~

As you can see there are so many warnings to take care of.

Take warnings seriously and fix them.

Doing that the code could be:

#include <stdio.h>

char dest[32];

int function1(char* newstring) {
    // Or simply use strcpy instead of a loop... 
    int i = 0;
    while(newstring[i] != '\0')
    {
      dest[i] = newstring[i];
      ++i;
    }
    dest[i] = '\0';
    return 0;
}

int main() {
    char* test="newstring";
    function1(test);
    int i = 0;
    while (dest[i] != '\0') {
      printf ("[%d] %c\n", i, dest[i]);
      ++i;
    }
    return 0;
}

Output:

[0] n
[1] e
[2] w
[3] s
[4] t
[5] r
[6] i
[7] n
[8] g

note You should also check for buffer overflow, i.e. that i is always less than 32. For clarity I omitted that but make sure to add that before the code is considered done..