CODEWITHSUNDEEP
cfile-io
ChaniLastnamé
ChaniLastnamé

Reputation: 533

Read file byte by byte using read()

I am trying to wrap my head around the read() system call.

How can I read an actual file byte by byte using read()?

The first parameter is the file descriptor which is of type int.
How can I pass a file to the read() call?

Upvotes: 2

Views: 11345

Answers (3)

chakra t
chakra t

Reputation: 37

The concept of read() system call to Kernel is this (In simple english)

read (from this file (file descriptor), into this buffer in the memory, of this size )

Example: Read a character by character from a file which is in the disk into this buffer BUFF 

int fd // initialize the File Descriptor 
fd = open ("file_name", O_RDONLY); //open a file with file name in read only mode.
char BUFF;

read (fd,&BUFF,sizeof(char)); // read file with file descriptor into the address of the BUFF buffer in the memory of a character of size CHAR data type.

Upvotes: 0

Jonathan Leffler
Jonathan Leffler

Reputation: 753475

You open the file with open(); you pass the file descriptor returned by open() to read().

int fd;
if ((fd = open(filename, O_RDWR)) >= 0)
{
    char c;
    while (read(fd, &c, 1) == 1)
        putchar(c);
}

There are other functions that return file descriptors: creat(), pipe(), socket(), accept(), etc.

Note that while this would work, it is inefficient because it makes a lot system calls. Normally, you read large numbers of bytes at a time so as to cut down on the number of system calls. The standard I/O libraries (in <stdio.h>) handle this automatically. If you use the low-level open(), read(), write(), close() system calls, you have to worry about buffering etc for yourself.

Upvotes: 6

Andr&#233; Fratelli
Andr&#233; Fratelli

Reputation: 6068

The last argument to read() is the number of bytes to read from the file, so passing 1 to it would do it. Before that, you use open() to get a file handle, something like this (untested code):

int fh = open("filename", O_RDONLY);
char buffer[1];
read(fh, buffer, 1);

However, it's usually not recommended to read files byte by byte, as it affects performance significantly. Instead, you should buffer your input and process it in chunks, like so:

int fh = open("filename", O_RDONLY);
char buffer[BUFFER_SIZE];
read(fh, buffer, BUFFER_SIZE);

for (int i=0 ; i < BUFFER_SIZE ; ++i) {
   // process bytes at buffer[i]
}

You would finally wrap your reads in a loop until EOF is reached.

Upvotes: 1

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