actual-problem.c

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

void create_file(char* f) {
  int fd = open(f, O_WRONLY| O_CREAT | O_EXCL);
  char* msg = "write some bytes";

  write(fd, msg, 16);
  close(fd);
}

int copy_with_sendfile(char* from, char* to) {
  int fd1 = open(from, O_RDONLY);    
  struct stat fd1_info;                
  fstat(fd1, &fd1_info);               
  int fd2 = open(to, O_WRONLY | O_CREAT|O_EXCL,  0100644);

  int bytes = sendfile(fd2, fd1, NULL, fd1_info.st_size); 

  close(fd1);
  close(fd2);                                                                

  return bytes;         
}

int copy_with_cfr(char* from, char* to) {
  int fd1 = open(from, O_RDONLY);                         
  struct stat fd1_info;                                   
  fstat(fd1, &fd1_info);                                  
  int fd2 = open(to, O_WRONLY | O_CREAT|O_EXCL,  0100644);
                                                          
  int bytes = copy_file_range(fd1, 0, fd2, NULL, fd1_info.st_size, 0);

  close(fd1);                                                                                                                                           
  close(fd2);

  return bytes;                                                  
}


int main() {
  create_file("test");  

  int og_sf_bw = copy_with_sendfile("test", "test-sf");
  int sf_cfr_bw = copy_with_cfr("test-sf", "test-cfr");
  int og_cfr_bw = copy_with_cfr("test", "test-cfr-og");
  int cfr_cfr_bw = copy_with_cfr("test-cfr-og", "test-cfr-cfr");
  
  printf("\tsendfile bytes written: %d\n\tsendfile -> copy_file_range bytes written: %d\n\tcopy_file_range bytes written: %d\n\tcfr->cfr bytes written: %d\n", 
        og_sf_bw, 
        sf_cfr_bw,
        og_cfr_bw,
        cfr_cfr_bw);

  return 0;
}

copy-file.c

// apk add gcc gcompat strace musl-dev
#define _GNU_SOURCE
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

int main() {
  char* infile = getenv("IN_FILE");
  char* outfile = getenv("OUT_FILE");

  int fd = open(infile, O_RDONLY|O_LARGEFILE|O_CLOEXEC);
  int fd2 = open(outfile,  O_WRONLY|O_CREAT|O_LARGEFILE|O_CLOEXEC, 0100644);

  int bytes_written = copy_file_range(fd, 0, fd2, NULL, 12, 0);
  printf("Bytes written %d\n", bytes_written);

  close(fd);
  close(fd2);

  return 0;
}

do-test.sh

// apk add gcc gcompat strace musl-dev

mkdir /tmp/copy_file_range_test
cd /tmp/copy_file_range_test

echo "hello" > testa
cp testa testb

wget https://gist.githubusercontent.com/dpchamps/ff36bb8a7767f6a2d40dbbf5a4817c86/raw/f7d7bb3a90d8af323ec0548fd5d46d7aaa13d8d8/copy-file.c

gcc copy-file.c -o ./copy-file

IN_FILE=testa OUT_FILE=testa-copy ./copy-file
echo "testa-copy contents:"
cat testa-copy

IN_FILE=testb OUT_FILE=testb-copy ./copy-file
echo "testb-copy contents:"
cat testb-copy

Dockerfile

FROM node:16-alpine3.12

RUN apk add gcc gcompat strace musl-dev

RUN wget https://gist.githubusercontent.com/dpchamps/ff36bb8a7767f6a2d40dbbf5a4817c86/raw/45e930dddcc36e0b7bf83bffb24de3f6b47f6d8f/do-test.sh

CMD ["sh", "./do-test.sh"]

run-test

wget -O - https://gist.githubusercontent.com/dpchamps/ff36bb8a7767f6a2d40dbbf5a4817c86/raw/45e930dddcc36e0b7bf83bffb24de3f6b47f6d8f/do-test.sh | sh

test1 strace

/cstuff # strace ./copy_file_test
execve("./copy_file_test", ["./copy_file_test"], 0x7ffc0ea35eb0 /* 9 vars */) = 0
arch_prctl(ARCH_SET_FS, 0x7f7ac4ed2b48) = 0
set_tid_address(0x7f7ac4ed2f90)         = 334
brk(NULL)                               = 0x559680da6000
brk(0x559680da8000)                     = 0x559680da8000
mmap(0x559680da6000, 4096, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x559680da6000
mprotect(0x7f7ac4ecf000, 4096, PROT_READ) = 0
mprotect(0x55967fec0000, 4096, PROT_READ) = 0
open("./foo/test", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
open("./foo/test2", O_WRONLY|O_CREAT|O_LARGEFILE|O_CLOEXEC, 0100644) = 4
fcntl(4, F_SETFD, FD_CLOEXEC)           = 0
copy_file_range(3, NULL, 4, NULL, 12, 0) = 6
close(3)                                = 0
close(4)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++

test1.c

#include <fcntl.h>
#include <unistd.h>


int main() {
  int fd = open("./foo/test", O_RDONLY|O_LARGEFILE|O_CLOEXEC);
  int fd2 = open("./foo/test2",  O_WRONLY|O_CREAT|O_LARGEFILE|O_CLOEXEC, 0100644);
  copy_file_range(fd, 0, fd2, NULL, 12, 0);
  close(fd);
  close(fd2);

  return 0;
}

test2 strace

cstuff # strace ./copy_file_test
execve("./copy_file_test", ["./copy_file_test"], 0x7ffd25f9b4a0 /* 9 vars */) = 0
arch_prctl(ARCH_SET_FS, 0x7f3b9b8fbb48) = 0
set_tid_address(0x7f3b9b8fbf90)         = 349
brk(NULL)                               = 0x555709f0b000
brk(0x555709f0d000)                     = 0x555709f0d000
mmap(0x555709f0b000, 4096, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x555709f0b000
mprotect(0x7f3b9b8f8000, 4096, PROT_READ) = 0
mprotect(0x555708fb8000, 4096, PROT_READ) = 0
open("./bar/test", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
open("./bar/test2", O_WRONLY|O_CREAT|O_LARGEFILE|O_CLOEXEC, 0100644) = 4
fcntl(4, F_SETFD, FD_CLOEXEC)           = 0
copy_file_range(3, NULL, 4, NULL, 12, 0) = 0
close(3)                                = 0
close(4)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++

test2.c

#include <fcntl.h>
#include <unistd.h>


int main() {
  int fd = open("./bar/test", O_RDONLY|O_LARGEFILE|O_CLOEXEC);
  int fd2 = open("./bar/test2",  O_WRONLY|O_CREAT|O_LARGEFILE|O_CLOEXEC, 0100644);
  copy_file_range(fd, 0, fd2, NULL, 12, 0);
  close(fd);
  close(fd2);

  return 0;
}