使用 splice 系统调用没有输出

Question

I have the following code:

#define _GNU_SOURCE
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>

int main(int argc, char **argv) {
  if(argc < 2) {
    fputs("Error: You must provide an filename\n", stderr);
    return 1;
  }

  FILE *handle = fopen(argv[1], "r");
  if(!handle) {
    perror("Error opening file:");
    goto error;
  }

  int handle_fd = fileno(handle);

  int filedes[2];
  if(pipe(filedes)) {
    perror("Error creating pipe:");
    goto error;
  }

  while(1) {
    ssize_t rc = splice(handle_fd, NULL, filedes[1], NULL, BUFSIZ, 0);

    if(rc == -1) {
      // Error occurred
      perror("Error copying data:");
      goto error;
    } else if(rc == 0) {
      break;
    }

    splice(filedes[0], NULL, STDOUT_FILENO, NULL, BUFSIZ, 0);
  }

  return 0;

error:
  if(fclose(handle)) {
    perror("Error closing file:");
  }

  return 1;
}

And when I run it I don't get any output. None of the perror calls are triggered either and I can't figure out why it doesn't work.

When I run the program in GDB it works, however it doesn't display anything when running just plain from the shell

Answer 1

You should check that STDOUT_FILENO is not opened in append mode, or the second splice(2) will fail, as documented in its manpage :

EINVAL The target file is opened in append mode.

And yes that will happen even if it's aa tty, where the O_APPEND flag shouldn't make any difference, which looks very much as a bug . Or at least a nuisance, especially since splice doesn't care when the O_APPEND flag is set on a socket or pipe.

Try running your program as

./your_program file >/dev/tty

or #include <fcntl.h> and add this at the beginning of your main() function:

if(isatty(1)) fcntl(1, F_SETFL, fcntl(1, F_GETFL) & ~O_APPEND);

---

Notice that the O_APPEND flag may get accidentally turned on on your terminal: to check if it's on, look into /proc/<pid>/fdinfo/<fd> :

$ grep flags /proc/self/fdinfo/1
flags:  0102002
           ^ here it is

One curious example of a program which turns the O_APPEND flag on its stdout is GNU make ;-)

$ strace -e trace=fcntl make
fcntl(1, F_GETFL) = 0x48002 (flags O_RDWR|O_LARGEFILE|O_NOATIME)
fcntl(1, F_SETFL, O_RDWR|O_APPEND|O_LARGEFILE|O_NOATIME) = 0
...

---

A testcase, scat.c :

/*
 * a simple program which copies its input to its output via splice(2)
 * if given a '1' argument, it will turn the O_APPEND flag on stdout
 * if given a '0' argument, it will turn it off
 * otherwise it will leave it as it is
 */
#define _GNU_SOURCE     1
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <err.h>
int main(int ac, char **av){
        ssize_t z; int fl;
        if(ac > 1){
                if((fl = fcntl(1, F_GETFL)) == -1) err(1, "fcntl(1, getfl)");
                if(atoi(av[1])) fl |= O_APPEND;
                else fl &= ~O_APPEND;
                if(fcntl(1, F_SETFL, fl)) err(1, "fcntl(1, setfl, %x)", fl);
        }
        while((z = splice(0, 0, 1, 0, 65536, 0)))
                if(z < 0) err(1, "splice");
}

$ cc -Wall scat.c -o scat

$ echo yup | ./scat
yup
$ echo yup | ./scat 1
scat: splice: Invalid argument
$ echo yup | ./scat
scat: splice: Invalid argument
$ echo yup | ./scat 0
yup
$ echo yup | ./scat
yup

OK with a pipe or socket as output:

$ echo yup | ./scat 1 | cat
yup
$ nc -l -p 9999 &
[4] 23952
$ echo yup | ./scat 1 | nc -q0 localhost 9999
yup
[4]-  Done                    nc -l -p 9999