Linux exec function: what is the arg0 parameter used for?

Question

Here is the prototype of the function execlp:

int execlp(const char *file, const char *arg, ...);

The man page says that the first argument of arg(i.e. arg0), "by convention, should point to the filename associated with the file being executed."

Then I did these experiments:

/*These three lines all produce the expected result:
 .  ..  a.out  main.c */
execlp("ls", "ls", "-a", 0);
execlp("ls", "arg0 is meaningless", "-a" , 0);
execlp("ls", "", "-a" , 0);

/*But this one does not work:
 a.out  main.c */
execlp("ls", "-a" , 0);

So the question is, is the arg0 parameter meaningful under any circumstances? Why the interface was designed like this?

Answer 1

The main function signature is

int main(int argc, char ** argv);

Where argv[0] is the name of the executable (arg0 in your case), so the application expects its command line from argv[1].

In some cases single binary can have multiple names (busybox, for example, sometimes uses symbolic links with the different names, pointing to the single binary). In such cases argv[0] is used to determine which link was used to call the binary.

Answer 2

Programs can use argv[0] to behave differently depending on how they are called. For example, see this snippet from args.c from xz-utils:

        const char *name = strrchr(argv[0], '/');
    if (name == NULL)
        name = argv[0];
    else
        ++name;

    // Look for full command names instead of substrings like
    // "un", "cat", and "lz" to reduce possibility of false
    // positives when the programs have been renamed.
    if (strstr(name, "xzcat") != NULL) {
        opt_mode = MODE_DECOMPRESS;
        opt_stdout = true;
    } else if (strstr(name, "unxz") != NULL) {
        opt_mode = MODE_DECOMPRESS;
    } else if (strstr(name, "lzcat") != NULL) {
        opt_format = FORMAT_LZMA;
        opt_mode = MODE_DECOMPRESS;
        opt_stdout = true;
    } else if (strstr(name, "unlzma") != NULL) {
        opt_format = FORMAT_LZMA;
        opt_mode = MODE_DECOMPRESS;
    } else if (strstr(name, "lzma") != NULL) {
        opt_format = FORMAT_LZMA;
    }