Programming

tail -n50 something.txt

/* last100 */
/* usage - last100 <filename> lines_to_keep */
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

typedef int fildes;
/* read a buffer */
ssize_t readall(fildes fd, void *buf, size_t nbyte){
     ssize_t nread = 0;
     ssize_t n=0;

     do {
         if ((n = read(fd, &((char *)buf)[nread], nbyte - nread)) == -1) {
             if (errno == EINTR)
                 continue;
             else
                 return (-1);
         }
         if (n == 0)
             return nread;
         nread += n;
     } while (nread < nbyte);
     return nread;
}

char  *process_file(fildes in, char *buf, size_t *len)
{
	struct stat st;
    if (fstat(in,&st)==(-1) )          /* get file size in bytes */
    {
        perror("Cannot stat file");    /* error */
        exit(EXIT_FAILURE);
    }
    *len=(size_t)st.st_size+1;
    buf=malloc((size_t)st.st_size+1);          /* allocate memory for whole file */
    if(buf==NULL)
    {
        perror("Memory allocation failure");	/* mem error */
        exit(EXIT_FAILURE);
    }
    memset(buf,0x0,(size_t)st.st_size+1);
    if(readall(in,buf,(size_t) st.st_size)==(-1)) /* read in whole file */
    {
        perror("Error on file read");   /* error */
        exit(EXIT_FAILURE);	
    }
	return buf;
}

char *position_ptr(char *p,size_t *len, long linecount)
{
    size_t pos=*len;	
    while (pos && linecount>(-1))
    {
    	if( *(p+pos--)=='\n') linecount--;    	
    }
    p+=pos;
    p++; /* skip first newline */
    *len-=pos;
    return p;
}

int main(int argc, char *argv[])
{
    
    size_t len=0;
    char *buf=NULL;
    char *p=NULL;
    fildes in=open(argv[1],O_RDONLY);  /* low-level file open */
    
    if(in==(-1))                      /* error check */
    {
        perror("Error opening input file");
        exit(EXIT_FAILURE);
    }
    buf=process_file(in,buf,&len);
    p=position_ptr(buf,&len,atol(argv[2]));
    fprintf(stdout,"%s",p);
    free(buf);                        /* clean up and exit */    
    if(close(in))
    {
         perror("File close error");
         exit(EXIT_FAILURE);	
    }    
    return 0;
}