Read File Introduction

The unistd.h library contains methods for working with files:

#include<unistd.h>
ssize_t read(int fd, void *buf, size_t len);

• Each call reads len number of bytes into buf from the current position of the file specified in the file descriptor fd.

• On success, it returns the number of bytes written to buf.

• On error, it returns -1, and error information is written to errno.

Actually, the read method can lead to many possibilities:

• Returns a value equal to len and all read bytes are stored in buf. The results are as expected.

• Returns a value less than len but greater than zero. The read bytes are stored in buf. This can happen because a signal interrupted the read in the middle, an error occurred in the middle of reading, more than zero but less than the value of len bytes are available, or EOF is reached before len bytes are read. Reissuing the read (with correspondingly updated buf and len values) will read the remaining bytes into the rest of the buffer or indicate the cause of the problem.

• Returns a value of 0. This indicates EOF. There is nothing to read.

• The method call blocks because no data is available. This will not happen in non-blocking mode.

• Returns a value of -1 and the error is set to EINTR. This indicates that a signal was received before any bytes were read.

• Returns a value of -1 and the error is set to EAGAIN. This indicates that the read would block because no data is currently available. This only happens in non-blocking mode.

• Returns a value of -1 and the error is set to a value other than EINTR or EAGAIN. This indicates a more serious error.