int argc — the number of strings in argv (always at least 1).
char *argv[] — an array of pointers to null-terminated C strings.

argv[0] is always the name used to invoke the program.
argv[argc] is always a NULL pointer — marks the end of the list.

Since argv[0] holds the name used to invoke the program, you can create multiple hard links to one executable and have it behave differently based on its name. Classic example: gzip, gunzip, and zcat are all the same binary on disk, distinguished only by argv[0].

You can read the command-line arguments of any process (given its PID) from the kernel’s virtual filesystem:

$ cat /proc/self/cmdline | tr '\0' ' '
cat /proc/self/cmdline

$ cat /proc/1234/cmdline | tr '\0' ' '
/usr/bin/python3 myscript.py --verbose

# Each argument is separated by a null byte (\0)
# tr '\0' ' '  replaces nulls with spaces for display

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    printf("Number of arguments (argc): %d\n\n", argc);

    for (int i = 0; i < argc; i++) {
        printf("argv[%d] = \"%s\"\n", i, argv[i]);
    }

    /* argv[argc] is always NULL — safe sentinel */
    printf("\nargv[%d] = %s (NULL terminator)\n",
           argc, argv[argc] == NULL ? "NULL" : "not NULL");

    return 0;
}

/* Compile: gcc -o myecho myecho.c
   Run:     ./myecho hello world 42

   Output:
   Number of arguments (argc): 4

   argv[0] = "./myecho"
   argv[1] = "hello"
   argv[2] = "world"
   argv[3] = "42"

   argv[4] = NULL (NULL terminator) */

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    if (argc < 2) {
        fprintf(stderr, "Usage: %s num1 num2 ...\n", argv[0]);
        return 1;
    }

    long sum = 0;
    for (int i = 1; i < argc; i++) {
        long val = strtol(argv[i], NULL, 10);  /* safer than atoi */
        sum += val;
        printf("  Adding argv[%d] = %ld\n", i, val);
    }

    printf("Total sum = %ld\n", sum);
    return 0;
}

/* Run: ./sum 10 20 30 40
   Output:
     Adding argv[1] = 10
     Adding argv[2] = 20
     Adding argv[3] = 30
     Adding argv[4] = 40
   Total sum = 100 */

#include <stdio.h>
#include <string.h>

/* One binary that behaves like two different tools
   depending on the name it is invoked as. */
int main(int argc, char *argv[])
{
    /* Extract just the basename (strip directory path) */
    const char *prog = strrchr(argv[0], '/');
    prog = (prog != NULL) ? prog + 1 : argv[0];

    if (strcmp(prog, "say_hello") == 0) {
        printf("Hello, %s!\n", argc > 1 ? argv[1] : "World");
    } else if (strcmp(prog, "say_bye") == 0) {
        printf("Goodbye, %s!\n", argc > 1 ? argv[1] : "World");
    } else {
        printf("Unknown invocation name: %s\n", prog);
    }
    return 0;
}

/* Setup and run:
   $ gcc -o say_hello multi_name.c
   $ ln say_hello say_bye         (hard link)

   $ ./say_hello Ravi
   Hello, Ravi!

   $ ./say_bye Ravi
   Goodbye, Ravi!

   Both run the SAME binary, different behaviour via argv[0]. */