fileio/seek_io.c

+/* seek_io.c
+
+   Demonstrate the use of lseek() and file I/O system calls.
+
+   Usage: seek_io file {r<length>|R<length>|w<string>|s<offset>}...
+
+   This program opens the file named on its command line, and then performs
+   the file I/O operations specified by its remaining command-line arguments:
+
+           r<length>    Read 'length' bytes from the file at current
+                        file offset, displaying them as text.
+
+           R<length>    Read 'length' bytes from the file at current
+                        file offset, displaying them in hex.
+
+           w<string>    Write 'string' at current file offset.
+
+           s<offset>    Set the file offset to 'offset'.
+
+   Example:
+
+        seek_io myfile wxyz s1 r2
+*/
 #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;
     unsigned 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]);
 
             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(buf[j]) ?  buf[j] : '?');
                     else
                         printf("%02x ", 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]);
         }
     }
 
     if (close(fd) == -1)
         errExit("close");
 
     exit(EXIT_SUCCESS);
 }