Unix (in C) trying to write the tail for XV6

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Hi stackoverflow(ers)!

I'm learning Unix using the XV6 OS (documentation found here) and have been trying to write a tail function in C. The expected output of:

  1. tail is to give the last 10 lines of the file
  2. tail - is to give the last of lines of the file
  3. tail ... is to give the last 10 lines of files ...
  4. tail - ... is to give the last of lines of ...
  5. grep | tail is to give the last 10 sentences in which contain

I have written two versions of tail, one implemented using char* [] and the other by writing to a file and then reading from it (both posted below) My version which implements the tail using char* [] seems to be more accurate to the actual command. However in the version where I'm writing to a temporary file and then reading from it I'm getting more lines as output and I'm not sure why that is happening. My guess is, while reading from one file and writing to another the placement of '\n' are getting messed up. I'd highly appreciate help in figuring it out!

Please don't get mad at me if I'm doing something silly. I'm new to C in Unix and only trying to learn.

tail.c using char* []

#include "types.h"
#include "stat.h"
#include "user.h"
#include "fcntl.h"

char buf [512];

void tail (int fd, int toSub) {
  int n;
  int numLines = 0;
  int linesToPrint = 0;
  char *buffer;
  buffer = (char*) malloc (500000);
  int buffSize = 0;
  while ((n = read(fd, buf, sizeof(buf))) > 0) {
    for (int i = 0; i<n; i++) {
      buffer[buffSize] = (char)buf[i];
      buffSize++;
      if(buf[i] == '\n')
        numLines++;
    }
  }
  if (n < 0) {
    printf (1, "tail: read error \n");
    exit ();
  }
  if (numLines < toSub)
    linesToPrint = 0;
  linesToPrint = numLines - toSub;

  int counter = 0;
  for (int i = 0; i < buffSize; i++) {
    if (counter >= linesToPrint)
      printf(1,"%c",buffer[i]);
    if (buffer[i] == '\n')
      counter++;
  }
  free (buffer);
}

int main (int argc, char *argv[]) {
  int toSub = 10;
  int fd = -1;

  if (argc <= 1) {
    tail (0, toSub);
    exit();
  }
  else if (argc > 1 && argv[1][0] == '-') {
    char getToSub [10];
    for (int k=1; k<strlen(argv[1]); k++) {
      getToSub[k-1] = argv[1][k];
    }
    toSub = (atoi)(getToSub);
  }
  else {
    if((fd = open (argv[1], toSub)) < 0) {
      printf (1, "tail: cannot open %s\n", argv[1]);
      exit ();
    }
    tail (fd, toSub);
    close (fd);
  }
  if (argc > 2) {
    for (int i=2; i<argc; i++) {
      if((fd = open (argv[i], 0)) < 0) {
        printf (1, "tail: cannot open %s\n", argv[i]);
        exit ();
      }
      else {
        tail (fd, toSub);
        close (fd);
      }
    }
  }
  exit();
}

tail.c using write

#include "types.h"
#include "stat.h"
#include "user.h"
#include "fcntl.h"

char buf [512];

void tail (int fd, int toSub) {
  int n;
  int numLines;
  int linesToPrint;
  int ptrDump;
  ptrDump = open ("tailDump", O_CREATE | O_RDWR);
  while ((n = read(fd, buf, sizeof(buf))) > 0) {
    write (ptrDump, buf, sizeof(buf));
    for (int i = 0; i<n; i++) {
      if(buf[i] == '\n')
        numLines++;
    }
  }
  if (n < 0) {
    printf (1, "tail: read error \n");
    exit ();
  }
  if (numLines < toSub)
    linesToPrint = 0;
  linesToPrint = numLines - toSub;

  close (ptrDump);
  ptrDump = open ("tailDump", 0);

  int counter = 0;
  while ((n = read(ptrDump, buf, sizeof(buf))) > 0) {
    for (int i = 0; i<n; i++) {
      if (counter > linesToPrint)
        printf(1,"%c",buf[i]);
      if (buf[i] == '\n')
        counter++;
      }
    }
    close (ptrDump);
    unlink("tailDump");
}

int main (int argc, char *argv[]) {
  int toSub = 10;
  int fd = -1;

  if (argc <= 1) {
    tail (0, toSub);
    exit();
  }
  else if (argc > 1 && argv[1][0] == '-') {
    char getToSub [10];
    for (int k=1; k<strlen(argv[1]); k++) {
      getToSub[k-1] = argv[1][k];
    }
    toSub = (atoi)(getToSub);
  }
  else {
    if((fd = open (argv[1], toSub)) < 0) {
      printf (1, "tail: cannot open %s\n", argv[1]);
      exit ();
    }
    tail (fd, toSub);
    close (fd);
  }
  if (argc > 2) {
    for (int i=2; i<argc; i++) {
      if((fd = open (argv[i], 0)) < 0) {
        printf (1, "tail: cannot open %s\n", argv[i]);
        exit ();
      }
      else {
        tail (fd, toSub);
        close (fd);
      }
    }
  }
  exit();
}

I have the code put up on my Github (found here) as well in tail_using_str.c and tail_using_file.c


I think your problem is here:

  while ((n = read(fd, buf, sizeof(buf))) > 0) {
    write (ptrDump, buf, sizeof(buf));

You read in n bytes but when you write, you write sizeof(buf) bytes. In other words, you may write too many bytes.

Maybe you want this instead:

  while ((n = read(fd, buf, sizeof(buf))) > 0) {
    write (ptrDump, buf, n);
                         ^
                        note