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Q&A How kill a child process without read() hanging in the parent process?

Below is a new version of popen2() that works. Some pipe ends were not closed in the linked to original version. (see the branch for "pid != 0", where extra handles are closed - whether there is ...

posted 3y ago by sktpin‭ · edited 3y ago by sktpin‭

Answer

#5: Post edited by

sktpin‭ · 2021-10-07T09:41:42Z (about 3 years ago)

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Below is a new version of popen2() that works.
Some pipe ends were not closed in the linked to original version.
~~(see the branch for "pid != 0", where extra handles are closed - whethere there is an error or not, but it's the parent process)~~
(I also renamed the pipe variables to something more clear to me)
The exec_proc2() code from the 1st post remains unchanged.
```
pid_t popen2(const char *command, int *infp, int *outfp)
{
constexpr int READ = 0, WRITE = 1;
int tochild[2], fromchild[2];
if (pipe(tochild) != 0 || pipe(fromchild) != 0)
return -1;
const pid_t pid = fork();
if (pid != 0) // we're in the parent process
{ close(tochild[READ]); // Always close these ends, too
close(fromchild[WRITE]);
if (pid < 0) // ERROR
return pid;
}
else // if (pid == 0) // fork() returns the child process ID to the parent and returns 0 to the child process
{
dup2(tochild[READ], READ);
dup2(fromchild[WRITE], WRITE);
close(tochild[READ]); close(tochild[WRITE]);
close(fromchild[READ]); close(fromchild[WRITE]);
execl("/bin/sh", "sh", "-c", command, NULL);
perror("execl");
_exit(1);
}
if (infp == NULL)
close(tochild[WRITE]);
else
*infp = tochild[WRITE];
if (outfp == NULL)
close(fromchild[READ]);
else
*outfp = fromchild[READ];
return pid;
}
```

Below is a new version of popen2() that works.
Some pipe ends were not closed in the linked to original version.
(see the branch for "pid != 0", where extra handles are closed - whether there is an error or not, but it's the parent process)
(I also renamed the pipe variables to something more clear to me)
The exec_proc2() code from the 1st post remains unchanged.
```
pid_t popen2(const char *command, int *infp, int *outfp)
{
constexpr int READ = 0, WRITE = 1;
int tochild[2], fromchild[2];
if (pipe(tochild) != 0 || pipe(fromchild) != 0)
return -1;
const pid_t pid = fork();
if (pid != 0) // we're in the parent process
{ close(tochild[READ]); // Always close these ends, too
close(fromchild[WRITE]);
if (pid < 0) // ERROR
return pid;
}
else // if (pid == 0) // fork() returns the child process ID to the parent and returns 0 to the child process
{
dup2(tochild[READ], READ);
dup2(fromchild[WRITE], WRITE);
close(tochild[READ]); close(tochild[WRITE]);
close(fromchild[READ]); close(fromchild[WRITE]);
execl("/bin/sh", "sh", "-c", command, NULL);
perror("execl");
_exit(1);
}
if (infp == NULL)
close(tochild[WRITE]);
else
*infp = tochild[WRITE];
if (outfp == NULL)
close(fromchild[READ]);
else
*outfp = fromchild[READ];
return pid;
}
```

#4: Post edited by

sktpin‭ · 2021-10-07T09:39:28Z (about 3 years ago)

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Below is a new version of popen2() that works.
~~I found a crucial hint elsewhere with someone having a similar problem.~~
Some pipe ends were not closed in the linked to original version.
(I also renamed the pipe variables to something more clear to me)
The exec_proc2() code from the 1st post remains unchanged.
```
pid_t popen2(const char *command, int *infp, int *outfp)
{
constexpr int READ = 0, WRITE = 1;
int tochild[2], fromchild[2];
if (pipe(tochild) != 0 || pipe(fromchild) != 0)
return -1;
const pid_t pid = fork();
if (pid != 0) // we're in the parent process
{ close(tochild[READ]); // Always close these ends, too
close(fromchild[WRITE]);
if (pid < 0) // ERROR
return pid;
}
else // if (pid == 0) // fork() returns the child process ID to the parent and returns 0 to the child process
{
dup2(tochild[READ], READ);
dup2(fromchild[WRITE], WRITE);
close(tochild[READ]); close(tochild[WRITE]);
close(fromchild[READ]); close(fromchild[WRITE]);
execl("/bin/sh", "sh", "-c", command, NULL);
perror("execl");
_exit(1);
}
if (infp == NULL)
close(tochild[WRITE]);
else
*infp = tochild[WRITE];
if (outfp == NULL)
close(fromchild[READ]);
else
*outfp = fromchild[READ];
return pid;
}
```

