信息在计算机内部组件传递之核心mmap使用

mmap函数非常重要, alsa驱动 uac uvc 音视频还有这个整个应用程序变成进程，这个函数也起着关键性作用。

root@instance-tau7o2q5:~/app_modules# strace  ./a.out .
execve("./a.out", ["./a.out", "."], 0x7ffc78e587b8 /* 27 vars */) = 0
brk(NULL)                               = 0x559be8915000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=46616, ...}) = 0
mmap(NULL, 46616, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f4f4d50d000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260\34\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=2030544, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4f4d50b000
mmap(NULL, 4131552, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f4f4cf01000
mprotect(0x7f4f4d0e8000, 2097152, PROT_NONE) = 0
mmap(0x7f4f4d2e8000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1e7000) = 0x7f4f4d2e8000
mmap(0x7f4f4d2ee000, 15072, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f4f4d2ee000
close(3)                                = 0
arch_prctl(ARCH_SET_FS, 0x7f4f4d50c4c0) = 0
mprotect(0x7f4f4d2e8000, 16384, PROT_READ) = 0
mprotect(0x559be7d71000, 4096, PROT_READ) = 0
mprotect(0x7f4f4d519000, 4096, PROT_READ) = 0
munmap(0x7f4f4d50d000, 46616)           = 0
openat(AT_FDCWD, "./file_mmap", O_RDWR|O_CREAT, 0600) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=294, ...}) = 0
mmap(NULL, 294, PROT_READ|PROT_WRITE, MAP_SHARED, 3, 0) = 0x7f4f4d518000
close(3)                                = 0
munmap(0x7f4f4d518000, 294)             = 0
exit_group(0)                           = ?
+++ exited with 0 +++

做了一个文件修改的操作

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


struct Card_Messge
{
   char number1;
   char number2;
   char number3;
   char number4;
   char number5;

};

struct Card_Messge* Card ;
int main(int argc, char *argv[])
{
    int fd;
    char *buf;
    off_t len;
    struct stat sb;
    char *fname = "./file_mmap";
 
    fd = open(fname, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
    if (fd == -1)
    {
        perror("open");
        return 1;
    }
    if (fstat(fd, &sb) == -1)
    {
        perror("fstat");
        return 1;
    }
 
    buf = mmap(0, sb.st_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (buf == MAP_FAILED)
    {
        perror("mmap");
        return 1;
    }
    
    Card  = (struct Card_Messge*)buf;
    
    if (close(fd) == -1)
    {
        perror("close");
        return 1;
    }
 
    for (len = 0; len < sb.st_size; ++len)
    {
        buf[len] = toupper(buf[len]);
        /*putchar(buf[len]);*/
    }
    
    buf[0] = '1';
    buf[1] = '2';
    buf[2] = '3';
    buf[3] = '4';
    
    
    Card->number1 = 'a';
    Card->number2 = 'b';
    Card->number3 = 'c';
    Card->number4 = 'd';
    
 
    if (munmap(buf, sb.st_size) == -1)
    {
        perror("munmap");
        return 1;
    }
    return 0;

信息在计算机内部组件传递之核心mmap使用

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