rdwrn.c

/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2015.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU Lesser General Public License as published   *
* by the Free Software Foundation, either version 3 or (at your option)   *
* any later version. This program is distributed without any warranty.    *
* See the files COPYING.lgpl-v3 and COPYING.gpl-v3 for details.           *
\*************************************************************************/

/* Listing 61-1 */

/* rdwrn.c

   Implementations of readn() and writen().
*/
#include <unistd.h>
#include <errno.h>
#include "rdwrn.h"		/* Declares readn() and writen() */

/* Read 'n' bytes from 'fd' into 'buf', restarting after partial
   reads or interruptions by a signal handlers */

ssize_t readn(int fd, void *buffer, size_t n)
{
    ssize_t numRead;		/* # of bytes fetched by last read() */
    size_t totRead;		/* Total # of bytes read so far */
    char *buf;

    buf = buffer;		/* No pointer arithmetic on "void *" */
    for (totRead = 0; totRead < n;) {
	numRead = read(fd, buf, n - totRead);

	if (numRead == 0)	/* EOF */
	    return totRead;	/* May be 0 if this is first read() */
	if (numRead == -1) {
	    if (errno == EINTR)
		continue;	/* Interrupted --> restart read() */
	    else
		return -1;	/* Some other error */
	}
	totRead += numRead;
	buf += numRead;
    }
    return totRead;		/* Must be 'n' bytes if we get here */
}

/* Write 'n' bytes to 'fd' from 'buf', restarting after partial
   write or interruptions by a signal handlers */

ssize_t writen(int fd, const void *buffer, size_t n)
{
    ssize_t numWritten;		/* # of bytes written by last write() */
    size_t totWritten;		/* Total # of bytes written so far */
    const char *buf;

    buf = buffer;		/* No pointer arithmetic on "void *" */
    for (totWritten = 0; totWritten < n;) {
	numWritten = write(fd, buf, n - totWritten);

	/* The "write() returns 0" case should never happen, but the
	   following ensures that we don't loop forever if it does */

	if (numWritten <= 0) {
	    if (numWritten == -1 && errno == EINTR)
		continue;	/* Interrupted --> restart write() */
	    else
		return -1;	/* Some other error */
	}
	totWritten += numWritten;
	buf += numWritten;
    }
    return totWritten;		/* Must be 'n' bytes if we get here */
}