Invalid argument in sendfile() with two regular files
I'm trying to test the sendfile()
system call under Linux 2.6.32 to zero-copy data between two regular files.
As far as I understand, it should work: ever since 2.6.22, sendfile()
has been implemented using splice()
, and both the input file and the output file can be either regular files or sockets.
The following is the content of sendfile_test.c
:
#include <sys/sendfile.h>
#include <fcntl.h>
#include <stdio.h>
int main(int argc, char **argv) {
int result;
int in_file;
int out_file;
in_file = open(argv[1], O_RDONLY);
out_file = open(argv[2], O_WRONLY | O_CREAT | O_TRUNC, 0644);
result = sendfile(out_file, in_file, NULL, 1);
if (result == -1)
perror("sendfile");
close(开发者_运维百科in_file);
close(out_file);
return 0;
}
And when I'm running the following commands:
$ gcc sendfile_test.c
$ ./a.out infile outfile
The output is
sendfile: Invalid argument
And when running
$ strace ./a.out infile outfile
The output contains
open("infile", O_RDONLY) = 3
open("outfile", O_WRONLY|O_CREAT|O_TRUNC, 0644) = 4
sendfile(4, 3, NULL, 1) = -1 EINVAL (Invalid argument)
What am I doing wrong?
You forgot to check that argc
is equal to 3
, i.e. you are opening the output file by the name argv[2]
but only give your program one argument (and you are not checking for errors after open(2)
.)
You can use strace(1)
to find out which system call fails.
Edit:
This looks like older kernel to me. Same source (modulo error checking) works fine here under 2.6.33.4 #3 SMP
. Also, any particular reason you are copying just one byte?
#include <sys/sendfile.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main( int argc, char *argv[] )
{
int in_file, out_file;
if ( argc != 3 )
{
fprintf( stderr, "usage: %s <in-file> <out-file>\n", argv[0] );
exit( 1 );
}
if (( in_file = open( argv[1], O_RDONLY )) == -1 ) err( 1, "open" );
if (( out_file = open( argv[2], O_WRONLY | O_CREAT | O_TRUNC, 0644 )) == -1 )
err( 1, "open(2)" );
if ( sendfile( out_file, in_file, NULL, 4096 ) == -1 ) err( 1, "sendfile" );
exit( 0 );
}
Trace:
nickf@slack:~/csource/linux/splice$ cc -Wall -pedantic -ggdb -g3 -o sf sndf.c
nickf@slack:~/csource/linux/splice$ strace ./sf Makefile mm
...
open("Makefile", O_RDONLY) = 3
open("mm", O_WRONLY|O_CREAT|O_TRUNC, 0644) = 4
sendfile(4, 3, NULL, 4096) = 239
exit_group(0) = ?
nickf@slack:~/csource/linux/splice$ diff Makefile mm
nickf@slack:~/csource/linux/splice$
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