I have a program that I start from a command line with a file pushed to stdin:
main.exe < file.bin
I read all the contents from the file like so:
freopen(NULL, "rb", stdin); // it's a binary file
long* buffer = new long[16000];
while ( fread(buffer, sizeof(long), 16000, stdin) == 16000);
After all numbers are read, I would like the user to confirm continuation in the program by pressing any key but at that point stdin probably contains EOF and the program skips the confirmation from the user.
printf("Press any key to continue");
getchar(); // user doesn't get a chance to press anything, program flows right through
Here's the whole program if you want to reproduce it. But to reproduce you have to push a file to stdin before execution.
#include <stdio.h>
int main()
{
freopen(NULL, "rb", stdin);
long* buffer = new long[16000];
while ( fread(buffer, sizeof(long), 16000, stdin) == 16000);
printf("Press any key to continue");
getchar();
delete[] buffer;
return 0;
}
The question is how to make the getchar() call take an character from the user's keyboard and not the file that was already read.
The program runs as C++17 but uses a lot of C style code. If you know a way how to do this with C++ streams feel free to post that as well. I use Windows 10 but I would also like to see some portable code.
If you use a file file.bin as a stdin via main.exe < file.bin, you cannot simultaneously use console as a stdin using standard C or C++ libraries. Maybe you could try it with WinAPI https://learn.microsoft.com/en-us/windows/console/readconsoleinput
And using std::cin to read binary data is unfortulately not possible: Read binary data from std::cin
Related
I am trying to create a tester using googletest. the problem is that the function that I am testing is returning void and printing a result instead. I want to get the last string printed into the console so I can test the output. the string may include \n.
so I have the function itself:
void f_sequence(char sequenceStr[])
{
//logic...
if(condotion1)
printf("somthing1");
else if(condotion2)
printf("somthing2")
(...)
}
and then the tester:
TEST(TesterGroup, TesterName)
{
f_sequence("input");
EXPECT_EQ("somthing1", /*how do i get the output?*/);
}
Is it possible?
The functions I test are in c, while the Test function itself (the tester) is in c++. the output is printed using printf. I cannot change the function itself. I am using CLion latest version.
Redirect the standard output to a buffer.
Live on Coliru
#include <stdio.h>
#include <unistd.h>
#define BUFFER_SIZE 1024
int stdoutSave;
char outputBuffer[BUFFER_SIZE];
void replaceStdout()
{
fflush(stdout); //clean everything first
stdoutSave = dup(STDOUT_FILENO); //save the stdout state
freopen("NUL", "a", stdout); //redirect stdout to null pointer
setvbuf(stdout, outputBuffer, _IOFBF, 1024); //set buffer to stdout
}
void restoreStdout()
{
freopen("NUL", "a", stdout); //redirect stdout to null again
dup2(stdoutSave, STDOUT_FILENO); //restore the previous state of stdout
setvbuf(stdout, NULL, _IONBF, BUFFER_SIZE); //disable buffer to print to screen instantly
}
void printHelloWorld()
{
printf("hello\n");
printf("world");
}
int main()
{
replaceStdout();
printHelloWorld();
restoreStdout();
// Use outputBuffer to test EXPECT_EQ("somthing1", outputBuffer);
printf("Fetched output: (%s)", outputBuffer);
return 0;
}
References: http://kaskavalci.com/redirecting-stdout-to-array-and-restoring-it-back-in-c/
I don't know if it's possible to get what was last printed, but if you control the environment before your test function is called, you can redirect where standard output goes, which lets you write it to a file, which you can then check.
See this old answer which IMO was neglected. The example from it is modified here:
FILE *fp_old = stdout; // preserve the original stdout
stdout = fopen("/path/to/file/you/want.txt","w"); // redirect stdout to anywhere you can open
// CALL YOUR FUNCTION UNDER TEST HERE
fclose(stdout); // Close the file with the output contents
stdout=fp_old; // restore stdout to normal
// Re-open the file from above, and read it to make sure it contains what you expect.
Two ways:
If you're on a POSIX compatibile system, you can redirect the output of the program to a file using >, then read from the file later to confirm that the output is correct.
The other way is something like this:
freopen("output.txt", "w", stdout);
f_sequence();
freopen("/dev/tty", "w", stdout);
for POSIX compatible systems. On other platforms you'd need to change the "/dev/tty" to something else. I'm not aware of a completely portable way to do this.
And then read from output.txt. What the above snippet does is change what stdout is, so that it prints to a file instead of the normal stdout.
One solution: You can write a separate program that executes the function, and in the unit test, you can execute that program as a sub process and inspect the output. This can be done with std::system, but be very careful to not pass any non-constant input to it. You don't want shell injection vulnerability even in a unit test. System specific functions exist that avoid the use of shell in the subprocess.
Another solution, which is possible at least on POSIX: Replace the the standard out / err streams with file descriptors, and read the files afterwards.
Googletest specific: There seems to be testing::internal::CaptureStdout, which appears to implement the idea of replacing standard streams. But as the namespace implies, this is not official API, so may change in future.
