popen: intercepting user's input - c

Question

I have a code which runs bc thru popen(). I can intercept the calculator's output and prepend it with "Output=" text. But how can I intercept what user's is writing to bc?
#include <stdio.h>
#include <stdlib.h>
int main(void) {
FILE *in;
char buff[512];
if(!(in = popen("bc", "r"))){
exit(1);
}
while(fgets(buff, sizeof(buff), in)!=NULL){
printf("Output = %s", buff);
}
pclose(in);
return 0;
}

Answer 1

You can combine bc and echo with a pipe: echo '12*4' | bc
Example typing 12*4:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void) {
FILE *in;
char buff[512];
char cmd[512];
while (fgets(buff, sizeof(buff), stdin)!=NULL){
strcpy(cmd, "echo '");
strcat(cmd, buff);
strcat(cmd, "' | bc");
if(!(in = popen(cmd, "r"))){
exit(1);
}
fgets(buff, sizeof(buff), in);
printf("output:%s", buff);
}
pclose(in);
return 0;
}
Output:
david#debian:~$ ./demo
12*4
output:48

Answer 2

You need to use pipe() and fork/exec(). However, manual piping is quite complex:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(void) {
int write_pipe[2], read_pipe[2];
pipe(read_pipe); pipe(write_pipe);
#define PARENT_READ read_pipe[0]
#define CHILD_WRITE read_pipe[1]
#define CHILD_READ write_pipe[0]
#define PARENT_WRITE write_pipe[1]
int child = fork();
if (child == 0) { /* in child */
close(PARENT_WRITE);
close(PARENT_READ);
dup2(CHILD_READ, 0); close(CHILD_READ);
dup2(CHILD_WRITE, 1); close(CHILD_WRITE);
execl("/usr/bin/bc", "/usr/bin/bc");
} else { /* in parent */
close(CHILD_READ);
close(CHILD_WRITE);
write(PARENT_WRITE, "2+3\n", 4);
char buff[512];
int output_len=read(PARENT_READ, buff, sizeof(buff));
write(1, buff, output_len);
close(PARENT_READ);
}
return 0;
}

Answer 3

What you're looking to do is to start a subprocess, then simultaneously:
When activity occurs on standard input, execute some function on that input before passing it to the subprocess.
When activity occurs on the subprocess output, execute some function on that output before passing it to standard output.
The system call that allows you to wait for activity on two handles is called poll, but before we do that, we need to create the handles and start the subprocess:
int a[2], b[2];
if(pipe(a)==-1)abort(); // for communicating with subprocess input
if(pipe(b)==-1)abort(); // for communicating with subprocess output
switch(fork()) {
case -1: abort();
case 0: dup2(a[0],0), dup2(b[1],1), execlp("/usr/bin/bc", "bc", 0); exit(1);
};
Note how pipe works: Data written to fildes[1] appears on (i.e., can be read from) fildes[0]. This means we want to read from the standard output of our subprocess, b[0] and write to the standard input of our subprocess a[1].
Before we do that, we can use the poll instruction to wait for activity on either standard input (fd #0), or the subprocess output (b[0]):
for(;;) {
struct pollfd p[2]={0};
p[0].fd = 0; p[1].fd = b[0];
p[0].events = p[1].events = POLLIN;
while (poll(p,2,-1) <= 0);
At this point, there is activity on at least one of these file descriptors. You can see which one by examining the .revents member.
if(p[0].revents & POLLIN) {
r = read(0, buffer, sizeof(buffer));
write(a[1], buffer, r); // check for errors, or perhaps modify buffer
}
if(p[1].revents & POLLIN) {
r = read(b[0], buffer, sizeof(buffer));
write(1, buffer, r); // check for errors, or perhaps modify buffer
}
Note especially we use the opposite member a[1] and b[0] from the member we dup2'd onto the subprocesses standard input (0) and standard output (1).
At this point you can loop back up to poll again:
}
Disconnects (like EOF, program crash, etc) will be presented as read() returning 0, so watch carefully for this case, and break; out of the loop if so desired.