Question

What I only want written to the file descriptor is the input string. However, it also writes the error messages passed in fprintf. Can someone explain why it is behaving like this? This small program shows the behavior.

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

int main(void)
{
    int fd;
    mode_t mode = S_IRUSR | S_IWUSR;
    char *filename = "file.txt";
    char *input = "hello world";
    char output[20];

    // create and close
    if((fd = open(filename, O_RDWR | O_CREAT | O_TRUNC, mode)) == -1)
    {
        fprintf(stderr, "Error.\n");
        return -1;
    }
    close(fd);

    // just open
    if((fd = open(filename, O_RDWR, mode)) == -1)
    {
        fprintf(stderr, "Error.\n");
        return -1;
    }

    if(write(fd, input, 200) == -1)
    {
        fprintf(stderr, "Error.\n");
        return -1;
    }

    // move back to beginning
    if(lseek(fd, 0, SEEK_SET) == -1)
    { 
        return -1;
    }

    if(read(fd, &output, 200) == -1)
    {
        fprintf(stderr, "Error.\n");
        return -1;
    }

    printf("%s\n", output);

    return 0;
}

Answer 1

write(fd, input, 200)

//...

read(fd, &output, 200)

You’re writing and reading well past the arrays that contain valid data – neither input nor output are anywhere close to 200 bytes. So who knows what’s going to end up in the file (and I’m not sure why the read() doesn’t crash your program)?