There is a solution ( in C ) for an API call ( cmd_rsp ) with source code here, that when called in your program, creates a separate process, and pipes to both stdin and stdout, from which it accepts a command and returns the response via an auto sizing buffer. Similar in concept to popen(...).
A simple use case:
char *buf = NULL;
/// test cmd_rsp
buf = calloc(BUF_SIZE, 1);
if(!buf)return 0;
if (!cmd_rsp("dir /s", &buf, BUF_SIZE))//note the first argument can be any legal command that
//can be sent via the CMD prompt in windows,
//including a custom executable
{
printf("%s", buf);
}
else
{
printf("failed to send command.\n");
}
free(buf);
I'm trying to use freopen() multiple times to read and close different files.
So here's what I did inside my main function:
if (argc != 5) {
std::cerr << "Wrong format of arguments given." << endl;
return -1;
}
std::string command, command2;
freopen(argv[1], "r", stdin);
// do something...
fclose(stdin);
freopen(argv[2], "r", stdin);
freopen(argv[3], "w", stdout);
while (std::cin >> command) {
std::cin >> command2;
// run some function...
}
fclose(stdin);
fclose(stdout);
But it turns out that the first part, // do something..., works fine (reads from std::cin without a problem) but the while loop in the second doesn't seem to run.
The input files are of correct formats, so I don't know why std::cin >> command returns false.
In line freopen(argv[2], "r", stdin); you are trying to reopen stdin. But you have already closed stdin in line fclose(stdin); just before that. Also, stdin is now dangling pointer after closing the file.
Following is an extract from www.cplusplus.com:
If a new filename is specified, the function first attempts to close any file already associated with stream (third parameter) and disassociates it. Then, independently of whether that stream was successfuly closed or not, freopen opens the file specified by filename and associates it with the stream just as fopen would do using the specified mode.
You should use fopen() function after closing stdin.
Say I have an .exe, lets say sum.exe. Now say the code for sum.exe is
void main ()
{
int a,b;
scanf ("%d%d", &a, &b);
printf ("%d", a+b);
}
I wanted to know how I could run this program from another c/c++ program and pass input via stdin like they do in online compiler sites like ideone where I type the code in and provide the stdin data in a textbox and that data is accepted by the program using scanf or cin. Also, I wanted to know if there was any way to read the output of this program from the original program that started it.
The easiest way I know for doing this is by using the popen() function. It works in Windows and UNIX. On the other way, popen() only allows unidirectional communication.
For example, to pass information to sum.exe (although you won't be able to read back the result), you can do this:
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *f;
f = popen ("sum.exe", "w");
if (!f)
{
perror ("popen");
exit(1);
}
printf ("Sending 3 and 4 to sum.exe...\n");
fprintf (f, "%d\n%d\n", 3, 4);
pclose (f);
return 0;
}
In C on platforms whose name end with X (i.e. not Windows), the key components are:
pipe - Returns a pair of file descriptors, so that what's written to one can be read from the other.
fork - Forks the process to two, both keep running the same code.
dup2 - Renumbers file descriptors. With this, you can take one end of a pipe and turn it into stdin or stdout.
exec - Stop running the current program, start running another, in the same process.
Combine them all, and you can get what you asked for.
This is my solution and it worked:
sum.cpp
#include "stdio.h"
int main (){
int a,b;
scanf ("%d%d", &a, &b);
printf ("%d", a+b);
return 0;
}
test.cpp
#include <stdio.h>
#include <stdlib.h>
int main(){
system("./sum.exe < data.txt");
return 0;
}
data.txt
3 4
Try this solution :)
How to do so is platform dependent.
Under windows, Use CreatePipe and CreateProcess. You can find example from MSDN :
http://msdn.microsoft.com/en-us/library/windows/desktop/ms682499(v=vs.85).aspx
Under Linux/Unix, you can use dup() / dup2()
One simple way to do so is to use a Terminal (like command prompt in windows) and use | to redirect input/output.
Example:
program1 | program2
This will redirect program1's output to program2's input.
To retrieve/input date, you can use temporary files, If you don't want to use temporary files, you will have to use pipe.
For Windows, (use command prompt):
program1 <input >output
For Linux, you can use tee utility, you can find detail instruction by typing man tee in linux terminal
It sounds like you're coming from a Windows environment, so this might not be the answer you are looking for, but from the command line you can use the pipe redirection operator '|' to redirect the stdout of one program to the stdin of another. http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/redirection.mspx?mfr=true
You're probably better off working in a bash shell, which you can get on Windows with cygwin http://cygwin.com/
Also, your example looks like a mix of C++ and C, and the declaration of main isn't exactly an accepted standard for either.
How to do this (you have to check for errors ie. pipe()==-1, dup()!=0, etc, I'm not doing this in the following snippet).
This code runs your program "sum", writes "2 3" to it, and than reads sum's output. Next, it writes the output on the stdout.
