sockets/rdwrn.c

/* 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 */
}