Do streams have to be closed when using popen

Question

As the title says , I am unsure if I should close a stream that was opened using popen.

The reason I am unsure is because every time i call pclose on a stream that was opened using popen I get a -1 return code.

If I make a call to perror after that I get the following message.

pclose: No Child Processes

The code that I am using below is basically to run a command and capture its output.I get the error from the last line (return pclose(fileListingStream);)

int executeCommand(char *command) {
    //The Stream to read that will contain the output of the command
    FILE *fileListingStream;
    char path[PATH_MAX];

    //Run the commmand in read mode
    fileListingStream = popen(command,"r");

    //Ensure that its not null before continuing
    if (fileListingStream == NULL)
        return EXIT_FAILURE;

    //Get the data from the stream and then print to the the console
    while (fgets(path, PATH_MAX, fileListingStream) != NULL)
        printf("%s", path);

    //Close the stream and return its return code
    return pclose(fileListingStream);
}

Does your program ignore or handle `SIGCHLD` by any chance? – caf Nov 30 '10 at 01:31 — caf, Nov 30 '10 at 01:31

score 5 · Accepted Answer · edited May 23 '17 at 12:18

Yes you should. See this answer for an explanation on the inner workings of pclose(). Furthermore you should note that errors in wait4() can be the cause of an apparent failure in pclose().

Update0

If the FILE * is valid (internally this is signified by the file descriptor not being -1), pclose() and fclose() will not cause leaks if there is an error. It's worth noting that if the FILE * is not valid, there's nothing to clean up anyway. As discussed in the answer I linked to, there is extra behaviour for pclose(), namely removing the FILE * from the proc file chain, and then waiting for the child to terminate. The internal wait is actually the second last thing done for a pclose(), everything has already been cleaned up by this point. Immediately after waiting, the contents of the FILE are trashed to signify its invalidity, this occurs regardless of any error in waitpid().

Given the error you are receiving, ECHILD, I can definitively say that there is no memory leak for pclose() under eglibc-2.11.1, and likely any glibc-derived library for at least the past 1-4 years.

If you wish to be completely certain, just run your program under valgrind, and trigger the ECHILD error. Valgrind will inform you if anything was leaked.

Do I still need to worry about memory leaks if the pclose fails ? , also what else can I do to close the file. — RC1140, Nov 29 '10 at 15:37

score 2 · Answer 2 · edited Nov 14 '12 at 09:50

2

If your application modify the disposition of SIGCHLD, it could interfere with popen()'s ability to wait for the shell to exit.

edited Nov 14 '12 at 09:50

Stephan

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answered Aug 17 '11 at 02:39

san

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score 0 · Answer 3 · answered Nov 29 '10 at 14:57

That error is ECHILD - meaning that waitpid returned an error, it could not get the return status of the child process. That can be caused by what 'command' actually int erms of process creation.

As every one else mentioned you should pclose, because it leaves a 'dangling' file descriptor, and can cause child processes not to be cleaned up promptly.

Do streams have to be closed when using popen

3 Answers3

Update0