How to properly error trap read in c to get byte n

Question

I am currently writing a small dummy program to try and get the hang of properly using the read in c. I made a small function called readdata to read from the file descriptor and store in a buffer then return the number of bytes read. My problem is I am trying to correctly error handle and trap things so that there is no buffer overflow but I keep doing something from.

Here is the tester:

#include <stdio.h>
#include <string.h>
#include <unistd.h>

#define BUFSIZE 10

int readdata(int fd, char *buf, int bsize);

int main(void) {
   char buf[BUFSIZE];
   int returnval;
   int length;
   returnval = readdata(STDIN_FILENO, buf, BUFSIZE);
   printf("%s",buf);
   length = strlen(buf);
   fprintf(stderr,"The return value is %d\n", returnval);
   fprintf(stderr,"The string is %s\n",buf);
   fprintf(stderr,"The length of the string is %d\n",length);
   return 0;
}

Here is the small function:

#include <stdio.h>
#include <stdlib.h>

int readdata(int fd, char *buf, int bufsize){
   int n = 0;
   if(fd < 0){
     return 1;
   }

   while((n=read(fd,buf,(bufsize-1)))>0){
      if(n == -1) {
         perror( "Read failed" );
         return 1;
      }
      else{
         buf[bufsize] = 0;
         return n;
      }
   }
}

If I run

 cc -o test test.c readdata.c

And then put

echo "Hello" | ./test

It works fine. But if I pass the bufsize limit like this:

echo "1234567891" | ./getdatatest

It gives me this weird output where it says "the string is 123456789[some weird symbol]" . So I am not sure where to handle this error or why it is still incorrectly putting in the buffer when reading.

Answer 1

You do know that read() can return less characters than you requested? Also, buf[bufsize] is just past the end of buf. Your readdata function should also return something like -1 on error instead of 1 so you can distinguish the condition “one byte read” from “IO error.”

Consider something like this:

for (;;) {
    n = read(fd, buf, (bufsize - 1));

    if(n == -1) {
       perror( "Read failed" );
       return -1;
    } else {
       buf[n] = 0;
       return n;
    }
}