00111
01011
01101
01110
10011
10101
10110
11001
11010
11100

##</code><code>##

11100 [0 1 2]
11010 [0 1 3]
11001 [0 1 4]
10110 [0 2 3]
10101 [0 2 4]
10011 [0 3 4]
01110 [1 2 3]
01101 [1 2 4]
01011 [1 3 4]
00111 [2 3 4]


##</code><code>##

typedef struct {
    int N, M;
    char *posns.
} POSNS;

POSNS *initGenerator( int N, int M ) {
     POSNS *p = malloc( sizeof( POSNS ) );

     p->N = N; p->M = M; p->posns = malloc( M );

     for( i = 0; i < M; ++i ) p->posns[ i ] = N - M + i;
     return posns;
}

char *iterGenerator( POSNS *p ) {
    if( done) {
        free( p->posns );
        free( p );
        return NULL;
    }
    // modify p->posns;
    ...
    return p->posns;
}

POSNS p = initGenerator( n, m );

while( c = iterGenerator( p ) ) {
    // use c[].
}