pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1) 详解

企业开发 2022-06-14 12:58:26 阅读次数: 0

发现很多文章都没有把pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1) 详细计算过程写出来，自己专门计算了一遍，附在文末。本例为man mmap中的原样实例程序，加上了一些额外的无关代码及打印输出信息 让总字节数超过一页大小sysconf(_SC_PAGE_SIZE)以方便查看调试信息

#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define handle_error(msg) \
    do {
      
       perror(msg); exit(EXIT_FAILURE); } while (0)

int main(int argc, char *argv[])
{
    
    
    char *addr;
    int fd;
    struct stat sb;
    off_t offset, pa_offset;
    size_t length;
    ssize_t s;

    if (argc < 3 || argc > 4) {
    
    
        fprintf(stderr, "%s file offset [length]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    fd = open(argv[1], O_RDONLY);
    if (fd == -1)
        handle_error("open");

    if (fstat(fd, &sb) == -1)           /* To obtain file size */
        handle_error("fstat");

    offset = atoi(argv[2]);
    pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);
        /* offset for mmap() must be page aligned */
     if (offset >= sb.st_size) {
    
    
        fprintf(stderr, "offset is past end of file\n");
        exit(EXIT_FAILURE);
    }

    if (argc == 4) {
    
    
        length = atoi(argv[3]);
        if (offset + length > sb.st_size)
            length = sb.st_size - offset;
                /* Can't display bytes past end of file */

    } else {
    
        /* No length arg ==> display to end of file */
        length = sb.st_size - offset;
    }

    addr = mmap(NULL, length + offset - pa_offset, PROT_READ,
                MAP_PRIVATE, fd, pa_offset);
    if (addr == MAP_FAILED)
        handle_error("mmap");

    printf("length = %ld\n", length);
	printf("offset = %ld\n", offset);
	printf("pa_offset = %ld\n", pa_offset);
	printf("length + offset - pa_offset = %ld\n", length + offset - pa_offset);
    
    s = write(STDOUT_FILENO, addr + offset - pa_offset, length);
    if (s != length) {
    
    
        if (s == -1)
            handle_error("write");

        fprintf(stderr, "partial write");
        exit(EXIT_FAILURE);
    }

    munmap(addr, length + offset - pa_offset);
    close(fd);

    exit(EXIT_SUCCESS);
}

//**额外增加的代码开始
//**加上了一些额外的无关代码及打印输出信息 让总字节数超过一页大小sysconf(_SC_PAGE_SIZE)以方便查看调试信息**开始
/* 
#include "head.h"
struct msgbuff{
    long mtype;
    char mtext[512];
};

void send_msg(int qid, int type, char *m_msg, int size){
    struct msgbuff msg;
    msg.mtype = type;
    strcpy(msg.mtext, m_msg);
    if(msgsnd(qid, (void *)&msg, sizeof(msg.mtext), IPC_NOWAIT) == -1){
        perror("msgsnd");
        exit(1);
    } 
}

void get_msg(int qid, int type){
    while (1){
        struct msgbuff msg;
        bzero(&msg, sizeof(msg));//清空缓冲区，也可用memmet实现
        if (msgrcv(qid, (void *)&msg, sizeof(msg.mtext),type, MSG_NOERROR) == -1) {
            perror("msgrcv");
            exit(1);
        }
        printf("抢到了资源: <%d> <%s>\n", type, msg.mtext);

