mmap() System call

mmap(void * start, size_t length, int prot, int flags, int sg_fd, off_t offset). This system call returns a pointer to the beginning of the reserved buffer associated with the sg file descriptor 'sg_fd'. The 'start' argument is a hint to the kernel and is ignored by this driver; best set it to 0. The 'length' argument should be less than or equal to the size of the reserved buffer associated with 'sg_fd'. If it exceeds the reserved buffer size (after 'length' has been rounded up to a page size multiple) then MAP_FAILED is returned and ENOMEM is placed in errno. The 'prot' argument should either be PROT_READ or (PROT_READ | PROT_WRITE). The 'flags' argument should contain MAP_SHARED. In a sense, the user application is "sharing" data with the sg driver. The MAP_PRIVATE flag does not play well with compiler optimization flags such as '-O2'. The 'offset' argument must be set to 0 (or NULL).

The mmap() system call can be made multiple times on the same sg_fd. The munmap() system call is not required if close() is called on sg_fd. Mmap-ed IO is well-behaved when a process is fork()-ed (or the equivalent finer grained clone() system call is made). In the case of a fork(), 2 processes will be sharing the same memory mapped area together with the sg driver for a sg_fd and the last one to close the sg_fd (or exit) will cause the shared memory to be freed.
It is assumed that if the default reserved buffer size of 32 KB is not sufficient then a ioctl(SG_SET_RESERVED_SIZE) call is made prior to any calls to mmap(). If the required size is not a multiple of the kernel's page size (returned by getpagesize() system call) then the size passed to ioctl(SG_SET_RESERVED_SIZE) should be rounded up to the next page size multiple.
Mmap-ed IO is requested by setting (or or-ing in) the SG_FLAG_MMAP_IO constant into the flag member of the the sg_io_hdr structure prior to a call to write() or ioctl(SG_IO). The logic to do mmap-ed IO _assumes_ that an appropriate mmap() call has been made by the application



Example of open file for read mode


#include
#include
#include
#include
#include
#include
#include

#define FILEPATH "/home/tarun/c_prog/mmap_test.txt"
#define NUMINTS (10)
#define FILESIZE (NUMINTS * sizeof(int))

int main(int argc, char *argv[])
{
int i;
int fd;
int *map; /* mmapped array of int's */

fd = open(FILEPATH, O_RDONLY);
if (fd == -1) {
perror("Error opening file for reading");
exit(EXIT_FAILURE);
}

map = mmap(0, FILESIZE, PROT_READ, MAP_SHARED, fd, 0);
if (map == MAP_FAILED) {
close(fd);
perror("Error mmapping the file");
exit(EXIT_FAILURE);
}

/* Read the file int-by-int from the mmap
*/
for (i = 1; i <=NUMINTS; ++i) {
printf("%d: %d\n", i, map[i]);
}

if (munmap(map, FILESIZE) == -1) {
perror("Error un-mmapping the file");
}
close(fd);
return 0;
}