C- Need to simulate stdin within the code when calling a function

Question

while( (reader=fread(array, sizeof(char), size, stdin))>0 )

I have this kind of guard what I need is within this cycle when I call a function I want to simulate that I'm giving my fread something.

Sorry about the scary English.

sizeof(char) is guaranteed to be 1 by the C standard. If you want to keep the code flexible, you could write sizeof array[0], which will change appropriately if you change the type of the array. Otherwise, I'd just use 1. (But it's personal preference.) — Chris Lutz
– Chris Lutz, Commented Dec 8, 2010 at 23:41
Change fread to my_fread and write my_fread to do whatever you want. — Eric Postpischil
– Eric Postpischil, Commented Sep 1, 2021 at 16:42

Fred Foo · Accepted Answer · 2011-06-29 06:34:43Z

4

@Fritschy's answer with @Ben Voigt's comment constitute the portable solution.

If you're on Unix/Linux/similar, you can actually write things into stdin by

int p[2];

// error return checks omitted
pipe(p);
dup2(p[0], STDIN_FILENO);

FILE *stdin_writer = fdopen(p[1], "w");

Now write into stdin_writer.

See pipe, dup2, fdopen in the POSIX standard.

answered Jun 29, 2011 at 6:34

Fred Foo

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Marcus Borkenhagen · Accepted Answer · 2010-12-08 23:35:04Z

3

You can replace stdin with a FILE* of your choice for testing:

FILE *test = fopen("mytestFile.txt", "r");
/* your code here... */

Or when need of stdin:

FILE *test = stdin;
/* your code here... */

answered Dec 8, 2010 at 23:35

Marcus Borkenhagen

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3 Comments

Unzi Over a year ago

how can i make this work with my guard in execution time, lets say i call i SIGINT and i want to pop out of my while but im waiting for the stdin input...

Marcus Borkenhagen Over a year ago

You could open files O_NONBLOCK or use select(2) or poll(2). Those however work only on plain FDs. You can get the FD from a FILE* with the fileno(3) function. See the manpages for more ;)

Ben Voigt Over a year ago

or, you could even call freopen on stdin. Not recommended for production code, though in a unit test it could be useful.

Levon · Accepted Answer · 2023-01-09 23:26:50Z

3

This answer is constructed on @Ben Voigt's comment. Create input.txt with some text and main.c with this code. The code will inject input.txt text into stdin and then send it into stdout.

#include <stdlib.h>
#include <stdio.h>

int main() {
    freopen("input.txt", "r", stdin);

    unsigned int BLOCK_SIZE = 3;
    char buffer[BLOCK_SIZE];

    for (;;) {
        size_t bytes = fread(buffer, sizeof(char), BLOCK_SIZE, stdin);
        fwrite(buffer, sizeof(char), bytes, stdout);
        fflush(stdout);
        if (bytes < BLOCK_SIZE)
            if (feof(stdin))
                break;
    }

    return 0;
}

edited Jan 9, 2023 at 23:26

answered Jul 31, 2018 at 15:47

Levon

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