    }
}

int main(int argc, char **argv){
    int opt, mode = 2, msgtype = 1;
    int msgq_id;
    char mtext[512] = {0};
    //mode == 1-> send
    while((opt = getopt(argc, argv, "st:rm:")) != -1){
        switch(opt){
            case 's':
                mode = 1;
                break;
            case 'r':
                mode = 2;
                break;
            case 't':
                msgtype = atoi(optarg);
                break;
            case 'm':
                strcpy(mtext, optarg);
                break;
            default:
                fprintf(stderr, "Usage: %s -[s|r] -t type -m msg\n", argv[0]);
                exit(1);
        }
    }
    //int msgget(key_t key, int msgflg);
    if((msgq_id = msgget(2021, IPC_CREAT | 0666)) < 0){
        perror("msgget");
        exit(1);
    }
    if (mode == 1){
        send_msg(msgq_id, msgtype, mtext, sizeof(mtext));
    } else {
        get_msg(msgq_id, msgtype);
    }

    return 0;
}
// ./a.out -s -t 1 -m "I'm the 1th!"
// ./a.out -r -t 1.msgq.c
// #include "head.h"
struct msgbuff{
    long mtype;
    char mtext[512];
};

void send_msg(int qid, int type, char *m_msg, int size){
    struct msgbuff msg;
    msg.mtype = type;
    strcpy(msg.mtext, m_msg);
    if(msgsnd(qid, (void *)&msg, sizeof(msg.mtext), IPC_NOWAIT) == -1){
        perror("msgsnd");
        exit(1);
    }
}

void get_msg(int qid, int type){
    while (1){
        struct msgbuff msg;
        bzero(&msg, sizeof(msg));//清空缓冲区，也可用memmet实现
        if (msgrcv(qid, (void *)&msg, sizeof(msg.mtext),type, MSG_NOERROR) == -1) {
            perror("msgrcv");
            exit(1);
        }
        printf("抢到了资源: <%d> <%s>\n", type, msg.mtext);

    }
}

int main(int argc, char **argv){
    int opt, mode = 2, msgtype = 1;
    int msgq_id;
    char mtext[512] = {0};
    //mode == 1-> send
    while((opt = getopt(argc, argv, "st:rm:")) != -1){
        switch(opt){
            case 's':
                mode = 1;
                break;
            case 'r':
                mode = 2;
                break;
            case 't':
                msgtype = atoi(optarg);
                break;
            case 'm':
                strcpy(mtext, optarg);
                break;
            default:
                fprintf(stderr, "Usage: %s -[s|r] -t type -m msg\n", argv[0]);
                exit(1);
        }
    }
    //int msgget(key_t key, int msgflg);
    if((msgq_id = msgget(2021, IPC_CREAT | 0666)) < 0){
        perror("msgget");
        exit(1);
    }
    if (mode == 1){
        send_msg(msgq_id, msgtype, mtext, sizeof(mtext));
    } else {
        get_msg(msgq_id, msgtype);
    }

    return 0;
}
// ./a.out -s -t 1 -m "I'm the 1th!"
// ./a.out -r -t 1.msgq.c#include "head.h"
struct msgbuff{
    long mtype;
    char mtext[512];
};

void send_msg(int qid, int type, char *m_msg, int size){
    struct msgbuff msg;
    msg.mtype = type;
    strcpy(msg.mtext, m_msg);
    if(msgsnd(qid, (void *)&msg, sizeof(msg.mtext), IPC_NOWAIT) == -1){
        perror("msgsnd");
        exit(1);
    } 
}

void get_msg(int qid, int type){
    while (1){
        struct msgbuff msg;
        bzero(&msg, sizeof(msg));//清空缓冲区，也可用memmet实现
        if (msgrcv(qid, (void *)&msg, sizeof(msg.mtext),type, MSG_NOERROR) == -1) {
            perror("msgrcv");
            exit(1);
        }
        printf("抢到了资源: <%d> <%s>\n", type, msg.mtext);

