Simple `redirect_stdout` example

General Usage
1

I am struggling to get any example with redirect_stdout to work. There isn’t any documentation on it. E.g.

julia> f, io = mktemp();

julia> fout = open(f, "w+");

julia> redirect_stdout(fout) do
       println("Hello!")
       end;

julia> close(fout)

julia> read(fout)
UInt8[]

julia> read(fout, String)
""

Any ideas? most of the examples I find on the discourse are too old, and no longer work

2

Don’t read from a closed file handle or the end of a file. After the redirect block, try seek(fout, 0); read(fout, String).

2 Likes
3

Thanks! This works with Julia I/O, but not when calling C. In fact something strange happens where it dumps the C stdout upon exiting Julia:

% julia
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.10.10 (2025-06-27)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> f, ios = mktemp();

julia> fout = open(f, "w+");

julia> redirect_stdout(fout) do
         @ccall printf("hello!"::Cstring)::Cint;
       end
6

julia> seek(fout, 0); read(fout, String)
""

julia> exit()
hello!%
4

“Something strange” is probably buffering of the I/O on the C side? Try calling Libc.flush_cstdio() after the printf.

2 Likes
5

That worked! Thank you!

6

Might have to flush on the Julia side too, I’ve been taking the apparent timeliness of REPL printing for granted but writes can be delayed by buffering otherwise.