1. The problem statement, all variables and given/known data:
Basically, I'm trying to search for a particular string pattern within a text file and print "textfile: line_no line". I need to read this in line at a time, but I'm restricted to using unbuffered I/O functions. I'm not sure if I'm missing something minor or if I'm misunderstanding how this works. The idea was to read() a small buffer (size 50), then to move the buffer over character at a time into a temp array until a newline is reached. Then use strstr to determine if the pattern occurs in the temp array, and if so, print the temp array to the screen.
I am reasonably sure that the problem lies within moving the buffer over to a temp array, and possibly with memset. Also, possible with read, because I keep getting a byte of trash every time I read into buf.
The output I'm getting for buf makes it appear that lseek is working perfectly fine, however, the temp array appears to be getting buf twice.
2. Relevant commands, code, scripts, algorithms:
Command used in bash shell: ./mycp <pattern> <filename>
3. The attempts at a solution (include all code and scripts):
I've attempted altering this line several times:
Code:
while (buf[i] != NULL)
In place of "NULL", I've also used '/0' and '/n'. I've also tried strcpy() instead of copying over one character at a time. I get similar output, but amount of trash varies.
Code:
// includes
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
// defines
#define max 50
int main(int argc, char *argv[])
{
char *buf[max];
char *temp[max];
char *ptr;
int *infile;
int n;
int i = 0;
//int j = 0;
int count = 0; // track line number
int occurences = 0; // track # of occurences
int offsetSum = 0; // track file offset position
if (argc != 3)
{
printf("usage: ./mygrep pattern filename\n");
exit(1);
}
if ((infile = open(argv[2], O_RDONLY)) == -1)
{
printf("Unable to open file: %s\n", argv[2]);
exit(1);
}
memset(temp, '\0', max); // set for first use
while ((n = read(infile, buf, max)) > 0 /*&& j < 10*/) // while !EOF
{
count++; // keep up with line number
while (buf[i] != NULL) // while in buf & not end of line
{
temp[i] = buf[i]; // copy one char at time into temp
i++;
offsetSum++;
}
if ((ptr = strstr(temp, argv[1])) != NULL); // if pattern found
{
printf("%s: %d %s\n", argv[2], count, temp);
occurences++;
}
if (lseek(infile, offsetSum, SEEK_SET) < 0) // reset file position for
error("Error while reading file: ", infile); // next read
i = 0; // reset for next use
memset(temp, '\0', max); // reset for next use
//j++;
}
printf("Number of occurences of %s : %d\n", argv[1], occurences);
close(infile);
return 0;
}
4. Complete Name of School (University), City (State), Country, Name of Professor, and Course Number (Link to Course):
Arkansas State University, Jonesboro, AR, USA - Dr. Jiang CS4223
read() doesn't add NULLs to the end of your array, so checking for NULL won't work. read() DOES return how many bytes it read, so you could just stop there instead, but --
read() doesn't stop at the end of a line, either. You should read one by one and check for newlines yourself. You could try something like this:
Code:
// reads a line of text in a manner sort of like fgets, and adds null.
// returns the # of bytes read, 0 means error.
int my_getline(int fd, char *buf, int max)
{
int n;
// read until end of file, or newline, or max
for(n=0; n<(max-1); n++)
{
ssize_t b=read(fd, buf+n, 1); // read one byte
if(b <= 0) // end of file or error
{
buf[n]='\0'; // add null
return(n);
}
if(buf[n] == '\n') // end of line
{
buf[n+1]='\0'; // add null AFTER \n
return(n);
}
}
buf[n]='\0';
return(n);
}
Thank you for the help! Using your suggestion, I rewrote my own getline to serve the purpose I needed:
Code:
int getline(int infile, char *buf, char *pattern)
{
int n;
int count = 0; // track line number
int occurences = 0; // track number of occurences
for (n = 0; n < (max - 1); n++) // while not EOF--b will grab each read
{
// point of this section is to read one byte at a time until end of line
ssize_t b = read(infile, buf + n, 1); // read one byte at a time
if (b <= 0) // EOF or error
{
buf[n] = '\0';
if (b == -1)
error("Error while reading file %s\n", infile);
}
if (buf[n] == '\n') // end of line
{
count++;
buf[n+1] = '\0';
occurences += checkPattern(pattern, buf, count);
n = -1;
}
}
buf[n] = '\0';
return occurences;
}
You forgot to break on error or end-of-line, if you don't your for() loop won't actually stop.
Also this: buf[n+1] = '\0'; Remember, the code you copied that from added the \n to the data. That's why it needed +1, to not overwrite th \n. When not adding the \n, n+1 goes one too far, leaving a garbage byte in your string.
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grep Simulation Using Only Unbuffered I/O - C
04-17-2011