    }
}

int main(int argc, char **argv){
    int opt, mode = 2, msgtype = 1;
    int msgq_id;
    char mtext[512] = {0};
    //mode == 1-> send
    while((opt = getopt(argc, argv, "st:rm:")) != -1){
        switch(opt){
            case 's':
                mode = 1;
                break;
            case 'r':
                mode = 2;
                break;
            case 't':
                msgtype = atoi(optarg);
                break;
            case 'm':
                strcpy(mtext, optarg);
                break;
            default:
                fprintf(stderr, "Usage: %s -[s|r] -t type -m msg\n", argv[0]);
                exit(1);
        }
    }
    //int msgget(key_t key, int msgflg);
    if((msgq_id = msgget(2021, IPC_CREAT | 0666)) < 0){
        perror("msgget");
        exit(1);
    }
    if (mode == 1){
        send_msg(msgq_id, msgtype, mtext, sizeof(mtext));
    } else {
        get_msg(msgq_id, msgtype);
    }

    return 0;
}
// ./a.out -s -t 1 -m "I'm the 1th!"
// ./a.out -r -t 1.msgq.c#include "head.h"
struct msgbuff{
    long mtype;
    char mtext[512];
};

void send_msg(int qid, int type, char *m_msg, int size){
    struct msgbuff msg;
    msg.mtype = type;
    strcpy(msg.mtext, m_msg);
    if(msgsnd(qid, (void *)&msg, sizeof(msg.mtext), IPC_NOWAIT) == -1){
        perror("msgsnd");
        exit(1);
    } 
}

void get_msg(int qid, int type){
    while (1){
        struct msgbuff msg;
        bzero(&msg, sizeof(msg));//清空缓冲区，也可用memmet实现
        if (msgrcv(qid, (void *)&msg, sizeof(msg.mtext),type, MSG_NOERROR) == -1) {
            perror("msgrcv");
            exit(1);
        }
        printf("抢到了资源: <%d> <%s>\n", type, msg.mtext);

    }
}

int main(int argc, char **argv){
    int opt, mode = 2, msgtype = 1;
    int msgq_id;
    char mtext[512] = {0};
    //mode == 1-> send
    while((opt = getopt(argc, argv, "st:rm:")) != -1){
        switch(opt){
            case 's':
                mode = 1;
                break;
            case 'r':
                mode = 2;
                break;
            case 't':
                msgtype = atoi(optarg);
                break;
            case 'm':
                strcpy(mtext, optarg);
                break;
            default:
                fprintf(stderr, "Usage: %s -[s|r] -t type -m msg\n", argv[0]);
                exit(1);
        }
    }
    //int msgget(key_t key, int msgflg);
    if((msgq_id = msgget(2021, IPC_CREAT | 0666)) < 0){
        perror("msgget");
        exit(1);
    }
    if (mode == 1){
        send_msg(msgq_id, msgtype, mtext, sizeof(mtext));
    } else {
        get_msg(msgq_id, msgtype);
    }

    return 0;
}
// ./a.out -s -t 1 -m "I'm the 1th!"
// ./a.out -r -t 1.msgq.c#include "head.h"
struct msgbuff{
    long mtype;
    char mtext[512];
};

void send_msg(int qid, int type, char *m_msg, int size){
    struct msgbuff msg;
    msg.mtype = type;
    strcpy(msg.mtext, m_msg);
    if(msgsnd(qid, (void *)&msg, sizeof(msg.mtext), IPC_NOWAIT) == -1){
        perror("msgsnd");
        exit(1);
    } 
}

void get_msg(int qid, int type){
    while (1){
        struct msgbuff msg;
        bzero(&msg, sizeof(msg));//清空缓冲区，也可用memmet实现
        if (msgrcv(qid, (void *)&msg, sizeof(msg.mtext),type, MSG_NOERROR) == -1) {
            perror("msgrcv");
            exit(1);
        }
        printf("抢到了资源: <%d> <%s>\n", type, msg.mtext);

