CITS2002 Systems Programming  
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Raw input and output

We've recently seen how C11 employs arrays of characters to represent strings, treating the NULL-byte with special significance.

At the lowest level, an operating system will only communicate using bytes, not with higher-level integers or floating-point values. C11 employs arrays of characters to hold the bytes in requests for raw input and output.

File descriptors - reading from a file

Unix-based operating systems provide file descriptors, simple integer values, to identify 'communication channels' - such as files, interprocess-communication pipes, (some) devices, and network connections (sockets).

In combination, our C11 programs will use integer file descriptors and arrays of characters to request that the operating system performs input and output on behalf of the process - see man 2 open. 

#include  <stdio.h>
#include  <fcntl.h>
#include  <stdlib.h>
#include  <unistd.h>

#define  MYSIZE      10000

void read_using_descriptor(char filename[])        
{
//  ATTEMPT TO OPEN THE FILE FOR READ-ONLY ACCESS
    int fd    = open(filename, O_RDONLY);

//  CHECK TO SEE IF FILE COULD BE OPENED
    if(fd == -1) {
        printf("cannot open '%s'\n", filename);
        exit(EXIT_FAILURE);
    }

//  DEFINE A CHARACTER ARRAY TO HOLD THE FILE'S CONTENTS
    char   buffer[MYSIZE];
    size_t got;

//  PERFORM MULTIPLE READs OF FILE UNTIL END-OF-FILE REACHED  
    while((got = read(fd, buffer, sizeof buffer)) > 0) {  
        .....
    }

//  INDICATE THAT THE PROCESS WILL NO LONGER ACCESS FILE
    close(fd);
}

Note that the functions open, read, and close are not C11 functions but operating system system-calls, providing the interface between our user-level program and the operating system's implementation.

 

CITS2002 Systems Programming, Lecture 7, p1, 12th August 2024.