Using memory mapping in C for reading binary

Question

I am trying to read data from a very large binary file and process it using memory mapping, so it can be read byte by byte. I am getting a few compiler errors while doing this, and I can't figure out what is causing them. I am doing this on a linux platform, for the record.

#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include<stdio.h>
#include<stdlib.h>
int fd;
char *data;

fd = open("data.bin", O_RDONLY);
pagesize = 4000;
data = mmap((caddr_t)0, pagesize, PROT_READ, MAP_SHARED, fd, pagesize);

The errors I get are as follows:

caddr not initialized

R_RDONLY not initialized

mmap has too few arguments.

I am using a Makefile to compile it, which looks like this:

all: order_book
CC = gcc 
CFLAGS = -std=c99

order_book: main.c
    $(CC) $(CFLAGS) -o order_book main.c
clean: 
    rm -f order_book

What am I doing wrong, and what can I do to fix it?

Answer 1

Several errors, if this is indeed the entire piece of code that fails:

1. O_RDONLY requires fcntl.h to be included.
2. The code is defined outside of any function.
3. The first argument to mmap is a void *, so just use NULL.
4. pagesize is not declared.

The following compiles:

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

int fd;
char *data;

void main(int argc, char *argv[]) {
  fd = open("data.bin", O_RDONLY);
  int pagesize = 4000;
  data = mmap(NULL, pagesize, PROT_READ, MAP_SHARED, fd, pagesize);
}