#include <iostream>
#include <sys/wait.h>
#include <unistd.h>
int main() {
int parent_to_child[2], child_to_parent[2];
pipe(parent_to_child);
pipe(child_to_parent);
char name[] = "sum";
char *args[] = {name, NULL};
switch (fork()) {
case 0:
// replace stdin with reading from parent
close(fileno(stdin));
dup(parent_to_child[0]);
close(parent_to_child[0]);
// replace stdout with writing to parent
close(fileno(stdout));
dup(child_to_parent[1]);
close(child_to_parent[1]);
close(parent_to_child[1]); // dont write on this pipe
close(child_to_parent[0]); // dont read from this pipe
execvp("./sum", args);
break;
default:
char msg[] = "2 3\n";
close(parent_to_child[0]); // dont read from this pipe
close(child_to_parent[1]); // dont write on this pipe
write(parent_to_child[1], msg, sizeof(msg));
close(parent_to_child[1]);
char res[64];
wait(0);
read(child_to_parent[0], res, 64);
printf("%s", res);
exit(0);
}
}
I'm doing what #ugoren suggested in their answer:
Create two pipes for communication between processes
Fork
Replace stdin, and stdout with pipes' ends using dup
Send the data through the pipe
Based on a few answers posted above and various tutorials/manuals, I just did this in Linux using pipe() and shell redirection. The strategy is to first create a pipe, call another program and redirect the output of the callee from stdout to one end of the pipe, and then read the other end of the pipe. As long as the callee writes to stdout there's no need to modify it.
In my application, I needed to read a math expression input from the user, call a standalone calculator and retrieve its answer. Here's my simplified solution to demonstrate the redirection:
#include <string>
#include <unistd.h>
#include <sstream>
#include <iostream>
// this function is used to wait on the pipe input and clear input buffer after each read
std::string pipeRead(int fd) {
char data[100];
ssize_t size = 0;
while (size == 0) {
size = read(fd, data, 100);
}
std::string ret = data;
return ret;
}
int main() {
// create pipe
int calculatorPipe[2];
if(pipe(calculatorPipe) < 0) {
exit(1);
}
std::string answer = "";
std::stringstream call;
// redirect calculator's output from stdout to one end of the pipe and execute
// e.g. ./myCalculator 1+1 >&8
call << "./myCalculator 1+1 >&" << calculatorPipe[1];
system(call.str().c_str());
// now read the other end of the pipe
answer = pipeRead(calculatorPipe[0]);
std::cout << "pipe data " << answer << "\n";
return 0;
}
Obviously there are other solutions out there but this is what I can think of without modifying the callee program. Things might be different in Windows though.
Some useful links:
https://www.geeksforgeeks.org/pipe-system-call/
https://www.gnu.org/software/bash/manual/html_node/Redirections.html
I try to redirect output in my c++ program from stdout with the following:
freopen(cmd.c_str(),"w",stdout);
Then I call system to execute cmd. I have also tried forking and then calling execvp. Either way, when program control returns to my program, things written to stdout are no longer showing. How can normal behavior be restored?
Here is solution for stdin if doing in loop, needed to figure this out for a program wherein freopen of stdin happens in a loop on some condition. Took some time for me to figure out (with help of search and all) and so posting here
savestdin = dup(STDIN_FILENO);
while (1) {
.
.
if (inputfile) {
savestdin = dup(savestdin);
freopen(inputfile, "r", stdin);
restorestdin = TRUE;
}
.
.
if (restorestdin) {
fflush(stdin);
fclose(stdin);
stdin = fdopen(savestdin, "r");
restorestdin = FALSE;
}
.
.
}
Do this:
fclose(stdout);
stdout = fdopen(1, "w"); //reopen: 1 is file descriptor of std output
If you can use STDOUT_FILENO from <unistd.h>, instead of 1 as first argument to fdopen.
Is that even possible ?
Lets say that the code has a lot of scanf lines. Instead of manually running and adding values by hand when debugging, is it possible to "feed" stdin with data so that when the scanf starts reading, it will read the inputted data without any need to interact with the terminal.
Put the test lines into a file, and run the program like this:
myprogram < mytestlines.txt
Better than hacking your program to somehow do that itself.
When you're debugging the code, you can set up the debugger to run it with that command line.
To make your program a little more versatile, you might want to consider rewriting your program to use fscanf, fprintf, etc. so that it can already handle file IO as opposed to just console IO; then when you want to read from stdin or write to stdout, you would just do something along the lines of:
FILE *infile, *outfile;
if (use_console) {
infile = stdin;
outfile = stdout;
} else {
infile = fopen("intest.txt", "r");
outfile = fopen("output.txt", "w");
}
fscanf(infile, "%d", &x);
fprintf(outfile, "2*x is %d", 2*x);
Because how often do programs only handle stdin/stdout and not allow files? Especially if you end up using your program in shell scripts, it can be more explicit to specify input and outputs on the command line.
int fd[2];
pipe(fd);
close(0); // 0:stdin
dup(fd[0], 0); // make read pipe be stdin
close(fd[0]);
fd[0] = 0;
write(fd[1], "some text", 9); // write "some text" to stdin