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Top Forums UNIX for Dummies Questions & Answers How to write to stdin of another program (program A -> [stdin]program B) Post 302220958 by redoubtable on Saturday 2nd of August 2008 12:32:50 PM
Old 08-02-2008
/*
* here's how you can do it...
* using popen()
*
#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define READ 0
#define WRITE 1

pid_t
popen2(const char *command, int *infp, int *outfp)
{
int p_stdin[2], p_stdout[2];
pid_t pid;

if (pipe(p_stdin) != 0 || pipe(p_stdout) != 0)
return -1;

pid = fork();
if (pid < 0)
return pid;
else if (pid == 0)
{
close(p_stdin[WRITE]);
dup2(p_stdin[READ], READ);
close(p_stdout[READ]);
dup2(p_stdout[WRITE], WRITE);
execl("/bin/sh", "sh", "-c", command, NULL);
perror("execl");
exit(1);
}

if (infp == NULL)
close(p_stdin[WRITE]);
else
*infp = p_stdin[WRITE];
if (outfp == NULL)
close(p_stdout[READ]);
else
*outfp = p_stdout[READ];
return pid;
}

/*
* now in main... infp will be the stdin (in file descriptor)
* and outfp will be the stdout (out file descriptor)
* have fun
*/

int
main(int argc, char **argv)
{
int infp, outfp;
char buf[128];

if (popen2("your-program-B", &infp, &outfp) <= 0)
{
printf("Unable to exec your-program-B\n");
exit(1);
}

memset (buf, 0x0, sizeof(buf));

write(infp, "Z\n", 2);
write(infp, "D\n", 2);
write(infp, "A\n", 2);
write(infp, "C\n", 2);
close(infp);
read(outfp, buf, 128);
printf("buf = '%s'\n", buf);
return 0;
}
 

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FD(4)							   BSD Kernel Interfaces Manual 						     FD(4)

NAME
fd, stdin, stdout, stderr -- file descriptor files DESCRIPTION
The files /dev/fd/0 through /dev/fd/# refer to file descriptors which can be accessed through the file system. If the file descriptor is open and the mode the file is being opened with is a subset of the mode of the existing descriptor, the call: fd = open("/dev/fd/0", mode); and the call: fd = fcntl(0, F_DUPFD, 0); are equivalent. Opening the files /dev/stdin, /dev/stdout and /dev/stderr is equivalent to the following calls: fd = fcntl(STDIN_FILENO, F_DUPFD, 0); fd = fcntl(STDOUT_FILENO, F_DUPFD, 0); fd = fcntl(STDERR_FILENO, F_DUPFD, 0); Flags to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR are ignored. IMPLEMENTATION NOTES
By default, /dev/fd is provided by devfs(5), which provides nodes for the first three file descriptors. Some sites may require nodes for additional file descriptors; these can be made available by mounting fdescfs(5) on /dev/fd. FILES
/dev/fd/# /dev/stdin /dev/stdout /dev/stderr SEE ALSO
tty(4), devfs(5), fdescfs(5) BSD
June 9, 1993 BSD
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