How to send a simple string between two programs using pipes

Question

I tried searching on the net  but there are hardly any resources  A small example would suffice   EDIT I mean  two different C programs communicating with each other  One program should send  Hi  and the other should receive it  Something like that

User · Answer

This answer might be helpful for a future Googler.

#include <stdio.h>
#include <unistd.h>

int main(){     
     int p, f;  
     int rw_setup[2];   
     char message[20];      
     p = pipe(rw_setup);    
     if(p < 0){         
        printf("An error occured. Could not create the pipe.");  
        _exit(1);   
     }      
     f = fork();    
     if(f > 0){
        write(rw_setup[1], "Hi from Parent", 15);    
     }  
     else if(f == 0){       
        read(rw_setup[0],message,15);       
        printf("%s %d\n", message, r_return);   
     }  
     else{      
        printf("Could not create the child process");   
     }      
     return 0;

}

You can find an advanced two-way pipe call example here.

User · Answer

A regular pipe can only connect two related processes   It is created by a process and will vanish when the last process closes it  A named pipe  also called a FIFO for its behavior  can be used to connect two unrelated processes and exists independently of the processes  meaning it can exist even if no one is using it   A FIFO is created using the mkfifo   library function  Example writer c  include  lt fcntl h gt   include  lt sys stat h gt   include  lt sys types h gt   include  lt unistd h gt   int main         int fd      char   myfifo    quot  tmp myfifo quot           create the FIFO  named pipe         mkfifo myfifo  0666           write  quot Hi quot  to the FIFO        fd   open myfifo  O WRONLY       write fd   quot Hi quot   sizeof  quot Hi quot         close fd           remove the FIFO        unlink myfifo        return 0     reader c  include  lt fcntl h gt   include  lt stdio h gt   include  lt sys stat h gt   include  lt unistd h gt    define MAX BUF 1024  int main         int fd      char   myfifo    quot  tmp myfifo quot       char buf MAX BUF           open  read  and display the message from the FIFO        fd   open myfifo  O RDONLY       read fd  buf  MAX BUF       printf  quot Received   s n quot   buf       close fd        return 0     Note  Error checking was omitted from the above code for simplicity

User · Answer

First  have program 1 write the string to stdout  as if you d like it to appear in screen   Then the second program should read a string from stdin  as if a user was typing from a keyboard  then you run     program 1   program 2

User · Answer

dup2  STDIN FILENO  newfd     And read   char reading  1025    int fdin   0  r control  if  dup2  STDIN FILENO  fdin    lt  0        perror   dup2             exit  errno      memset  reading    0   1025    while    r control   read  fdin  reading  1024      gt  0        printf    lt  s gt    reading        memset  reading    0   1025      if  r control  lt  0       perror   read             close  fdin          But  I think that fcntl can be a better solution  echo  salut    code

User · Answer

What one program writes to stdout can be read by another via stdin  So simply  using c  write prog1 to print something using printf   and prog2 to read something using scanf    Then just run    prog1     prog2

User · Answer

Here s a sample   int main         char buff 1024     0       FILE  cvt      int status         Launch converter and open a pipe through which the parent will write to it        cvt   popen  converter    w        if   cvt                printf  couldn t open a pipe  quitting n            exit 1            printf  enter Fahrenheit degrees           fgets buff  sizeof  buff   stdin     read user s input           Send expression to converter for evaluation        fprintf cvt    s n   buff       fflush cvt          Close pipe to converter and wait for it to exit        status pclose cvt          Check the exit status of pclose          if   WIFEXITED status           printf  error on closing the pipe n        return 0      The important steps in this program are    The popen   call which establishes the association between a child process and a pipe in the parent  The fprintf   call that uses the pipe as an ordinary file to write to the child process s stdin or read from its stdout  The pclose   call that closes the pipe and causes the child process to terminate

User · Answer

From Creating Pipes in C  this shows you how to fork a program to use a pipe   If you don t want to fork    you can use named pipes     In addition  you can get the effect of prog1   prog2 by sending output of prog1 to stdout and reading from stdin in prog2   You can also read stdin by opening a file named  dev stdin  but not sure of the portability of that                                                                                    Excerpt from  Linux Programmer s Guide - Chapter 6    C opyright 1994-1995  Scott Burkett                                                                                  MODULE  pipe c                                                                                   include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   include  lt unistd h gt   include  lt sys types h gt   int main void            int     fd 2   nbytes          pid t   childpid          char    string      Hello  world  n           char    readbuffer 80            pipe fd            if  childpid   fork       -1                            perror  fork                    exit 1                      if childpid    0                               Child process closes up input side of pipe                    close fd 0                        Send  string  through the output side of pipe                    write fd 1   string   strlen string  1                    exit 0                     else                              Parent process closes up output side of pipe                    close fd 1                        Read in a string from the pipe                    nbytes   read fd 0   readbuffer  sizeof readbuffer                    printf  Received string   s   readbuffer                      return 0

[c] How to send a simple string between two programs using pipes?

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