Testing C code from the command line: gccrun
I wrote a little utility the other day before I found myself in need of it: it takes its argument, compiles it as the body of a C program, and runs it.
This is way more convenient than you’d think at first glance: it turns out that, for one- or two-line programs where you’re just testing how something works, when 80% of your program is include boilerplate and making a source file and running gcc followed by the program, you’re somewhat disinclined to test small things. When you have a utility like this, you end up using it a lot…
How many file descriptors can I open?
dr-wily:~ geofft$ gccrun 'int fd; while ((fd = open("/dev/null", 0)) > 0) printf("%d\n", fd);' | tail -1 1023
What error does that get me?
dr-wily:~ geofft$ gccrun 'while (open("/dev/null", 0) > 0); perror("open");' open: Too many open files
Can I go higher by not having things open on lower fds, and just opening something on fd 1024?
dr-wily:~ geofft$ gccrun 'dup2(0, 1023); perror("dup2");' dup2: Success dr-wily:~ geofft$ gccrun 'dup2(0, 1024); perror("dup2");' dup2: Bad file descriptor`
What is errno 2?
dr-wily:~ geofft$ gccrun 'printf("%s\n", strerror(2));' No such file or directory
What is my “session leader”?
dr-wily:~ geofft$ gccrun 'printf("%d\n", getsid(0));' 18291 dr-wily:~ geofft$ ps 18291 PID TTY STAT TIME COMMAND 18291 pts/28 Ss 0:00 /bin/bash
This is, of course, no substitute for reading formal docs or writing rigorous tests, but it’s great for getting an intuition about how something works.
gccrun is a very tiny shell script. Most of the convenience comes from it including a bunch of headers by default; recommendations on headers to add, and patches in general, welcome.