与指针算术混淆

谁能帮我理解这两行在做什么

buf = (char *)(malloc(2 * pagesize) & pagemask);
buf = (char *)(((long)buf + pagesize) & ∼pagemask);

我了解 malloc，但不确定&操作在两个表达式中试图实现什么

页面大小和页面掩码定义如下

pagesize = sysconf(_SC_PAGESIZE);
pagemask = pagesize - 1;

谢谢！

编辑1

这段代码来自Steve D. Pate的"Unix FileSystems"一书。

编辑2

这是完整的代码

#include <sys/unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include “sys/fs/vx_ioctl.h”
#define MB (1024 * 1024)
main(int argc, char argv[])
{
char        *buf;
int         i, fd, advisory;
long        pagesize, pagemask;
if (argc != 2) {
exit(1);
}
if (strcmp(argv[1], “VX_SEQ”) == 0) {
advisory = VX_SEQ;
} else if (strcmp(argv[1], “VX_RANDOM”) == 0) {
advisory = VX_RANDOM;
} else if (strcmp(argv[1], “VX_DIRECT”) == 0) {
advisory = VX_DIRECT;
}
pagesize = sysconf(_SC_PAGESIZE);
pagemask = pagesize - 1;
buf = (char *)(malloc(2 * pagesize) & pagemask);
buf = (char *)(((long)buf + pagesize) & ∼pagemask);
fd = open(“myfile”, O_RDWR);
ioctl(fd, VX_SETCACHE, advisory);
for (i=0 ; i<MB ; i++) {
read(fd, buf, 4096);
}
}