If you wanted to avoid <string.h>, you could use the poor man's strlen(), snprintf(0,0,"%s",argv[1]). For full input validation without adding any more statements, the best I can get (in ISO C) is

      uint8_t number = (argc<2||sscanf(*++argv,"%*[0123456789]%n",&argc)||argc--[*argv]?printf("bad\n"):argc[*argv])-'0'; // No problems here

Though with either function you may run into issues if the OS allows arguments longer than INT_MAX. To be defensive, you could use "%32767s" or "%*32767[0123456789]%n" instead, at the cost of failing inputs longer than 32KiB.