c - Redirect child process's stdin and stdout to pipes -

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edit: solution

 int c1=dup2(pipes[0][1],stdout_fileno);  int c2=dup2(pipes[1][0],stdin_fileno);    setvbuf(stdout,null,_ionbf,0); 

it setvbuf set stdout non-buffered. though printing newline character if destination not actual screen, guess, becomes buffered.

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edit: when put fflush(stdout) after line 1 , fflush(fout) after line 4 works expected. however, not work without fflush(stdout) after line 1. problem not able put fflush in program planning run.

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i trying start program process. don't have access code know uses stdin , stdout user interaction. trying start program creating 2 pipes, fork-ing , redirecting child's stdin/stdout proper pipe ends. points parent should able communicate child via file descriptors, while stdin/stdout should intact. popen syscall opens unidirectional pipe. following code works.

there 4 lines marked line 1..4.

line 1 child sending pipe, line 2 child receiving pipe, line 3 parent sending pipe, line 4 parent receiving pipe,

this toy example make sure things work. issue is 4 lines line1..4 uncommented output see on terminal

parent1: -1 fd: 1 0    4 5    0 1 debug1: 0 debug2: 0 

while if line 1 , line 3 uncommented see continuous stream of data. same happens if if line 2 , line 4 uncommented. however, want full bidirectional communication. adding commented sleep not change behavior.

what issue here. wonder why there no bidirectional popen.

int pid; int pipes[2][2];  pipe(pipes[0]); pipe(pipes[1]);  pid=fork();  if(pid==0)   {   //usleep(1000000);   close(pipes[0][0]);   close(pipes[1][1]);    int c1=dup2(pipes[0][1],stdout_fileno);   int c2=dup2(pipes[1][0],stdin_fileno);   //int c2=dup2(stdin_fileno,pipes[1][0]);    fprintf(stderr,"fd: %d %d    %d %d    %d %d\n",c1,c2,pipes[0][1],pipes[1][0],stdin_fileno,stdout_fileno);    //file*fout=fdopen(pipes[0][1],"w");   //file*fin =fdopen(pipes[1][0],"r");   while(1)     {     static int c1=0;     fprintf(stderr,"debug1: %d\n",c1);     printf("%d\n",c1);                      // line 1     fprintf(stderr,"debug2: %d\n",c1);     scanf("%d",&c1);                        // line 2     fprintf(stderr,"debug3: %d\n",c1);     c1++;     }   //fclose(fout);   //fclose(fin);   return 0;   }  close(pipes[0][1]); close(pipes[1][0]);  char buffer[100]; file*fin=fdopen(pipes[0][0],"r"); file*fout=fdopen(pipes[1][1],"w"); while(1)   {   int c1=-1;   printf("parent1: %d\n",c1);   fscanf(fin,"%d",&c1);                         // line 3   printf("recv: %d\n",c1);    fprintf(fout,"%d\n",c1+1);                    // line 4   printf("parent3: %d\n",c1+1);   } fclose(fin); fclose(fout); 

your code quite long i'm not sure have understand why don't use select ? want redirect output of child in tird process or use in parent process ?

the following exemple cat in child process.

#include <unistd.h> #include <stdlib.h>  int     main() {   pid_t pid;   int   p[2];     pipe(p);   pid = fork();   if (pid == 0)     {       dup2(p[1], 1); // redirect output (stdout pipe)       close(p[0]);       execlp("cat", "cat", null);       exit(exit_failure);     }   else     {       close(p[1]);       fd_set rfds;       char      buffer[10] = {0};         while (1)         {           fd_zero(&rfds);           fd_set(p[0], &rfds);            select(p[0] + 1, &rfds, null, null, null); //wait changes on p[0]           if(fd_isset(p[0], &rfds))             {               int       ret = 0;               while ((ret = read(p[0], buffer, 10)) > 0) //read on pipe                 {                   write(1, buffer, ret); //display result                   memset(buffer, 0, 10);                 }             }         }     } }