c - Printf printing garbage after read() call. The offset is always printed as 0

Question

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <stdint.h>

int main()
{
    int file;
    off_t offset;

    if((file=open("testfile.txt",O_RDONLY)) < -1)
            return 1;

    char buffer[19];

    if(read(file,buffer,19) != 19)  return 1;
    fprintf(stdout,"%s
",buffer);


    if(offset = lseek(file,-19,SEEK_END) < 0) return 1;
    fprintf(stdout,"%jd
",(intmax_t)offset);

    if(read(file,buffer,19) != 19)  return 1;
    fprintf(stdout,"%s
",buffer);

    return 0;
}

The Output is as follows:

This is a test file??

0

his is a test file

??

testfile.txt :

This is a test file Testing how SEEK_END works This is a test file

I have tried different formatting for offset, such as %ld,%d, but the output is still the same. Can't figure out why garbage appears at the end of first line and last line. Please help.

Answer 1

You need to leave room for an end of line character, '';

So make char buffer[19]; char buffer[20]; then also add buffer[19] = ''; - remember it's zero based counting. Then it shouldn't have the garbage data.

The reason is because printf doesn't know where the end is of the character array. So it keeps printing until it finds a '' in garbage memory.