第六题:

#include "csapp.h"

int main(int argc, char *argv[])
{
    int fd1, fd2, fd3;
    char *fname = argv[1];
    fd1 = Open(fname, O_CREAT|O_TRUNC|O_RDWR, S_IRUSR|S_IWUSR);
    Write(fd1, "pqrs", 4);    

    fd3 = Open(fname, O_APPEND|O_WRONLY, 0);
    Write(fd3, "jklmn", 5);
    fd2 = dup(fd1);  /* Allocates new descriptor */
    Write(fd2, "wxyz", 4);
    Write(fd3, "ef", 2);

    Close(fd1);
    Close(fd2);
    Close(fd3);
    return 0;
}

运行结果:

pqrswxyznef