Question

I am trying to use the system calls read() and write(). The following program creates a file and writes some data into it. Here is the code..

int main()
{
    int fd;
        open("student",O_CREAT,(mode_t)0600);

    fd=open("student",O_WRONLY);
    char data[128]="Hi nikhil, How are u?";

    write(fd,data,128);

}

Upon the execution of the above program i got a file with name student created with size as 128 bytes.

int main()
{
    int fd=open("student",O_WRONLY);
    char data[128];
    read(fd,data,128);
    cout<<(char*)data<<endl;
}

But the output i get is junk characters….why is this so?

I wrote a small read program to read data from the file. Her is the code.

But the output

Answer 1

You’re not checking whether read() returns an error. You should do so, because that’s probably the case with the code in your question.

Since you’re opening the file write-only in the first place, calling read() on it will result in an error. You should open the file for reading instead:

char data[128];
int fd = open("student", O_RDONLY);
if (fd != -1) {
    if (read(fd, data, sizeof(data)) != -1) {
        // Process data...
    }
    close(fd);
}