In the picture above, the code can simply be run from the terminal. pipeclient and pipeserver files are run. pipeclient file takes command.txt as an input using the < symbol.
Now, if I don't want to run the pipeclient file from the terminal, but want to run from a C program, how would I go about it? Will the exec set of functions help me? How would I run the pipeclient file with the command.txt input file from a C program ?
The low-level way to run a program with input taken from a file is to:
opendup2 system call to duplicate the open file handle over top of the handle for standard input (always handle 0)close the old handle (the copy duplicated on top of handle 0 will stay open)execve or another function from the exec family to switch to the new program, which will now have its standard input opened to the file.This is the same way the shell itself implements the < file input redirection.
Here's an example that runs /usr/bin/rev using input from a commands.txt file in the current directory:
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
extern char **environ;
#define ERROR(msg) do { perror(msg); exit(1); } while(0)
int main()
{
// open the input file
int fd = open("commands.txt", O_RDONLY);
if (fd == -1)
ERROR("open");
// "dup" the handle to standard input (handle 0)
if (dup2(fd, 0) == -1)
ERROR("dup2");
// close the old handle
close(fd);
// exec the program
char *args[] = {"rev", NULL};
execve("/usr/bin/rev", args, environ);
// the program never gets here, unless the exec fails
ERROR("execve");
return -1;
}
You can also use the system command which executes a shell command including redirection, so the program:
#include <stdio.h>
#include <stdlib.h>
#define ERROR(msg) do { perror(msg); exit(1); } while(0)
int main()
{
if (system("/usr/bin/rev <commands.txt"))
ERROR("system");
// this *will* return after completion
return 0;
}
will work as well. Here, the system call is actually invoking a copy of a shell ("/bin/sh") to process the command line as if it was a shell command.
This is more convenient, but you have less control over the process of invoking the child program (e.g., setting up its environment, cleaning its argument list, etc.). There are also complex potential security issues with using system that can be a problem if your program will run as root, though that may not be an issue here.
you can use system function to call your 2nd program for 1st c program.like
1st.c
#include <stdio.h>
#include<ctype.h>
#include <stdlib.h>
int main()
{
printf("I m 1st program");
system("./2nd.out\n");
}
2nd.c
#include <stdio.h>
#include<ctype.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
printf("Hello there i m 2nd program\n")
}
