whats wrong with this code of reading with mmap? - c

Question

I am trying to run this code, while I am ending with -
value: 1
value: 0.000000
My question is why the both results are different??
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <string.h>
#include <unistd.h>
int main ()
{
int fd;
struct stat mystat;
void *pmap;
int i,integer;
double *values;
int32_t *mapped_baseA;
int32_t fdA;
fd = open("test.txt",O_RDWR); // a text file containing- 1 2 3 4 5
if(fd==-1)
{
perror("open");
exit(1);
}
if(fstat(fd,&mystat)<0)
{
perror("fstat");
close(fd);
exit(1);
}
pmap = mmap(0,mystat.st_size,PROT_READ | PROT_WRITE,MAP_SHARED,fd,0);
if(pmap==MAP_FAILED)
{
perror("mmap failed");
close(fd);
exit(1);
}
//strncpy(pmap,"That is my name",15);
sscanf (pmap, " %d", &integer);
printf("value: %d \n", integer);
//printing the values after scanning from string.
values = (double *) mmap(0,mystat.st_size,PROT_READ | PROT_WRITE,MAP_SHARED,fd,0);
printf("value: %lf \n", values[1]);
//printing the values from pointer
munmap (pmap, mystat.st_size);
close(fd);
return 0;
}

Answer 1

Read carefully (and several times) mmap(2). Notice:
A file is mapped in multiples of the page size.
and in the ERRORS section
EINVAL We don't like addr, length, or offset (e.g., they are too
large, or not aligned on a page boundary).
Consider also using strace(1) on your executable.
Of course, a memory mapping is just giving a view (as raw sequence of bytes) of the mapped file into the process by modifying its virtual address space. It obviously won't do any conversion (you might use sscanf(3) or strtol(3) on parts of that view to make such a conversion from an UTF8 or ASCII string representation of a number into its machine representation).