can't read from stream until child exits?
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by BobTurbo
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Published on 2010-04-26T11:22:11Z
Indexed on
2010/04/26
11:23 UTC
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OK I have a program that creates two pipes -> forks -> the child's stdin and stdout are redirected to one end of each pipe -> the parent is connected to the other ends of the pipes and tries to read the stream associated with the child's output and print it to the screen (and I will also make it write to the input of the child eventually).
The problem is, when the parent tries to fgets the child's output stream, it just stalls and waits until the child dies to fgets and then print the output. If the child doesn't exit, it just waits forever. What is going on? I thought that maybe fgets would block until SOMETHING was in the stream, but not block all the way until the child gives up its file descriptors.
Here is the code:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
FILE* fpin;
FILE* fpout;
int input_fd[2];
int output_fd[2];
pid_t pid;
int status;
char input[100];
char output[100];
char *args[] = {"/somepath/someprogram", NULL};
fgets(input, 100, stdin); // the user inputs the program name to exec
pipe(input_fd);
pipe(output_fd);
pid = fork();
if (pid == 0) {
close(input_fd[1]);
close(output_fd[0]);
dup2(input_fd[0], 0);
dup2(output_fd[1], 1);
input[strlen(input)-1] = '\0';
execvp(input, args);
}
else {
close(input_fd[0]);
close(output_fd[1]);
fpin = fdopen(input_fd[1], "w");
fpout = fdopen(output_fd[0], "r");
while(!feof(fpout)) {
fgets(output, 100, fpout);
printf("output: %s\n", output);
}
}
return 0;
}
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