How do you read gzip output from 'C' program?

This is a discussion on How do you read gzip output from 'C' program? within the C Programming forums, part of the General Programming Boards category; Hi, I'm coding in "C" and I am using the PIPE method to invoke the running of a gzip command ...

  1. #1
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    How do you read gzip output from 'C' program?

    Hi,
    I'm coding in "C" and I am using the PIPE method to invoke the running of a gzip command from another program. Its a backup program and part of the program is to verify the integrity of the gzipped backup file. To do this I try to execute the command:
    "gzip -vt <backupfile>.tar.gz" via opening a PIPE.
    The pipe opens and executes this command fine however the resulting output doesn't get sent back up the PIPE to my program, it just gets printed out in the shell. E.g. "<backupfile>.tar.gz: OK" shows on the screen, but my program doesn't get this output sent back to it. So my program never knows the result of this test. How can I get my program to read this shell outputted result?

    Any help greatly appreciated.

  2. #2
    and the hat of wrongness Salem's Avatar
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    For some reason, gzip writes to stderr rather than stdout.

    How exactly are you invoking it?

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    Follow Up

    Just a basic PIPE:

    Code:
    int c1=0;
    FILE *pi;
    char temp[1000]={'\x00'};
    char backupfilename[1000]={"backup.tar.gz"};
    char c;
    
    sprintf(temp, "gzip -vt %s", backupfilename);
    pi=popen(temp, "r");
    if(pi != NULL)
            {
            c=fgetc(pi);
            while(c != '\xff')
                    {
                    temp[c1]=c;
                    c=fgetc(pi);
                    c1++;
                    }
            temp[c1]='\x00';
            pclose(pi);
            }
    The code then goes on the parse whats in the temp array, looking for the string "OK", so the program knows its ok. But of course the temp array is empty as the output from gzip isn't sent to STDOUT as you've said.

    So how can I get the output from gzip into my temp array?

    By the way many many thanks for your assistance, really is appreciated!!!

  4. #4
    and the hat of wrongness Salem's Avatar
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    > sprintf(temp, "gzip -vt %s", backupfilename);
    This might work to redirect stderr to stdout.
    sprintf(temp, "gzip -vt %s 2>&1", backupfilename);

    If not, then you'd need to use fork()/execl()/dup() to get control of the stderr descriptor for yourself.

  5. #5
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    Yes

    Yes it WORKS

    THANK YOU, THANK YOU, THANK YOU.

    You are the best, I always get excellent help around these boards. Thanks a million !!!!!!!!!!!!!!!!!!!!!!!!!!

  6. #6
    cwr
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    Why don't you just use system("gzip -t <filename>") and look at the return value from system() to see if the archive is okay or not?

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