本文共 9868 字，大约阅读时间需要 32 分钟。
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#include #include #include #include #include #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);}/*#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*/
编译命令：
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gcc mmap.c
./a.out mmap.c 10000
//执行结果，示例length = 2266offset = 10000pa_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) = 22664096(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) =61440pa_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转载地址：http://lcmii.baihongyu.com/