Demonstration of read() , write() , and lseek()

Listing 4-3: Demonstration of read() , write() , and lseek()
––––––––––––––––––––––––––––––––––––––––––––––––––––––––– fileio/seek_io.c
#include <sys/stat.h>
#include <fcntl.h>
#include <ctype.h>
#include "tlpi_hdr.h"
int
main(int argc, char *argv[])
{
size_t len;
off_t offset;
int fd, ap, j;
char *buf;
ssize_t numRead, numWritten;
if (argc < 3 || strcmp(argv[1], "--help") == 0)
usageErr("%s file {r<length>|R<length>|w<string>|s<offset>}...\n",
argv[0]);
fd = open(argv[1], O_RDWR | O_CREAT,
S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP |
S_IROTH | S_IWOTH); /* rw-rw-rw- */
if (fd == -1)
errExit("open");
for (ap = 2; ap < argc; ap++) {
switch (argv[ap][0]) {
case 'r': /* Display bytes at current offset, as text */
case 'R': /* Display bytes at current offset, in hex */
len = getLong(&argv[ap][1], GN_ANY_BASE, argv[ap]);
File I/O: The Universal I/O Model 85
buf = malloc(len);
if (buf == NULL)
errExit("malloc");
numRead = read(fd, buf, len);
if (numRead == -1)
errExit("read");
if (numRead == 0) {
printf("%s: end-of-file\n", argv[ap]);
} else {
printf("%s: ", argv[ap]);
for (j = 0; j < numRead; j++) {
if (argv[ap][0] == 'r')
printf("%c", isprint((unsigned char) buf[j]) ?
buf[j] : '?');
else
printf("%02x ", (unsigned int) buf[j]);
}
printf("\n");
}
free(buf);
break;
case 'w': /* Write string at current offset */
numWritten = write(fd, &argv[ap][1], strlen(&argv[ap][1]));
if (numWritten == -1)
errExit("write");
printf("%s: wrote %ld bytes\n", argv[ap], (long) numWritten);
break;
case 's': /* Change file offset */
offset = getLong(&argv[ap][1], GN_ANY_BASE, argv[ap]);
if (lseek(fd, offset, SEEK_SET) == -1)
errExit("lseek");
printf("%s: seek succeeded\n", argv[ap]);
break;
default:
cmdLineErr("Argument must start with [rRws]: %s\n", argv[ap]);
}
}
exit(EXIT_SUCCESS);
}
––––––––––––––––––––––––––––––––––––––––––––––––––––––––– fileio/seek_io.c