Program gets stuck while trying to read a file using read() system call

Question

Here is my code snippet:

int fd;
bufsize = 30;
char buf[bufsize];
char cmd[100] = "file.txt";
int newfd = 1;

if (fd = open(cmd,O_RDONLY) >=0){
    puts("wanna read");
    while (read(fd,&bin_buf,bufsize)==1){
        puts("reading");
        write(newfd,&bin_buf,bufsize);
    }
    close(fd);
}

So here the program prints "wanna read" but never prints "reading". I have also tried opening using nonblock flag, but no use. Can anybody help me? I must use open() and read() system calls only. Thanks. Edit: I have made some clarifications in the code. Actually the newfd that I'm writing to is a socket descriptor, but I don't think that is important for this problem because it sticks on the read which is before the write.

Answer 1

Your read() call attempts to read bufsize bytes and returns the number of bytes actually read. Unless bufsize ==, it is quite unlikely read() will return 1, so the block is almost always skipped and nothing get written.

Also note that if (fd = open(cmd, O_RDONLY) >= 0) is incorrect and would set fd to 1, the handle for standard output, if the file exists, causing the read to fail as standard input is most likely not opened for reading.

Note that reading with the read system call is tricky on some environments, because a return value of -1 may be restartable.

Here is an improved version:

int catenate_file(const char *cmd, int newfd, size_t bufsize) {
    int fd;
    char buf[bufsize];

    if ((fd = open(cmd, O_RDONLY)) >= 0) {
        puts("wanna read");
        ssize_t nc;
        while ((nc = read(fd, buf, bufsize)) != 0) {
            if (nc < 0) {
                if (errno == EINTR)
                    continue;
                else
                    break;
            }
            printf("read %zd bytes\n", nc);
            write(newfd, buf, nc);
        }
        close(fd);
        return 0;
    }
    return -1;
}