Barnoid’s Knowledge Base

Consider the following C++ code:

class A
{
int do(string key, bool b)         { cout << "do('" << key << "', " << (b ? "true" : "false") << ")" << endl; };
int do(string key, string value) { cout << "do('" << key << "', '" << value << "')" << endl; };
};

...

A *a = new A();
a->do(”do”, “something”);

What is printed? Well, not what I expected:

# ./reason1
do('do', true)

Intuitively, the second method, do(string string), should be executed, but gcc (4.1) executes the first method, do(string, bool). This is because the second ’string’, “something”, is interpreted as ‘char *’.

One down, 1000 more reasons to come.

Sometimes you’ve got data in a compressed form, but you need to read and process the (uncompressed) data in a program. If, as in my case, the uncompressed data (a program execution trace) is around 8 GB, and the compressed trace only 400MB big, uncompressing that data to disk, reading it and, after processing, deleting it is a big hassle.

It would be nice to directly uncompress and read the data from within the program…and luckly this is actually very simple:

Let’s say, we have a compressed file, trace.dat.gz, then we need to read from.

The code to read the decompressed data goes something like this:

FILE *trace = fopen(filename, "r");

while ((res = fscanf(trace, "%s "lx %lx", &status, &adr, &data)) == 3) {
    /* process data */
}

fclose(trace);

The trick to directly read from the compressed file is to execute a process that decompresses the data and directly feeds it into our program. That’s what pipes & fork are for:

int pfd[2];
FILE *trace = NULL;
if (pipe(pfd) != 0) return false;
pid_t cpid = fork();
if (cpid == 0) {
    // child process: redirect pipe to stdout & start the decompressor
    close(pfd[0]); dup2(pfd[1], 1); close(pfd[1]);
    excelp("gunzip", "gunzip", "-c", filename, NULL);
    exit(0);
} else {
    // parent: read from the pipe
    close(pfd[1]);
    trace = fdopen(pfd[0], "r");
}

while ((res = fscanf(trace, "%s "lx %lx", &status, &adr, &data)) == 3) {
    /* process data */
}

fclose(trace);
close(pfd[0]);

That’s it! Our program now automatically runs “gunzip” on the compressed data before piping it into our program.