node -p cuts off output when piped

[mohd-akram]

Version

v24.12.0

Platform

macOS 26.2

Subsystem

No response

What steps will reproduce the bug?

1. seq -s ' ' 50000 >file
2. node -p 'require("fs").readFileSync("file", "utf-8")' | cat

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior? Why is that the expected behavior?

Prints numbers up to 50000

What do you see instead?

Prints numbers up to 12773

Additional information

For comparison, run cat file | cat, which works correctly.

[aduh95]

I can't reproduce with node -p 'Array.from({length:5000}, (_, i) => i + 1).join(" ")' | cat, it outputs all the numbers for me

[juanarbol]

I don't think this is a Node.js problem itself, but the underlying OS rules.

The pipe buffer is simply full by its first read.

As you may see, Node.js requests 288.895bytes write into stdout. Only 65.536 (64kib) were written. As the event loop is done, Node.js will simply exit.

strace -e trace=write,poll,epoll_ctl,epoll_wait node -p 'require("fs").readFileSync("/tmp/file", "utf-8")' | cat > /dev/null
write(6, "*", 1)                        = 1
write(6, "*", 1)                        = 1
write(1, "1 2 3 4 5 6 7 8 9 10 11 12 13 14"..., 288895) = 65536
write(1, "4 12775 12776 12777 12778 12779 "..., 223359) = 65536
epoll_ctl(15, EPOLL_CTL_DEL, 1, 0x7ffc67383f54) = -1 ENOENT (No such file or directory)
epoll_ctl(15, EPOLL_CTL_DEL, 21, 0x7ffc67383f54) = -1 ENOENT (No such file or directory)
write(6, "*", 1)                        = 1
write(14, "\1\0\0\0\0\0\0\0", 8)        = 8
+++ exited with 0 +++

If you decide to use streams instead, your loop will be busy waiting for the "drain" event. So $ node -e 'require("fs").createReadStream("/tmp/file").pipe(process.stdout)' | c at will fix your issue.

strace -e trace=write,poll,epoll_ctl,epoll_wait node -e 'require("fs").createReadStream("/tmp/file").pipe(process.stdout)' | cat > /dev/null
write(6, "*", 1)                        = 1
write(1, "1 2 3 4 5 6 7 8 9 10 11 12 13 14"..., 65536) = 65536
write(1, "4 12775 12776 12777 12778 12779 "..., 65536) = 65536
write(1, "697 23698 23699 23700 23701 2370"..., 65536) = 65536
write(1, "34620 34621 34622 34623 34624 34"..., 65536) = 65536
write(1, "2 45543 45544 45545 45546 45547 "..., 26750) = 26750
epoll_ctl(15, EPOLL_CTL_DEL, 1, 0x7ffd9f82e124) = -1 ENOENT (No such file or directory)
write(14, "\1\0\0\0\0\0\0\0", 8)        = 8
+++ exited with 0 +++

It just prints everything. The loop is gonna wait for the buffer to drain. And write again, and again... until done.

Or doing more weird stuff like "waiting for drain", will fix your issue.

$ node -e 'const d=require("fs").readFileSync("/tmp/file"); if(!process.stdout.write(d)) process.stdout.once("drain", () => {});' | cat

Your're simply overflowing the pipe buffer.

[juanarbol]

It works nicely in 24.6 BTW

[mohd-akram]

I can't reproduce with node -p 'Array.from({length:5000}, (_, i) => i + 1).join(" ")' | cat, it outputs all the numbers for me

Try 50000.

Your're simply overflowing the pipe buffer.

Node is, not me :D

Simply doing node -e 'process.stdout.write(require("fs").readFileSync("file", "utf-8"))' resolves it for me. So, I'm not sure what -p is doing differently.

[mohd-akram]

It works nicely in 24.6 BTW

Didn't work for me. But Node.js 20.19.6 behaves correctly.

[juanarbol]

It works nicely in 24.6 BTW

Didn't work for me. But Node.js 20.19.6 behaves correctly.

I meant macOS version.

I honestly don't have too much more to say about this issue. This may helps a bit v24.12.0...v20.19.6

[mohd-akram]

Might be this - a8b21fd (#52172)

The problem happens from 22.0 onwards. Maybe a synchronous print needs to happen there (or use the beforeExit event).