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- Programming (https://www.linuxquestions.org/questions/programming-9/)

- - mmap (https://www.linuxquestions.org/questions/programming-9/mmap-509667/)

Hi all,
can anyone tell me how to implement mmap function.. with some sample application code..??
thanks...

This is some older code - note the open flags on the file. You cannot write the buffer back to the file without closing the fd first

Code:

/******************************

* filemap.c 

* provides a memory mapped file to caller

*

*******************************/

#include <sys/types.h>

#include <unistd.h>

#include <fcntl.h>

#include <sys/mman.h>

#include <stdlib.h>

#include <sys/stat.h>

#include <errno.h>



/* returns the size in bytes of the file */



size_t filesize(int fd)

{

        struct stat st;

        if(fstat(fd,&st)==(-1))

        {

                perror("Cannot stat input file");

                exit(EXIT_FAILURE);

        }

        return st.st_size;

}

/***********************

* mapfile maps a file into memory

* arg:

*    filename - name of the file to map

*    fd - files descriptor - mapfile sets this to an open fd

*    filelen - length of file - mapfile sets this to len of file in bytes 

* returns:

*    pointer to file (byte 0 of file) in memory

*

*    any changes made to file in memory are not reflected in the physical file.

***************************/

caddr_t *mapfile(const char *filename, int *fd, size_t *filelen)

{

    int filedesc=0; 

    size_t len=0;

    caddr_t *mapptr;

        if( (filedesc=open(filename,O_RDONLY))<0)

        {

                perror("Input file open failure");

                exit(EXIT_FAILURE);

        }

        *fd=filedesc;

        len=filesize(filedesc);        

        *filelen=len;

        mapptr=(caddr_t *)mmap(NULL, 

                              len,

                              PROT_READ|PROT_WRITE,

                              MAP_PRIVATE,  

                              filedesc, 

                              0);

        if(mapptr==MAP_FAILED)

        {

                perror("Memory mapping of file failed.");

                exit(EXIT_FAILURE);

        }    

        return mapptr;

}



/* close files and release memory */

void mapcleanup(caddr_t *ptr, int fd, size_t filelen )

{

        if(munmap(ptr,filelen)==(-1))

        {

                perror("Error releasing memory");

                exit(EXIT_FAILURE);

        }

        while(close(fd)==(-1))

        {

                if(errno=EINTR)

                {

                        continue;

                }

                else

                {

                        perror("File I/O Error");

                        exit(EXIT_FAILURE);

                }

        }                

}

sorry... repeated...

wow thanks a lot jim mcnamara...

I am thinking of implementing mmap system call in my driver. I need to map kernal buffer which is allocated by kmalloc to user space. I am able to map the buffer area but i could not read from that buffer in the user space....Will u please in this regard with a sample code of driver.
thanks again....

If what I posted is news to you, then writing kernel mode code or drivers may also be interesting for you.

Have you considered reading 'Linux Device Drivers' by Corbet, Rubini & Kroah-Hartman?

Or some other primary material like that? Using mmap is covered in
http://www.advancedlinuxprogramming.com/alp-folder Advanced Linux Programming - a free online book.

thanks jim mcnamara...
ya ya i have referred the same book, linux device drivers by Rubini.. i have implemented the code referring to that book.. but when is try to read the data from the user space i am not getting the actual data..that is the problem.... and also in the link given the only application code is implemented... so i am searching the driver side code for that...can u help me ???