How do I suppress output while using a dynamic library?

Question

I actually have a solution to this problem, but I'm wondering if there is a slicker one.

I have the need to load in a library to my utility using dlopen and then call one of the functions.

Unfortunately, the function spews a whole bunch of information out onto STDOUT and this I do not want.

I have a solution that is non-portable and I'm wondering if there's a better, more generic solution that I could use.

Here's what I have (NB: This is C) :

/*
 * Structure for retaining information about a stream, sufficient to
 * recreate that stream later on
 */
struct stream_info {
    int fd;
    fpos_t pos;
};
#define STDOUT_INFO 0
#define STDERR_INFO 1

struct stream_info s_info[2];
point_stream_to_null(stdout, &s_info[STDOUT_INFO]);
point_stream_to_null(stderr, &s_info[STDERR_INFO]);

void *output = noisy_function();

reset_stream(stderr, &s_info[STDERR_INFO]);
reset_stream(stdout, &s_info[STDOUT_INFO]);

/*
 * Redirects a stream to null and retains sufficient information to restore the stream to its original location
 *** NB ***
 * Not Portable
 */
void point_stream_to_null(FILE *stream, struct stream_info *info) {
    fflush(stream);
    fgetpos(stream, &(info->pos));
    info->fd = dup(fileno(stream));
    freopen("/dev/null", "w", stream);
}

/*
 * Resets a stream to its original location using the info provided
 */
void reset_stream(FILE *stream, struct stream_info *info) {
    fflush(stream);
    dup2(info->fd, fileno(stream));
    close(info->fd);
    clearerr(stream);
    fsetpos(stream, &(info->pos));
}

Any suggestions?

looking for a portable solution. What I have is just fine for Unix. — Dancrumb, Jan 22 '11 at 09:51

Null Set · Answer 1 · 2011-01-21T16:44:04.790

You could try using setvbuf to set stdout to have a very large buffer and be fully buffered. Then, after every call to noisy_function, clear out the buffer before flushing it to the stream. I think this invokes undefined behavior though.

Another way would be to redirect stdout to a temp file, like with this macro function.

#include <stdio.h>

#define QUIET_CALL(noisy) { \
    FILE* tmp = stdout;\
    stdout = tmpfile();\
    (noisy);\
    fclose(stdout);\
    stdout = tmp;\
}

int main(){
    QUIET_CALL(printf("blah blah"));
    printf("bloo bloo\n");
    return 0;
}

score 2 · Accepted Answer · edited Feb 04 '20 at 17:47

2

I have a suggestion, which lets you use the preprocessor for portability, or perhaps "portability".

If you try something like

#if defined __unix__
#define DEVNULL "/dev/null"
#elif defined _WIN32
#define DEVNULL "nul"
#endif

(ignoring other OSes, else case, error directive, etc.) and then reopen the file as before

FILE *myfile = freopen(DEVNULL, "w", stream);

then that may give you what you want.

I haven't tried this at home, though. The "nul" file exists; see /dev/null in Windows. And you can get predefined macros at "Pre-defined C/C++ Compiler Macros".

edited Feb 04 '20 at 17:47

Dancrumb

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answered Mar 29 '11 at 13:43

JXG

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Thanks JXG: That's the closest I got to a portable answer... I'll take "portable" anyday :) – Dancrumb Mar 29 '11 at 23:52

score 1 · Answer 3 · answered Feb 21 '19 at 21:27

1

For Windows console applications:

freopen("NUL", "w", stdout); // redirect stdout to the windows version of /dev/null
NoisyDllFunction();
freopen("CON", "w", stdout); // redirect stdout back to the console

Worked for me.

answered Feb 21 '19 at 21:27

user11098380

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1

score 0 · Answer 4 · answered Jan 21 '11 at 15:04

0

In Windows you can redirect streams too. See http://support.microsoft.com/kb/110930/en-us

answered Jan 21 '11 at 15:04

bratao

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Those redirects are shell redirects. As a result, this does nothing to separate my desired output from the undesired junk that the library function generates – Dancrumb Jan 23 '11 at 00:53

score 0 · Answer 5 · answered Jan 21 '11 at 15:05

0

Unfortunately, freopening to a platform-specific null filename is about the closest you can get in standard C. You could also consider modifying the library itself to not spew so much output on stdout.

That said, in practice, the only OSes you need to worry about are either unix-based (including MacOS) or Windows - in the case of Windows, stdout is hidden by default, so you can just skip the redirection step, and for *nix you have the code already.

answered Jan 21 '11 at 15:05

bdonlan

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Can you clarify 'stdout is hidden by default'? Where is printf directed to in that case? – Dancrumb Jan 21 '11 at 15:20
Unless you have a debugger attached, or someone goes to the trouble of otherwise attaching to your stdout, or you initialize a console, in a Win32 subsystem app anything sent to stdout is discarded by default. – bdonlan Jan 21 '11 at 17:14
Understood. However, the utility is run from the command prompt, so we always have a console attached to STDOUT – Dancrumb Jan 23 '11 at 00:52

How do I suppress output while using a dynamic library?

5 Answers5