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How do I capture stdout/stderr output from program running in a new process using apr?
apr_procattr_t *attr;
apr_proc_t newproc;

const char *progname;
const char *args[100];

// progname and args are populated with data here

apr_procattr_create(&attr, p);
apr_procattr_io_set(attr, APR_CHILD_BLOCK, APR_CHILD_BLOCK, APR_CHILD_BLOCK);
apr_procattr_cmdtype_set(attr, APR_PROGRAM_PATH);
apr_proc_create(&newproc, progname, args, NULL, attr, p);

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 * this work for additional information regarding copyright ownership.
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static void test_file_redir(abts_case *tc, void *data)
{
    apr_file_t *testout = NULL;
    apr_file_t *testerr = NULL;
    apr_off_t offset;
    apr_status_t rv;
    const char *args[2];
    apr_procattr_t *attr;
    apr_file_t *testfile = NULL;
    apr_size_t length;
    char *buf;

    testfile = NULL;
    rv = apr_file_open(&testfile, "data/stdin",
                       APR_FOPEN_READ | APR_FOPEN_WRITE | APR_FOPEN_CREATE | APR_FOPEN_EXCL,
                       APR_OS_DEFAULT, p);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    rv = apr_file_open(&testout, "data/stdout",
                       APR_FOPEN_READ | APR_FOPEN_WRITE | APR_FOPEN_CREATE | APR_FOPEN_EXCL,
                       APR_OS_DEFAULT, p);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    rv = apr_file_open(&testerr, "data/stderr",
                       APR_FOPEN_READ | APR_FOPEN_WRITE | APR_FOPEN_CREATE | APR_FOPEN_EXCL,
                       APR_OS_DEFAULT, p);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);

    length = strlen(TESTSTR);
    apr_file_write(testfile, TESTSTR, &length);
    offset = 0;
    rv = apr_file_seek(testfile, APR_SET, &offset);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    ABTS_ASSERT(tc, "File position mismatch, expected 0", offset == 0);

    rv = apr_procattr_create(&attr, p);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    rv = apr_procattr_child_in_set(attr, testfile, NULL);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    rv = apr_procattr_child_out_set(attr, testout, NULL);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    rv = apr_procattr_child_err_set(attr, testerr, NULL);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    rv = apr_procattr_dir_set(attr, "data");
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    rv = apr_procattr_cmdtype_set(attr, APR_PROGRAM_ENV);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);

    args[0] = "proc_child";
    args[1] = NULL;

    rv = apr_proc_create(&newproc, proc_child, args, NULL,
                         attr, p);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);

    rv = apr_proc_wait(&newproc, NULL, NULL, APR_WAIT);
    ABTS_INT_EQUAL(tc, APR_CHILD_DONE, rv);

    offset = 0;
    rv = apr_file_seek(testout, APR_SET, &offset);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);

    length = 256;
    buf = apr_pcalloc(p, length);
    rv = apr_file_read(testout, buf, &length);
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    ABTS_STR_EQUAL(tc, TESTSTR, buf);


    apr_file_close(testfile);
    apr_file_close(testout);
    apr_file_close(testerr);

    rv = apr_file_remove("data/stdin", p);;
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    rv = apr_file_remove("data/stdout", p);;
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
    rv = apr_file_remove("data/stderr", p);;
    ABTS_INT_EQUAL(tc, APR_SUCCESS, rv);
}