void remove_spaces(char* lpsz)
{
auto sptr = lpsz;
auto dptr = lpsz;
while (*dptr = *sptr) { // copy up to and including terminating NUL
    if (*dptr != ' ') dptr++; // but spaces take no slots in the destination
    sptr++;
}
}
 
#include <stdio.h>
int main(void)
{
    char test[] = { "   Here is  a test   string with  varying     number    of spaces mixed in .  " };
    printf("Before: \'%s\'\n", test);
    remove_spaces(test);
    printf("After : \'%s\'\n", test);
}

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