read()

Read bytes from a file

Synopsis:

#include <unistd.h>
#include <sys/uio.h>

ssize_t read( int fildes,
              void* buf,
              size_t nbyte );

Library:

libc

Description:

The read() function attempts to read nbyte bytes from the file associated with the open file descriptor, fildes, into the buffer pointed to by buf.

If nbyte is zero, read() returns zero, and has no other effect.

On a regular file or other file capable of seeking, read() starts at a position in the file given by the file offset associated with fildes. Before successfully returning from read(), the file offset is incremented by the number of bytes actually read.

The read() call ignores advisory locks that may have been set by the fcntl() function.

On a file not capable of seeking, read() starts at the current position.

When read() returns successfully, its return value is the number of bytes actually read and placed in the buffer. This number will never be greater than nbyte, although it may be less than nbyte for one of the following reasons:

• the number of bytes left in the file is less than nbyte
• the read() request was interrupted by a signal
• the file is a pipe (or FIFO) or a special file, and has fewer than nbyte bytes immediately available for reading. For example, reading from a file associated with a terminal may return one typed line of data.

If read() is interrupted by a signal before it reads any data, it returns a value of -1, and errno is set to EINTR. However, if read() is interrupted by a signal after it has successfully read some data, it will return the number of bytes read.

No data is transferred past the current end-of-file. If the starting position is at or after the end-of-file, read() returns zero. If the file is a device special file, the result of subsequent calls to read() will work, based on the then current state of the device (that is, the end of file is transitory).

If the value of nbyte is greater than INT_MAX, read() will return -1, and set errno to EINVAL. See <limits.h>.

When attempting to read from an empty pipe or FIFO:

1. If no process has the pipe open for writing, read() returns 0 to indicate end-of-file.
2. If a process has the pipe open for writing, and O_NONBLOCK is set, read() returns -1, and errno is set to EAGAIN.
3. If a process has the pipe open for writing, and O_NONBLOCK is clear, read() blocks until some data is written, or the pipe is closed by all processes that had opened it for writing.

When attempting to read from a file (other than a pipe or FIFO) that support nonblocking reads and has no data currently available:

1. If O_NONBLOCK is set, read() returns -1, and errno is set to EAGAIN.
2. If O_NONBLOCK is clear, read() blocks until some data is available.
3. The O_NONBLOCK flag has no effect if some data is available.

If read() is called on a portion of a file, prior to the end-of-file, that has not been written, it will return bytes with the value zero.

If read() succeeds, the st_atime field of the file is marked for update.

Returns:

The number of bytes actually read, or -1 (errno is set).

Errors:

EAGAIN

The O_NONBLOCK flag is set for the file descriptor, and the process would be delayed in the read operation.

EBADF

The file descriptor, fildes, isn't a valid file descriptor open for reading.

EINTR

The read operation was interrupted by a signal, and either no data was transferred, or the resource manager responsible for that file doesn't report partial transfers.

EIO

A physical I/O error occurred (for example, a bad block on a disk). The precise meaning is device-dependent.

ENOSYS

The read() function isn't implemented for the filesystem specified by filedes.

Examples:

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

int main( void )
{
    int fd;
    int size_read;
    char buffer[80];

    /* Open a file for input */
    fd = open( "myfile.dat", O_RDONLY );

    /* Read the text */
    size_read = read( fd, buffer,
                      sizeof( buffer ) );

    /* Test for error */
    if( size_read == -1 ) {
        perror( "Error reading myfile.dat" );
        return EXIT_FAILURE;
    }

    /* Close the file */
    close( fd );
    
    return EXIT_SUCCESS;
}

Classification:

POSIX 1003.1

See also:

close(), creat(), dup(), dup2(), errno, fcntl(), lseek(), open(), pipe(), readblock(), readv(), select(), write(), writev()