你可以从头文件stdlib.h中使用long strtol(const char* nptr, char** endptr, int base)来检查整个字符串是否可以转换为数字：

char* end;
strtol( argv[1], &end, 10 );
if( *end == '\0' ){
    printf( "String is a natural number\n" );
} else {
    printf( "String is not a natural number\n" );
}

另一种方法是检查不是'+'、'-'或数字的字符

bool valid = true;
for( const char* it = argv[1]; *it; ++it ){
    if(!(( *it >= '0' && *it <= '9' ) || *it == '+' || *it == '-' )){
        valid = false;
    }
}
printf( "String is%s a natural number\n", valid ? "" : " not" );