Linux mmap学习及遇到的问题解决

Linux mmap的学习

本来以前mmap是很简单的，看下参数用法就ok，结果遇到了一大堆麻烦，google了很多博客，都写得很浅显，就介绍一下参数然后再给了一个简单得不得了的代码，就完了，所以纠结了我差不多一天时间。

mmap怎么用我就不说了，网上很多，在这里就把我犯的错误经验分享出来，先贴一下我用mmap实现拷贝任意文件的代码：

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <assert.h>#include <unistd.h>#include <fcntl.h>#include <sys/mman.h>#include <sys/stat.h>#include <sys/types.h>int main(){int fdin,fdout, len = 0, pages = 0, offset = 0, lastPage = 0;char *src, *dst;size_t pagesize;struct stat statbuf;if((fdin = open("mmmap.c", O_RDWR)) == -1){printf("can't open fdin ");return 1;}if((fdout = open("mmmmap1.c",O_RDWR | O_CREAT | O_TRUNC,0644)) == -1){if(EEXIST == errno){fd = open("mmmmap1.c", O_RDWR);if(-1 == fd){perror("open");exit(-1);}}else{perror("open");exit(-1);}}//lseek(fd, 4096, SEEK_SET);//write(fd, " ", 1);pagesize = sysconf(_SC_PAGESIZE);printf("pagesize = %ld\n", pagesize);assert(fstat(fdin,&statbuf) == 0);printf("statbuf.st_size = %d\n", statbuf.st_size);pages = statbuf.st_size/pagesize;if((statbuf.st_size)%pagesize != 0){lastPage = (statbuf.st_size)%pagesize;pages += 1;printf("lastPage = %d\n", lastPage);}printf("pages = %d\n", pages);//assert(ftruncate(fdout, statbuf.st_size) == 0);if (lastPage != 0){len += lastPage;}else{len += pagesize;}while(pages--){assert(ftruncate(fdout, len) == 0);src = (char *)mmap(NULL, pagesize, PROT_READ | PROT_WRITE, MAP_SHARED, fdin, offset);if( src == MAP_FAILED ){printf("mmap error for fdin.\n");return 1;}dst = mmap(0,pagesize,PROT_READ | PROT_WRITE,MAP_SHARED,fdout,offset);if( dst == MAP_FAILED ){printf("mmap error for fdout.\n");return 1;}memcpy(dst,src, pagesize);offset += pagesize;if (pages == 1 && lastPage != 0){len += lastPage;}else{len += pagesize;}munmap(src, pagesize);munmap(dst, pagesize);}close(fdin);close(fdout);return 0;}

我遇到的问题：
1.报段错误，打开的读写方式和mmap里的参数不一致会报此错
2.bus error，文件为空，或者mmap了一页而你却要去操作第二页
3.新建的写入文件有时会打开异常，那是因为在mmap的时候往文件里写了不应该写入的字符，所以建议更新文件大小的时候不要用write，用ftruncate

反正记住mmap是一页一页映射的，len和offset参数要是页的倍数就行了...

有什么问题欢迎留言讨论..…^_^