void RemoveAllSpaces(char* str)
{
   . . .
}

MS>void RemoveAllSpaces(char* str)
MS>{
MS>   . . .
MS>}
MS>

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void RemoveAllSpaces(char* str)
{
    int i = 0, j = 0;

    while (1) {
        while (str[i] && str[i] == ' ')
            ++i;

        if (i != j)
            str[j] = str[i];

        if (!str[i])
            break;

        ++i;
        ++j;
    }
}

void test(const char *q, const char *a)
{
    char *s1 = strdup(q);

    RemoveAllSpaces(s1);
    if (strcmp(s1, a))
        printf("\"%s\" FAILED\n", q);
    else
        printf("OK\n");

    free(s1);
}

int main()
{
    test("abc ", "abc");
    test("   abc", "abc");
    test("", "");
    test(" ", "");
    test(" a bb c", "abbc");

    return 0;
}

{
  char *c = str;
  while(*c = *str)
  {
    if (*str != ' ') c++;
    str++;
  }
}