I code it like this below, but no one can work.
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bson b2[1];

char * buffer;
int size;

size = bson_size(b);
buffer = (char *)malloc(size);
memcpy(buffer, bson_data(b), size);

//bson_init_unfinished_data(b2, buffer, size, 0); // no effect
//bson_init_unfinished_data(b2, buffer, size, 1); // with crash
//bson_init_finished_data_with_copy(b2, bson_data(b)); // no effect
bson_append_int(b2, "age", 18);

bson_finish( b2 );
bson_print( b2 );

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best regard and thanks.