Below is a new version of popen2() that works.
Some pipe ends were not closed in the linked to original version.
(see the branch for "pid != 0", where extra handles are closed - whethere there is an error or not, but it's the parent process)
(I also renamed the pipe variables to something more clear to me)
The exec_proc2() code from the 1st post remains unchanged.
```
pid_t popen2(const char *command, int *infp, int *outfp)
{
constexpr int READ = 0, WRITE = 1;
int tochild[2], fromchild[2];
if (pipe(tochild) != 0 || pipe(fromchild) != 0)
return -1;
const pid_t pid = fork();
if (pid != 0) // we're in the parent process
{ close(tochild[READ]); // Always close these ends, too
close(fromchild[WRITE]);
if (pid < 0) // ERROR
return pid;
}
else // if (pid == 0) // fork() returns the child process ID to the parent and returns 0 to the child process
{
dup2(tochild[READ], READ);
dup2(fromchild[WRITE], WRITE);
close(tochild[READ]); close(tochild[WRITE]);
close(fromchild[READ]); close(fromchild[WRITE]);
execl("/bin/sh", "sh", "-c", command, NULL);
perror("execl");
_exit(1);
}
if (infp == NULL)
close(tochild[WRITE]);
else
*infp = tochild[WRITE];
if (outfp == NULL)
close(fromchild[READ]);
else
*outfp = fromchild[READ];
return pid;
}
```

#3: Post edited by

sktpin‭ · 2021-10-06T16:31:20Z (about 3 years ago)

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Markdown

Below is a new version of popen2() that works.
I found a crucial hint elsewhere with someone having a similar problem.
Some pipe ends were not closed in the linked to original version.
(I also renamed the pipe variables to something more clear to me)
The exec_proc2() code from the 1st post remains unchanged.
```
pid_t popen2(const char *command, int *infp, int *outfp)
{
constexpr int READ = 0, WRITE = 1;
int tochild[2], fromchild[2];
if (pipe(tochild) != 0 || pipe(fromchild) != 0)
return -1;
const pid_t pid = fork();
if (pid != 0) // we're in the parent process
{ close(tochild[READ]); // Always close these ends, too
close(fromchild[WRITE]);
if (pid < 0) // ERROR
return pid;
}
else // if (pid == 0) // fork() returns the child process ID to the parent and returns 0 to the child process
{
dup2(tochild[READ], READ);
dup2(fromchild[WRITE], WRITE);
close(tochild[READ]); close(tochild[WRITE]);
close(fromchild[READ]); close(fromchild[WRITE]);
execl("/bin/sh", "sh", "-c", command, NULL);
perror("execl");
_exit(1);
}
if (infp == NULL)
close(tochild[WRITE]);
else
*infp = tochild[WRITE];
if (outfp == NULL)
close(fromchild[READ]);
else
*outfp = fromchild[READ];
return pid;
}
```

Below is a new version of popen2() that works.
I found a crucial hint elsewhere with someone having a similar problem.
Some pipe ends were not closed in the linked to original version.
(I also renamed the pipe variables to something more clear to me)
The exec_proc2() code from the 1st post remains unchanged.
```
pid_t popen2(const char *command, int *infp, int *outfp)
{
constexpr int READ = 0, WRITE = 1;
int tochild[2], fromchild[2];
if (pipe(tochild) != 0 || pipe(fromchild) != 0)
return -1;
const pid_t pid = fork();
if (pid != 0) // we're in the parent process
{ close(tochild[READ]); // Always close these ends, too
close(fromchild[WRITE]);
if (pid < 0) // ERROR
return pid;
}
else // if (pid == 0) // fork() returns the child process ID to the parent and returns 0 to the child process
{
dup2(tochild[READ], READ);
dup2(fromchild[WRITE], WRITE);
close(tochild[READ]); close(tochild[WRITE]);
close(fromchild[READ]); close(fromchild[WRITE]);
execl("/bin/sh", "sh", "-c", command, NULL);
perror("execl");
_exit(1);
}
if (infp == NULL)
close(tochild[WRITE]);
else
*infp = tochild[WRITE];
if (outfp == NULL)
close(fromchild[READ]);
else
*outfp = fromchild[READ];
return pid;
}
```

