PUSH_SWAP | Notion

You might wonder why the operating system passes everything as strings instead of trying to figure out types. The answer is simplicity and flexibility. The command line is fundamentally text-based. The shell doesn't know what your program expects - maybe you want numbers, maybe you want filenames, maybe you want options like -v or --verbose. By passing everything as strings, your program has complete control over interpreting its arguments however it needs to.
int main (int argc, char ** argv) :

PS : Normally, File Descriptor 0 is connected to your keyboard (waiting for you to type).When you use the pipe |, the shell disconnects the keyboard and plugs the output of push_swap directly into File Descriptor 0.
Physically, a pipe is a temporary kernel memory buffer acting as a First-In-First-Out (FIFO) queue, not a file on the disk. When the shell sees a |, it requests this buffer via pipe(), creates two cloned processes via fork(), and uses dup2() to rewire their internal plumbing before they even start running: the first process (push_swap) has its Standard Output (FD 1) redirected to the pipe's write-end, and the second process (checker) has its Standard Input (FD 0) redirected to the pipe's read-end. This allows push_swap to printf directly into RAM and checker to read from that same RAM, with the OS kernel pausing and waking them in real-time to prevent the buffer from overflowing or underflowing, seamlessly passing data until the writer exits and sends an EOF.
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