    }
}

int main(int argc, char **argv){
    int opt, mode = 2, msgtype = 1;
    int msgq_id;
    char mtext[512] = {0};
    //mode == 1-> send
    while((opt = getopt(argc, argv, "st:rm:")) != -1){
        switch(opt){
            case 's':
                mode = 1;
                break;
            case 'r':
                mode = 2;
                break;
            case 't':
                msgtype = atoi(optarg);
                break;
            case 'm':
                strcpy(mtext, optarg);
                break;
            default:
                fprintf(stderr, "Usage: %s -[s|r] -t type -m msg\n", argv[0]);
                exit(1);
        }
    }
    //int msgget(key_t key, int msgflg);
    if((msgq_id = msgget(2021, IPC_CREAT | 0666)) < 0){
        perror("msgget");
        exit(1);
    }
    if (mode == 1){
        send_msg(msgq_id, msgtype, mtext, sizeof(mtext));
    } else {
        get_msg(msgq_id, msgtype);
    }

    return 0;
}
// ./a.out -s -t 1 -m "I'm the 1th!"
// ./a.out -r -t 1.msgq.c#include "head.h"
struct msgbuff{
    long mtype;
    char mtext[512];
};

void send_msg(int qid, int type, char *m_msg, int size){
    struct msgbuff msg;
    msg.mtype = type;
    strcpy(msg.mtext, m_msg);
    if(msgsnd(qid, (void *)&msg, sizeof(msg.mtext), IPC_NOWAIT) == -1){
        perror("msgsnd");
        exit(1);
    } 
}

void get_msg(int qid, int type){
    while (1){
        struct msgbuff msg;
        bzero(&msg, sizeof(msg));//清空缓冲区，也可用memmet实现
        if (msgrcv(qid, (void *)&msg, sizeof(msg.mtext),type, MSG_NOERROR) == -1) {
            perror("msgrcv");
            exit(1);
        }
        printf("抢到了资源: <%d> <%s>\n", type, msg.mtext);

    }
}

int main(int argc, char **argv){
    int opt, mode = 2, msgtype = 1;
    int msgq_id;
    char mtext[512] = {0};
    //mode == 1-> send
    while((opt = getopt(argc, argv, "st:rm:")) != -1){
        switch(opt){
            case 's':
                mode = 1;
                break;
            case 'r':
                mode = 2;
                break;
            case 't':
                msgtype = atoi(optarg);
                break;
            case 'm':
                strcpy(mtext, optarg);
                break;
            default:
                fprintf(stderr, "Usage: %s -[s|r] -t type -m msg\n", argv[0]);
                exit(1);
        }
    }
    //int msgget(key_t key, int msgflg);
    if((msgq_id = msgget(2021, IPC_CREAT | 0666)) < 0){
        perror("msgget");
        exit(1);
    }
    if (mode == 1){
        send_msg(msgq_id, msgtype, mtext, sizeof(mtext));
    } else {
        get_msg(msgq_id, msgtype);
    }

    return 0;
}
// ./a.out -s -t 1 -m "I'm the 1th!"
// ./a.out -r -t 1.msgq.c

*/
//**额外增加的结束
//加上了一些额外的无关代码及打印输出信息 让总字节数超过一页大小sysconf(_SC_PAGE_SIZE)以方便查看调试信息** 结束

编译命令：gcc mmap.c
程序执行格式： ./a.out file offset [length]
格式解析：./a.out +file 文件名 + offset偏移量 +length可选长度

./a.out mmap.c 10000

//执行结果，示例
length = 2266
offset = 10000
pa_offset = 8192

假设文件mmap.c文件大小为12266 ，
length =文件总大小(12266) - 偏移量offset(1000) = 2266

4096(sysconf(_SC_PAGE_SIZE)一页大小)

0001 0000 0000 0000 = 4096(sysconf(_SC_PAGE_SIZE)一页大小)
0000 1111 1111 1111 = (sysconf(_SC_PAGE_SIZE) - 1) =4095
~(0000 1111 1111 1111)
= 1111 0000 0000 0000 = ~ (sysconf(_SC_PAGE_SIZE) - 1) =61440

pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1)
=10000 & 61440
=(0010 0111 0001 0000) & (1111 0000 0000 0000)
=(0010 0000 0000 0000)
=8192 //pa_offset即内存中的页偏移量

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