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Homework and Emergencies Homework & Coursework Questions grep Simulation Using Only Unbuffered I/O - C Post 302514697 by Corona688 on Sunday 17th of April 2011 02:27:59 PM
Old 04-17-2011
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);
}

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GETS(3) 						     Linux Programmer's Manual							   GETS(3)

NAME
fgetc, fgets, getc, getchar, gets, ungetc - input of characters and strings SYNOPSIS
#include <stdio.h> int fgetc(FILE *stream); char *fgets(char *s, int size, FILE *stream); int getc(FILE *stream); int getchar(void); char *gets(char *s); int ungetc(int c, FILE *stream); DESCRIPTION
fgetc() reads the next character from stream and returns it as an unsigned char cast to an int, or EOF on end of file or error. getc() is equivalent to fgetc() except that it may be implemented as a macro which evaluates stream more than once. getchar() is equivalent to getc(stdin). gets() reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF, which it replaces with ''. No check for buffer overrun is performed (see BUGS below). fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A '' is stored after the last character in the buffer. ungetc() pushes c back to stream, cast to unsigned char, where it is available for subsequent read operations. Pushed - back characters will be returned in reverse order; only one pushback is guaranteed. Calls to the functions described here can be mixed with each other and with calls to other input functions from the stdio library for the same input stream. For non-locking counterparts, see unlocked_stdio(3). RETURN VALUE
fgetc(), getc() and getchar() return the character read as an unsigned char cast to an int or EOF on end of file or error. gets() and fgets() return s on success, and NULL on error or when end of file occurs while no characters have been read. ungetc() returns c on success, or EOF on error. CONFORMING TO
ANSI - C, POSIX.1 BUGS
Never use gets(). Because it is impossible to tell without knowing the data in advance how many characters gets() will read, and because gets() will continue to store characters past the end of the buffer, it is extremely dangerous to use. It has been used to break computer security. Use fgets() instead. It is not advisable to mix calls to input functions from the stdio library with low - level calls to read() for the file descriptor associ- ated with the input stream; the results will be undefined and very probably not what you want. SEE ALSO
read(2), write(2), ferror(3), fopen(3), fread(3), fseek(3), puts(3), scanf(3), unlocked_stdio(3) GNU
1993-04-04 GETS(3)
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