#2: Post edited by

sktpin‭ · 2021-10-06T16:31:05Z (about 3 years ago)

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Raw

Markdown

Below is a new version of popen2() that works.
I found a crucial hint elsewhere with someone having a similar problem.
Some pipe ends were not closed in the linked to original version.
(I also renamed the pipe variables to something more clear to me)
```
pid_t popen2(const char *command, int *infp, int *outfp)
{
constexpr int READ = 0, WRITE = 1;
int tochild[2], fromchild[2];
if (pipe(tochild) != 0 || pipe(fromchild) != 0)
return -1;
const pid_t pid = fork();
if (pid != 0) // we're in the parent process
{ close(tochild[READ]); // Always close these ends, too
close(fromchild[WRITE]);
if (pid < 0) // ERROR
return pid;
}
else // if (pid == 0) // fork() returns the child process ID to the parent and returns 0 to the child process
{
dup2(tochild[READ], READ);
dup2(fromchild[WRITE], WRITE);
close(tochild[READ]); close(tochild[WRITE]);
close(fromchild[READ]); close(fromchild[WRITE]);
execl("/bin/sh", "sh", "-c", command, NULL);
perror("execl");
_exit(1);
}
if (infp == NULL)
close(tochild[WRITE]);
else
*infp = tochild[WRITE];
if (outfp == NULL)
close(fromchild[READ]);
else
*outfp = fromchild[READ];
return pid;
}
```

Below is a new version of popen2() that works.
I found a crucial hint elsewhere with someone having a similar problem.
Some pipe ends were not closed in the linked to original version.
(I also renamed the pipe variables to something more clear to me)
The exec_proc2() code from the 1st post remains unchanged.
```
pid_t popen2(const char *command, int *infp, int *outfp)
{
constexpr int READ = 0, WRITE = 1;
int tochild[2], fromchild[2];
if (pipe(tochild) != 0 || pipe(fromchild) != 0)
return -1;
const pid_t pid = fork();
if (pid != 0) // we're in the parent process
{ close(tochild[READ]); // Always close these ends, too
close(fromchild[WRITE]);
if (pid < 0) // ERROR
return pid;
}
else // if (pid == 0) // fork() returns the child process ID to the parent and returns 0 to the child process
{
dup2(tochild[READ], READ);
dup2(fromchild[WRITE], WRITE);
close(tochild[READ]); close(tochild[WRITE]);
close(fromchild[READ]); close(fromchild[WRITE]);
execl("/bin/sh", "sh", "-c", command, NULL);
perror("execl");
_exit(1);
}
if (infp == NULL)
close(tochild[WRITE]);
else
*infp = tochild[WRITE];
if (outfp == NULL)
close(fromchild[READ]);
else
*outfp = fromchild[READ];
return pid;
}
```

#1: Initial revision by

sktpin‭ · 2021-10-06T16:29:19Z (about 3 years ago)

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Raw

Markdown

Below is a new version of popen2() that works.
I found a crucial hint elsewhere with someone having a similar problem.
Some pipe ends were not closed in the linked to original version.
(I also renamed the pipe variables to something more clear to me)

```
pid_t popen2(const char *command, int *infp, int *outfp)
{
    constexpr int READ = 0, WRITE = 1;
    int tochild[2], fromchild[2];
    if (pipe(tochild) != 0 || pipe(fromchild) != 0)
        return -1;

    const pid_t pid = fork();

    if (pid != 0) // we're in the parent process
    {	close(tochild[READ]); // Always close these ends, too
        close(fromchild[WRITE]);
        if (pid < 0) // ERROR
            return pid;
     }
    else // if (pid == 0) // fork() returns the child process ID to the parent and returns 0 to the child process
    {
        dup2(tochild[READ], READ);
        dup2(fromchild[WRITE], WRITE);
        close(tochild[READ]); close(tochild[WRITE]);
        close(fromchild[READ]); close(fromchild[WRITE]);

        execl("/bin/sh", "sh", "-c", command, NULL);
        perror("execl");
        _exit(1);
    }

    if (infp == NULL)
        close(tochild[WRITE]);
    else
        *infp = tochild[WRITE];

    if (outfp == NULL)
        close(fromchild[READ]);
    else
        *outfp = fromchild[READ];

    return pid;
}
```

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